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3.2.2 Timed Exposure Science Mode

3.2.2.1 Purpose

An observer uses Timed Exposure Science Mode to acquire, process and telemeter two dimensional images from the CCD array. An observer uses a Timed Exposure Parameter Block to configure Timed Exposure Science Mode, and "Start Timed Exposure Run" and "Stop Timed Exposure Run" commands to start and stop the run.

3.2.2.2 Scenario

To execute a Timed Exposure Science Run, an observer sends a "Start Timed-Exposure" command to ACIS. ACIS retrieves the parameter block referenced by the start command, dumps the parameter block(s), and proceeds to execute the science run, optionally computing the CCD bias values, and then processing the event data. As the event data is being processed and telemetered, ACIS will compress and trickle the computed bias values into the telemetry stream. The observer allows ACIS to acquire, process and telemeter CCD data for a period of time, after which the observer sends a "Stop Timed-Exposure" command to ACIS. ACIS then stops the run and telemeters a Science Run Report. Optionally, the observer may then decide to recompute the bias values and send a "Compute Timed-Exposure Bias" command. ACIS will then re-compute and telemeter the bias values and then stop.

If the observer requires parameters not already provided by the ACIS parameter library, the observer must issue a "Load Timed-Exposure Parameter Block" command to load the desired parameters for the run. If the loaded parameter block requires custom event processing windows, the observer must also issue a "Load 2-D Window List" command to place the desired window settings into the instrument.

Figure 17 illustrates the interactions involved to configure and start a Timed Exposure Science Run. Figure 18 illustrates some of the Front End processor interactions during the run and Figure 19 illustrates what the Back End processor does with the generated data. Whether or not the Bad Pixel and Column Maps are applied by the Front End or Back End Processors is left for the design phase, and is not shown in the diagrams. See Section 3.2.2.3.13 on page 64 for the detailed requirements of the functionality of the Bad Pixel and Column Map applications.

FIGURE 17. Timed Exposure Setup and Start Sequence Diagram


FIGURE 18. Timed Exposure Front End Processor Sequence Diagram


FIGURE 19. Timed Exposure Back End Event Processing Sequence Diagram


FIGURE 20. Timed Exposure Pixel Processing Data Flow


3.2.2.3 Functional Requirements
3.2.2.3.1 Timed Exposure Parameter Block Content

An observer configures Timed Exposure Mode using a Timed Exposure Mode Parameter Block. Table 11 describes the content of a Timed Exposure Parameter Block.

TABLE 11. Timed Exposure Parameter Block Content (Continued)

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Req      Item                 Description                                                                  
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 11-1    Parameter Block      This item is a ground-supplied identifier which is echoed by the instru      
         Identifier           ment software in key portions of the science telemetry.                      
 11-2    FEP CCD Selec        These items specify which CCD a given FEP should process. The items          
         tion                 are indexed by FEP Id. Any given CCD may be specified more than              
                              once in the set, at the cost of reducing the total number of CCDs being      
                              used. CCDs or FEPs which are not powered on (see Section 3.2.7 on            
                              page 99) are ignored and will not be used for the run. If there are no       
                              powered CCD being processed by a powered FEP, the science run will           
                              be aborted.                                                                  
 11-3    Subarray Readout     An observer uses the start row and row count values to effectively           
         Start Row and Row    shorten the time it takes to transfer an image from the CCDs to the          
         Count                Front End Processors by sacrificing some of the imaging area. The start      
                              row and row count specifies which contiguous horizontal swath of             
                              CCD rows are transferred from the Framestore back to the FEPs. These         
                              two parameters apply to all selected CCDs. The Start Row parameter           
                              can range from 0 to 1023, where 0 corresponds to image area row 2, 1         
                              to image area row 3, and so on. (NOTE: Image area rows 0 and 1 are           
                              never used), and the Row Count parameter can range from 1 to 1024. If        
                              Start Row plus Row Count extends beyond the image area, the software         
                              truncates the subarray row count to fit.                                     
 11-4    Primary and Sec      These items control the amount of time that the Image Area of each           
         ondary Exposure      CCD is integrating data. This does not include the time taken to trans       
         Times                fer the Image Area into the Framestore. These values can range from 0        
                              to 10 seconds in 0.1 second increments, and apply to all of the selected     
                              CCDs. They are accurate to within one SRAM Major Cycle (~10Ás).              
                              The CCDs are clocked using the primary exposure time for 1 exposure,         
                              followed by the secondary exposure time for "N" exposures. "N" is            
                              determined by the Exposure Duty Cycle parameter. If "N" is 0, the sec        
                              ondary exposure time is not used. If both exposure times are used, the       
                              bias computation algorithms only use data from exposures integrated          
                              over the Secondary Exposure time.                                            
                              NOTE: If Image Area to Framestore transfers need to be performed at          
                              different times on each CCD, and the specified exposure time is shorter      
                              than can be supported by the number of selected CCDs, the shortest           
                              possible exposure time will be used instead of the specified time (i.e.      
                              the Image to Frame transfer time multiplied by one less than the num         
                              ber of selected CCDs).                                                       
 11-5    Exposure Duty        This parameter controls how many secondary exposures separate pri            
         Cycle                mary exposures. If this parameter is 0, all exposures use the Primary        
                              Exposure Time (i.e. the Secondary Exposure Time is ignored). If 1,           
                              there is one Secondary Exposure between each Primary Exposure, if 2,         
                              there are two Secondaries for each Primary, and so on. This parameter        
                              can range from 0 to 15.                                                      
 11-6    Output Register      This item controls the clocking mode of all of the CCD's Serial Output       
         Clocking Mode        Shift Registers. This item can specify either; Full-Mode, AC-Mode,           
                              BD-Mode or Diagnostic-Mode. See Section 2.2.2 on page 20 for a               
                              description of these modes.                                                  
 11-7    Number of Over       This item specifies the number of extra pixel pairs to clock out of each     
         clock Pairs          of the Serial Output Shift Registers. Timed Exposure Mode uses these         
                              pixels to determine the bias introduced to the CCDs pixel pulse height       
                              by the analog electronics. This item can specify 0 to 15 pairs of pixels.    
 11-8    Video Chain          These items control the response of the Video Subsection in the Detec        
         Responses            tor Electronics Assembly for each CCD. This item selects 1 or 4 elec         
                              trons per Analog-to-Digital Unit (ADU). These items are indexed by           
                              FEP Id. If a single CCD is specified by more than 1 FEP entry, then the      
                              last corresponding chain response entry is used for the listed CCD.          
 11-9    On-chip Summing      This item controls whether or not 2x2 on-chip summing is performed.          
         Select               If disabled, no pixel summing is performed on the CCD. If this is            
                              enabled, then the charge from pairs of adjacent columns and rows are         
                              summed on the CCD. Each measured pixel output from the CCD is the            
                              sum of a 2x2 pixel array from the chip.                                      
 11-10   Reference to an      This is an optional item which allows a maintainer to by-pass the on-        
         explicit DEA Load    board SRAM/PRAM load builder, and load these areas directly. This            
                              reference points directly into BEP memory. If 0, then no explicit DEA        
                              load is to be used. If not 0, then the item points to the memory block       
                              which contains the load image to use. Post-launch DEA load images            
                              are uplinked into memory by the maintainer using the Write Memory            
                              function of the instrument (see Section 3.2.8.3.2 on page 108), or via a     
                              patch. This parameter is intended to be used by the maintainer of the        
                              instrument, and should be 0 during normal operations. The format of          
                              the referenced SRAM/PRAM load will be specified in the ACIS Instru           
                              ment Software Detailed Design Specification (AS-BUILT), MIT 36-              
                              53200, and shall support different loads into different DEA boards.          
 11-11   Front End Selec      This item specifies whether or not the Front End Processor should sup        
         tion Algorithm       ply all pixels to the Back End (Raw Mode), compute histograms of raw         
                              pixel data (Histogram Mode), or supply pixels to the Back End whose          
                              center is above the current threshold (Event-finding Mode) and whose         
                              pulse height is greater (or sometimes equal) to its immediate neighbors      
                              (3x3 local maxima).                                                          
 11-12   Threshold Set        These items control minimum pulse-height above or below bias a pixel         
         Points               should have to be considered as an event candidate by a given FEP.           
                              Each of these items applies to a specific CCD output node being pro          
                              cessed by a FEP. These values can range from -4096 to 4095. These            
                              values are not used when the FEPs are processing raw pixels or histo         
                              grams. One set of four values is specified for each FEP.                     
 11-13   Reference to Front   This value allows a maintainer to override the standard code and data        
         End Processor        loaded into the Front End Processor, and provide a special load for the      
         Code                 Science Run. This reference points directly into BEP memory. If 0,           
                              then no explicit FEP load is to be used. If not 0, then the item points to   
                              the memory block which contains the load image to use. Post-launch           
                              FEP load images are uplinked into memory by the maintainer using the         
                              Write Memory function of the instrument (see Section 3.2.8.3.2 on            
                              page 108), or via a patch. This parameter is intended to be used by the      
                              maintainer of the instrument, and should be 0 during normal opera            
                              tions. The format of the referenced FEP Load image will be specified in      
                              the ACIS Instrument Software Detailed Design Specification (AS-              
                              BUILT), MIT 36-53200.                                                        
 11-14   Event List Packing   This parameter is only used when the Front End Selection Algorithm is        
         Code                 in "Event Finding Mode". This value is not used when the Front End is        
                              processing raw pixels or histograms. This value selects the format to        
                              use when sending processed events. Currently, the following event-list       
                              formats are supported;                                                       
                              Faint-Mode, Faint-with-Bias, and Graded-Mode                                 
 11-15   Reference to a 2-D   This parameter is only used when the Front End Selection Algorithm is        
         Window Collection    in "Raw Mode" or "Event Finding Mode". This value is not used when           
                              the Front End is processing histograms. An observer uses this item to        
                              specify a set of 2-dimensional event selection and processing windows        
                              to use for the Science Run. If no windows are specified, Timed Expo          
                              sure Mode shall process all events from the CCDs using the processing        
                              parameters specified in this block. See Table 13, "2-Dimensional Win         
                              dow List Parameter Block," on page 58.                                       
 11-16   Split Threshold      These parameters are only used when the Front End Selection Algo             
         Set Points           rithm is in "Event Finding Mode". These values are not used when the         
                              Front End is processing raw pixels or histograms. These values specify       
                              the threshold above which a neighboring pixel's pulse height must be to      
                              be considered as part of an event. These values are used when grading        
                              events and computing the amplitude of an event. Each of these items          
                              applies to a specific CCD output node being processed from a particu         
                              lar FEP. These values can range from 0 to 4095, and each set is indexed      
                              by FEP Id.                                                                   
 11-17   Ignore Bad Pixel/    These parameters are only used when the Front End Selection Algo             
         Column Map Flags     rithm is in "Event Finding Mode". These values are not used when the         
                              Front End is processing raw pixels or histograms. These values control       
                              whether or not to discard events on the basis of the CCD's list of bad       
                              pixels and/or columns.                                                       
 11-18   Grading Selection    These parameters are only used when the Front End Selection Algo             
         Bit Map              rithm is in "Event Finding Mode". These values are not used when the         
                              Front End is processing raw pixels or histograms. This set of values         
                              select which event grades to accept. Accepted events whose grades are        
                              indicated in this list shall be telemetered. This bitmap consists of a 256   
                              bits, where each bit corresponds to one of the possible grade codes.         
 11-19   Event Amplitude      These parameters are only used when the Front End Selection Algo             
         Lower Bound and      rithm is in "Event Finding Mode". These values are not used when the         
         Range                Front End is processing raw pixels or histograms. These parameters           
                              select the minimum and maximum amplitudes of accepted events.                
                              Events whose total amplitude (function of the 3x3 grid around the            
                              event) is less than the lower bound value, or greater than or equal to the   
                              lower bound plus range are not telemetered. These field's ranges are         
                              governed by the minimum and maximum corrected event amplitudes.              
 11-20   Re-compute Bias      This item indicates if the software must re-compute the pixel bias val       
         Flag                 ues prior to starting data processing.                                       
 11-21   Trickle Bias Flag    This item indicates if the software must telemeter the current bias map      
                              to telemetry after it has been computed.                                     
 11-22   Bias Algorithm       This specifies the type of bias computation to be performed. This field      
         Mode                 may indicate that the bias be computed using an algorithm which uses         
                              entire image frames for each of its iterations (faster, but less accurate)   
                              called "Whole Frame Mode", or uses an algorithm which incrementally          
                              operates on sections of the bias map (slower, but more accurate), called     
                              "Strip" Mode. The remaining bias parameters depend on the selected           
                              mode. There is one set of settings for each FEP, and each set is indexed     
                              by FEP Id.                                                                   
 11-23   Whole Frame:         This specifies the number of conditioning exposures used to determine        
         Conditioning         the minimum pixel bias value. This field can vary from 0 to 200 expo         
         Count                sures.                                                                       
 11-24   Whole Frame:         This specifies the number of approximation exposures used to locate          
         Approximation        the actual bias value above the minimum value determined during the          
         Count                conditioning pass. This field can vary from 0 to 200 exposures.              
 11-25   Whole Frame: Low     This parameter is used to discover abnormally low pixel values after         
         Pixel Rejection      the conditioning process, and is used only if it's value is not zero. If a   
         Threshold            pixel's value is smaller than all but 1 of its adjacent neighbors by more    
                              than this value, it is replaced with the median value of its neighbors.      
                              This field's value can range from 0 to 4095.                                 
 11-26   Whole Frame:         During the approximation-to-the-mean phase, this parameter is used to        
         Event Rejection      discard pixel samples which potentially contain charge from an event         
         Threshold            or charged-particle. If a sample pixel's value exceeds the current bias      
                              estimate by more than this value, it and its immediate neighbors are not     
                              used to adjust the bias estimate. This field's value can range from 0 to     
                              4095.                                                                        
 11-27   Whole Frame:         This parameter is used when approximating the mean to prevent sam            
         Rejection Thresh     ples whose pulse heights are uncharacteristically high from affecting        
         old for approxima    the mean estimate. Pixel samples whose pulse heights are larger than         
         tion-to-mean         the current pulse height estimate by more than this amount are not           
                              included in the estimated mean. This field's value can range from 0 to       
                              4095.                                                                        
 11-28   Strip Mode: Expo     This specifies the total number exposures to use per strip of pixels. This   
         sures Count          field can vary from 0 to 40 exposures per strip.                             
 11-29   Strip Mode: Mean/    This specifies whether to base the bias value on a truncated mean or         
         Median Selection     median of the pixel samples.                                                 
 11-30   Strip Mode: Mean     When a mean algorithm is selected, pixel samples whose value is              
         Sigma Rejection/     greater than this value multiplied by the RMS variance are not included      
         Median Index         in the final mean computation. The value can range from 0 to 5. When         
                              the median algorithm is selected, this parameters indexes the pulse          
                              height to use for the bias out of the sorted list. In this case, the field   
                              value can range from 0 to 4095.                                              
 11-31   Raw Histogram        This item specifies how many exposures to use when building histo            
         Exposure Count       grams. This field can vary from 0 to 240,000 exposures                       
 11-32   Bias Compression     This indicates whether or not to compress pixel bias map values prior        
         Flag                 to being telemetered.                                                        
 11-33   Bias Compression     If bias map compression is enabled, these items specify which com            
         Selections           pression table to use for a given FEP. These selections are indexed by       
                              FEP Id.                                                                      
 11-34   Raw Compression      This indicates whether or not to compress raw pixel data values prior to     
         Flag                 being telemetered.                                                           
 11-35   Raw Compression      If raw pixel data compression is enabled, this item specifies which          
         Selection            compression table to use for all FEPs.                                       
 11-36   Initial Frames to    This specifies the number of whole exposure frames that the FEP is to        
         Ignore               ignore at the start of a bias calibration or science run that uses this      
                              parameter block. If this value is zero, each FEP will start processing all   
                              data pixels and overclocks received after the first vsync code.              
                                                                                                           
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3.2.2.3.2 2-D Window Lists

Events produced by Timed Exposure Clocking have an originating position within the CCD's Imaging Section. Timed Exposure Mode has the capability to select and process events based on this position. This selection and processing is controlled using a 2-D Window Parameter Block. Table 12 illustrates the content within a single 2-D Window, and Table 13 illustrates the content of a list of 2-D Windows.

TABLE 12. 2-Dimensional Window Information (Continued)

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Req     Item             Description                                                                   
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 12-1   CCD Identifier   This selects the CCD (out of all 10) to which the window applies.             
 12-2   Window Position  This specifies the window position in terms of the bottom-left pixel          
                         within the CCD's Imaging Section.                                             
 12-3   Window Width     This specifies the width of the window in terms of Imaging Section col        
                         umns (1-1024).                                                                
 12-4   Window Height    This specifies the height of the window in terms of Imaging Section           
                         rows (1-1024).                                                                
 12-5   Sample Cycle     If processing event data, this specifies the sampling frequency of the        
                         window. If zero, no events are processed in the window. If one, then          
                         every event is processed, if 2, every other event is processed, if 3, every   
                         third, and so on.                                                             
                         If processing raw pixel data, this specifies whether or not to send pixels    
                         who belong to the window. If zero, all pixels belonging to the window         
                         are discarded. If not zero, the pixels are telemetered.                       
 12-6   Lower Event      When processing events, these items set the lower and upper event             
        Amplitude and    amplitude bounds used to select candidate events. Events whose ampli          
        Range            tude is outside this range are rejected. When processing raw pixel data,      
                         this parameter is ignored.                                                    
                                                                                                       
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TABLE 13. 2-Dimensional Window List Parameter Block (Continued)

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Req     Item              Description                                                                 
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 13-1   Parameter Block   This item is a ground-supplied identifier which is echoed by the instru     
        Identifier        ment software in key portions of the science telemetry.                     
 13-2   Window Count      This item identifies the number of 2-D Windows contained in the Win         
                          dow List. This value can range from 0 to 36. This quantity may not actu     
                          ally be loaded nor telemetered, but may be calculated using the length of   
                          the enclosing command or telemetry packet.                                  
 13-3   Windows[Win       This item is an array of 2-D windows. The content of these windows is       
        dow Count]        described in Table 12. The number of windows in the array is deter          
                          mined by "Window Count."                                                    
                                                                                                      
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3.2.2.3.3 Building of DEA PRAM and SRAM Loads

Due to the expected size of SRAM and PRAM loads (greater than 2Kbytes), the ACIS software shall be capable of building the SRAM and PRAM loads needed to perform a Timed Exposure Clocking Mode, as described in Section 2.2.3 on page 22.

The following itemizes the inputs needed by the builder:

The output shall consist of a series of DEA SRAM and PRAM load commands to the DEA sequencer.

The CCD Selections specify which DEA boards need to be programmed. Since power constraints require the instrument to stagger large parallel clock transfers (such as Image Area to Framestore transfers), the builder uses knowledge of the total number of CCDs selected to prevent transfers on the different CCDs from overlapping.

The exposure times control the time between the clearing of a CCD's Imaging Section and the transfer of its rows to its Framestore, and the duty cycle controls number of iterations of each type of exposure time.

The subarray readout start and end rows control which contiguous rows of the CCD image to actually transfer out of the Serial Output Shift Registers to the DPA.

The On-chip summing selection controls how the Framestore is clocked into the Output Register and how each pixel is processed at the CCD output nodes.

The Output Register Clocking Mode specifies how to clock out the output registers.

The number of overclock pixels determines how many pixels to clock out of the output registers after each row of image data has been transferred.

The video chain response information controls how the pixel data is processed in the DEA's analog video processing section.

The optional DEA SRAM/PRAM load override instructs the builder to ignore all of the previous parameters and to load the explicitly specified SRAM and PRAM commands.

3.2.2.3.4 Normal Exposure CCD Clocking Operations

These operations address the condition where CCD's exposure time is longer than the time it takes to transfer the image from its Framestore to the DPA. In this mode, ACIS shall program the DEA's PRAM and SRAM to clock the CCDs in the following fashion:

1. Clear the Imaging Section, Framestore and Serial Output Shift Registers.

This is accomplished by clocking the Imaging Section and Framestore Rows 1026 times, and then clocking all pixels out the output registers a number of times, discarding the output. For Full-Mode and Diagnostic-Mode, (256 + 4) pixels must be shifted out of the output registers in order to clear the registers once. In AC-Mode and BD-Mode, (512 + 8) pixels must be shifted. If overclocks are specified, clock and discard the output registers an additional number of times.

2. Expose the Imaging Section for the duration specified by "Exposure Time."

The exposure effectively starts after the last row of the Imaging section has been clocked into the Framestore, and before the output registers have been flushed. On the first exposure of the run, this can be accomplished by just waiting for the specified period of time. On subsequent exposures, however, this wait period is overlapped with the steps listed below. The effective "do-nothing" time is the exposure time minus the time it takes to copy the Imaging Section to the Framestore and to transfer the image to the DPA.

3. Transfer the Imaging Section to the Framestore.

This is accomplished by clocking 1026 rows from the Imaging Section to the Framestore and from the Framestore into the output registers. This has the effect of summing any garbage data previously left in the Framestore into the output registers. This operation shall take no more than 60 msec. The goal is to perform this operation as quickly as the hardware allows.

4. Position start of subarray at row 0 of the Framestore.

This is accomplished by clocking "Start Row+2" Framestore rows into the output register. This places the first subarray row into Framestore row 0. This also has the effect of summing all rows prior to the subarray with the contents of the output registers.

5. Clock out output registers and discard.

This is accomplished by clocking the pixels out of the output registers and into the output nodes and discarding the data. For Full-Mode and Diagnostic-Mode, (256 + 4) pixels must be shifted out in order to clear the registers. In AC-Mode and BD-Mode, (512 + 8) pixels must be shifted. If overclocks are specified, clock and discard the output registers an additional number of times.

6. Clock desired number of image rows (i.e. "End Row - Start Row + 1") to the DPA

This is accomplished by clocking one row at a time from the Framestore to the output registers, and then clocking pixels from the output registers to the output nodes. As each pixel's data arrives at the output node, it is sampled, and converted to a 12 bit digital representation, which is then transmitted to the DPA. If overclock data is needed, clock and sample the output registers an additional number of times. The readout of the Framestore shall take no longer than 6.5 seconds. The goal is to perform this operation as quickly as the hardware allows.

7. Repeat starting from step 2.

NOTE: Rows after "End Row" in the Framestore are clocked and discarded as part of 3) and 5). As the Imaging Section is transferred into the Framestore, the rows left from the previous image are summed into the output registers. The summed rows are then discarded as part of 5).

3.2.2.3.5 Short Exposure CCD Clocking Operations

These operations apply when the configured CCD exposure time is less than the time it takes to transfer the image from the Framestore to the DPA (NOTE: This time depends on the subarray parameters). In general, the only difference between this situation and the "Normal Exposure CCD Clocking Operations" is that there is an extra "flush" of the Imaging Section prior to the start of an exposure. In this mode, ACIS shall program the DEA's PRAM and SRAM to clock the CCDs in the following fashion:

1. Clear the Imaging Section, Framestore and Serial Output Shift Registers.

This is accomplished by clocking the Imaging Section and Framestore Rows 1026 times, and then clocking out the output registers and discarding the output. For Full-Mode and Diagnostic-Mode, (256 + 4) pixels must be shifted out of the output registers in order to clear the registers. In AC-Mode and BD-Mode, (512 + 8) pixels must be shifted. If overclocks are specified, clock and discard the output registers an additional number of times.

2. Expose the Imaging Section for the duration specified by "Exposure Time."

This is accomplished by waiting for the desired period of time. The exposure effectively starts after the last row of the Imaging Section has been clocked into the Framestore, and before the output registers have been flushed.

3. Transfer the Imaging Section to the Framestore.

This is accomplished by clocking 1026 rows from the Imaging Section to the Framestore and from the Framestore into the output registers. This has the effect of summing any garbage data previously in the Framestore into the output registers. This operation shall take no more than 60 msec. The goal is to perform this operation as quickly as the hardware allows.

4. Position start of subarray at row 0 of the Framestore.

This is accomplished by clocking "Start Row+2" Framestore rows into the output register. This places the first subarray row into Framestore row 0. This also has the effect of summing all rows prior to the subarray with the contents of the output registers.

5. Clock out output registers and discard.

This is accomplished by clocking the pixels out of the output registers and into the output nodes and discarding the data. For Full-Mode and Diagnostic-Mode, (256 + 4) pixels must be shifted out in order to clear the registers. In AC-Mode and BD-Mode, (512 + 8) pixels must be shifted. If overclocks are specified, clock and discard the output registers an additional number of times.

6. Clock desired number of image rows (i.e. "End Row - Start Row + 1") to the DPA

This is accomplished by clocking one row at a time from the Framestore to the output registers, and then clocking pixels from the output registers to the output nodes. As each pixel's data arrives at the output node, it is sampled, converted and transmitted to the DPA. If overclock data is needed, clock and sample the output registers an additional number of times. The readout of the Framestore shall take no longer than 6.5 seconds. The goal is to perform this operation as quickly as the hardware allows.

7. Clear the Imaging Section

This is accomplished by clocking 1026 rows from the Imaging Section into the Framestore.

8. Repeat starting from step 2).

NOTE: Rows after "End Row" in the Framestore and the garbage transferred from the Imaging Section to the Framestore as part of 7) are clocked and discarded during 3) and 5).

3.2.2.3.6 Overclock Processing

If configured to do so, the Front End Processor software on ACIS shall be responsible for computing, reporting and using the overclock levels of its CCD. Overclock data is generated by clocking the Serial Output Shift Registers after all of a row's pixel data has been clocked out. The clocking operation is under control of the SRAM and PRAM loaded into the DEA's sequencers. As the overclock data arrives from the CCDs, the Front End hardware writes the selected overclock data to a set of special buffers.

The Front End Processor software then averages the data contained in these buffers to produce an output node-specific overclock level of the CCD. The average is rounded to the nearest integer value.

(EQ 3) Overclock Level = SUM(Overclock Data) / # Overclock Elements

ACIS uses the resulting overclock levels to control the threshold values for the subsequent exposure(s). The overclock levels are also included with the event data telemetry.

The Front End Processor software shall re-compute and apply the overclock levels every exposure.

3.2.2.3.7 Pixel Bias Map Determination

The pixel-by-pixel bias map values are computed when configured by the parameter block, instructed by a "Compute Bias" command, or when the bias maps are invalid. Prior to processing events, the Front End Processor shall use one or more exposures to compute output node-specific "Initial Overclock" values, pixel-by-pixel bias levels and store the results in the Front End's Pixel Bias Map.

(EQ 4) Pixel Bias Map Value = Computed Pixel Bias + Initial Overclock

NOTE: If the "Exposure Duty Cycle" parameter is not 0, then only exposures whose integration time corresponds to the "Secondary Exposure Time" parameter (see Table 11) are included in the bias computation.

The algorithm to use to compute the bias is determined by the "Bias Selection" parameter of the Timed Exposure Parameter Block. The chosen algorithm may be tuned using the "Bias Parameters" of the same block.

3.2.2.3.8 Threshold Determination

The Front End Processor software shall compute the output node-specific threshold register values according to the following formula:

(EQ 5) Threshold Register Value = Threshold Set Point + (Overclock Level - Initial Overclock)

The Front End Processor software shall set the threshold registers with the computed values prior to each exposure.

3.2.2.3.9 Pixel Threshold Processing

The Front End Processor contains a Pixel Threshold BitMap. This bit map indicates which pixels in the acquired CCD image have pulse heights greater than their respective output -node threshold registers plus their respective pixel bias map values. The bit corresponding to a pixel is set if:

(EQ 6) Pixel Pulse Height > Pixel Bias Map Value + Threshold Register Value

When the "Front End Selection Algorithm" field of the Timed Exposure Parameter Block indicates the Event-finding mode, the Front End Processor software shall scan this bitmap for each exposure. Any pixel marked as above its threshold shall be counted in an "above threshold" counter, and be tested as an event. This counter is telemetered with the event data. Refer to Section 3.2.2.3.10 for the requirements the Event-finding mode requirements.

The threshold map is ignored for all Front End Modes except "Event-finding Mode."

3.2.2.3.10 Front End Event-finding Mode

When the "Front End Selection Algorithm" field of the Timed Exposure Parameter Block indicates the Event-finding mode, the Front End processor selects events from candidates indicated by the hardware Pixel Threshold Map. If a pixel is indicated in this map, the Front End software compares the corrected pulse height (see Section 3.2.2.3.14 on page 65) of the pixel to the 8 surrounding pixels, as shown in Figure 21. In order to meet the criteria for an event, the center pixel (black square) must have a corrected pulse height greater than or equal to the previous row and the pixel immediately preceding it within its row (shaded squares), and must have a corrected pulse height greater than the pixels in the subsequent row, and the pixel immediately after it in its row (hatched squares).

FIGURE 21. 3x3 Event-finding Mode Pixel Diagram


If the pixels meet the criteria, the Front End processor forwards the 3x3 group of pixels to the Back End processor for further selection and processing.

Each Front End processor must be capable of testing at least 1000 candidate events per second. When put together, all 6 of the Front End processors must be capable of handling a total of at least 750 actual events per second.

3.2.2.3.11 Front End Raw Mode

When the "Front End Selection Algorithm" field of the Timed Exposure Parameter Block indicates "Raw Mode", the Front End processor merely passes raw pixel data to the Back End Processor as it is poured into the Front End's image buffer.

3.2.2.3.12 Front End Histogram Mode

When the "Front End Selection Algorithm" field of the Timed Exposure Parameter Block indicates "Histogram Mode", the Front End processor forms one histogram for each active CCD output node (i.e. If "Output Register Clocking Mode" is Full-Mode or Diagnostic-Mode, four histograms are produced. In AC-Mode or BD-Mode, only two histograms are produced).

Each histogram contains 4096 bins, one bin for each raw pixel pulse height, where each bin is 32-bits wide. As raw pixel data is poured into the Front End's image buffer, the Front End processor reads each acquired raw-pixel pulse height and increments the location indexed by pulse height value. It proceeds to do this for a user-selectable number of exposures (as specified in the "Histogram Parameters" field). Once the desired number of exposures have been processed, or if a "Stop Timed Exposure" command is received, the Front End processor stops processing, and forwards the histogram data to the Back End processor to be telemetered.

For each exposure added to the histogram, the Front End Processor shall compute the overclock values for the exposure, and maintain the minimum, maximum, mean and variance of the computed overclock values. Once the histograms are complete, the Front End processor shall forward the computed minimum, maximum, mean and variance values to the Back End processor, which includes the information in the telemetry stream.

NOTE: If the "Exposure Duty Cycle" is not zero, then only exposures whose integration time is defined by "Secondary Exposure Time" are accumulated into the histograms (see Table 11).

The Front End Processors shall be able to process all pixels from every other 2.65 second exposure. It is a goal to be able to keep up with each 2.65 second exposure.

3.2.2.3.13 Bad Pixel and Column Map Content and Application

The Back End Processor shall contain a list or bitmap of bad pixels and columns from each CCD. This Bad Pixel and Column Map is edited using maintenance commands from the ground (see Section 3.2.7 on page 99). The Bad Pixel and Column Map entries provide the information described in Table 14 and Table 15.

TABLE 14. Bad Pixel Entry Information (Continued)

-----------------------------------------------------------------------------------------------
Req     Item                     Description                                                     
-----------------------------------------------------------------------------------------------
 14-1   CCD Id                   The Bad Pixel Map must contain enough information to identify   
                                 the CCD to which the bad pixel entry applies.                   
 14-2   Pixel Row and Column Id  The Bad Pixel Map must contain enough information to identify   
                                 which pixel in the CCD is bad.                                  
                                                                                                 
-----------------------------------------------------------------------------------------------

TABLE 15. Bad Column Entry Information (Continued)

-------------------------------------------------------------------------------------------
Req     Item       Description                                                               
-------------------------------------------------------------------------------------------
 15-1   CCD Id     The Bad Column Map must contain enough information to identify the CCD    
                   to which the bad column applies.                                          
 15-2   Column Id  The Bad Column Map must contain enough information to identify which col  
                   umn in the CCD is bad.                                                    
                                                                                             
-------------------------------------------------------------------------------------------

When the "Front End Selection Algorithm" field of the Timed Exposure Parameter Block indicates the Event-finding mode, and the "Ignore Bad Pixel Map Flag" is de-asserted, the software shall test any events generated by the Front End to ensure that the center pixel of the 3x3 event is not listed in the Bad Pixel and Column Map. If the center pixel is in the list, no further processing on the event takes place. If the center of the event is not in the list, software accepts the event for further selection and processing.

Bad pixels or pixels belonging to a bad column which surround the center pixel of an event shall be treated as having a pulse-height of 0.

The Bad Pixel and Column maps are ignored for all Front End Modes except "Event-finding Mode."

3.2.2.3.14 Pulse Height Bias Correction and Event Amplitude

To obtain the corrected pulse height of a pixel, the ACIS Science Instrument Software subtracts the corresponding output node overclock level, computed pixel bias value (i.e. the stored bias map value minus the global initial overclock) from its raw pulse height

(EQ 7) Corrected Pixel PH = raw pixel ph - pixel bias map value - (overclock - initial overclock)

The amplitude of an event is the sum of the corrected pulse heights of the appropriate pixels in the group. The pixels whose pulse heights are to be included are shown in Figure 22, and are as follows:

FIGURE 22. Event Amplitude Pixel Diagram


3.2.2.3.15 Event Selection by Event Amplitude

When the "Front End Selection Algorithm" field of the Timed Exposure Parameter Block indicates the Event-finding mode, the Back End Processor shall compute the overall amplitude of 3x3 events received from the Front End, and compare the result with the "Event Amplitude Lower Bound and Range" specified in the Timed Exposure Parameter Block. If the value is less than the lower bound, or greater than or equal to the upper bound (lower bound plus range), the software shall increment a telemetered event amplitude rejection counter and discard the event. If the value is within range, the software shall accept the event for further selection and processing.

Event Amplitude selection only applies when the "Front End Selection Algorithm" field indicates "Event-finding Mode."

3.2.2.3.16 Event Grade Computation

When needed for event selection or telemetry, the Back End software shall compute the "grade" of a 3x3 pixel event. To accomplish this, the software compares the corrected pulse heights of the 8 outer edge pixels of the event to the "Split Threshold" specified in the Timed Exposure Parameter Block. Each pixel corresponds to a "bit" in the grade code. If a pixel's pulse height is greater than or equal to the threshold, the corresponding bit is marked as a `1'. If a pixel's pulse height is less than the threshold, the bit is marked as a `0'. Since there are 8 pixels on the edge of a 3x3 event, this leads to an 8 bit grade code. There are 256 possible "grades" for a 3x3 event. The actual pixel position to grade code bit assignments will be defined in the design.

Figure 23 illustrates an example 3x3 event, with pixel pulse heights drawn as vertical bars, and the split threshold drawn as a grid. The pixels whose pulse height is above the threshold are shown to the right as lightly shaded areas. The since the center pixel does not contribute to the grade, it is shown using a darker shade. The diagram then shows the mapping of the pixels to grading bits, and from grading bits to the resulting Grade Code.

FIGURE 23. 3x3 Event Grading Illustration


3.2.2.3.17 Event Selection by Grade

When the "Front End Selection Algorithm" field of the Timed Exposure Parameter Block indicates the Event-finding mode, and the "Grading Selection Bitmap" does not indicate that all possible grades are to be accepted, the Back End software shall compute the grade of an event and compare the computed code to the list of desired grades. If the computed grade is not in the list, the grade discard counter is advanced and the event is ignored. If the event's grade is in the list, the event is accepted for further selection and processing.

Grade-based event selection only applies when the "Front End Selection Algorithm" field indicates "Event-finding Mode," and when the "Grading Selection Bitmap" indicates that only a subset of grades are to be processed (i.e. if all grades are accepted, no grade-based selection operation is performed).

3.2.2.3.18 2-D Window Event Processing

The Back End software shall use 2-D windows to select events and pixels to report. If an event's center pixel, or an individual pixel's position is within the bounds of a 2-D window, it is selected according to the window. For a given event or pixel, each window in the window list is checked in the order they are presented in the list. If the event/pixel position is within the boundaries of the window, the event/pixel is processed by the window. If the position is outside the bounds of the window, the next window is checked. This continues until the list is exhausted.

If no window list is specified, or if the event/pixel does not intersect with any of the configured windows, the event/pixel is, by default, accepted for processing. This behavior can be thought of as acting as a default window which encompasses the entire CCD and whose property is to accept all events/pixels.

The possible selection modes for the windows are as follows:

For example, to accept only events which occur within a small region of a CCD, two windows are needed, as shown in Figure 24. The first window (white square) in the list specifies the region to accept, and has a processing mode which accepts all events. The second window (shaded square) encompasses the entire CCD, and has a processing mode which rejects all events. Events whose position intersects the smaller first window are accepted by the window. All remaining events will intersect only the second window, and are rejected.

FIGURE 24. Example Clipping Technique


Another example to perform the inverse of the previously described operation, as shown in Figure 25. Here, we want to reject events in the smaller window, and accept all other events. This scenario only requires a single window. This window identifies the region from which to reject events. Events which intersect this window are then rejected. All other events will be handled by the default CCD window, whose scope is the entire CCD, and whose property is to accept all events.

FIGURE 25. Example Inverse Clipping Technique


Figure 26 illustrates an fictitious layout which uses several windows.

FIGURE 26. Multiple Windows


3.2.2.3.19 Raw Mode Telemetry

The Back End software produces raw pixel telemetry when the "Front End Selection Algorithm" field of the Timed Exposure Parameter Block indicates a "Raw" mode. In this mode, contiguous regions of pixels are bit-packed into telemetry packets and telemetered as a set. For each CCD readout, zero or more Raw Mode Data Telemetry packets are produced, followed by one Raw Mode Exposure Record packet.

Table 20 describes the content of the CCD data telemetry when in this mode and Table 17 describes the exposure record telemetry.

TABLE 16. Raw Mode Data Telemetry Content (Continued)

-----------------------------------------------------------------------------------------------------
Req     Field            Description                                                                   
-----------------------------------------------------------------------------------------------------
 16-1   Packet Synch     This field marks the start of the ACIS packet within the telemetry stream.    
 16-2   Packet Word      This field indicates the number of 32-bit words in the packet.                
        Length                                                                                         
 16-3   Format Tag       This field indicates the type of data contained in the telemetry packet, in   
                         this case a Timed Exposure Raw Mode data packet.                              
 16-4   Packet           This field is used to sequentially number all ACIS telemetry packets.         
        Sequence Num                                                                                   
        ber                                                                                            
 16-5   CCD/FEP Iden     These identify which CCD and FEP produced the raw pixel data.                 
        tifiers                                                                                        
 16-6   Data Packet      Since data from different CCDs can be interleaved, and also mixed with        
        Number           other kinds of packets, this field sequentially numbers the data packet for   
                         a given CCD exposure.                                                         
 16-7   Compression      These fields indicate whether or not the raw data has been compress, and      
        Selection Flag   if so, by which compression table.                                            
        and Table Selec                                                                                
        tion                                                                                           
 16-8   Pixel Position   This specifies the row and column of the first pixel in the packet in a col   
                         lection of raw pixel data.                                                    
 16-9   Packed or com    This consists of an array of window-selected raw CCD pixel data. Pixels       
        pressed collec   filtered out by the window processing are dropped. This forces the ground     
        tion of raw 12-  to use knowledge of the chosen 2-D windows to reconstruct the image.          
        bit CCD pixel    The packed pixels are output row by row, starting from the first row          
        and overclock    clocked out of the CCD, with the overclock data for a given row sent          
        data             immediately after the pixel data for the row. Overclock pixels are always     
                         sent, even if the row is completely filtered out by the 2-D windows.          
                         Unused bits at the end of the telemetry packet (needed to fill out to the     
                         nearest 32-bit word boundary) will be zero.                                   
                                                                                                       
-----------------------------------------------------------------------------------------------------

TABLE 17. Raw Mode Exposure Record Telemetry (Continued)

---------------------------------------------------------------------------------------------------------
Req      Field               Description                                                                   
---------------------------------------------------------------------------------------------------------
 17-1    Packet Synch        This field marks the start of the ACIS packet within the telemetry            
                             stream.                                                                       
 17-2    Packet Word         This field indicates the number of 32-bit words in the packet.                
         Length                                                                                            
 17-3    Format Tag          This field indicates the type of data contained in the telemetry packet, in   
                             this case, a Timed Exposure Raw Mode Exposure Record packet.                  
 17-4    Packet Sequence     This field contains the packet sequence number.                               
         Number                                                                                            
 17-5    Science Run Start   This identifies the start time of the science run using the ACIS time-        
         Time                stamp latched by the hardware at the start of the run.                        
 17-6    Timed Exposure      This item is a copy of the Timed Exposure Parameter Block Identifier          
         Parameter Block     used to configure the current run.                                            
         Identifier                                                                                        
 17-7    Window Block        This item is a copy of the Window List Parameter Block Identifier used        
         Identifier          to configure the current run. If no Window List was specified, this field     
                             will be zero.                                                                 
 17-8    CCD/FEP Identi      These identify which CCD and FEP produced the exposure.                       
         fiers                                                                                             
 17-9    FEP Timestamp       This is the value of the FEP time-stamp counter when the exposure             
                             arrived from the CCD.                                                         
 17-10   Exposure Number     This uniquely identifies the clocked exposure during the run. This field      
                             must allow the ground to uniquely identify over 2.6 million exposures         
                             during a single run (0.1 second exposures for 72 hours).                      
 17-11   Pixel Count         This specifies the total # of pixels sent for the exposure from the CCD.      
                                                                                                           
---------------------------------------------------------------------------------------------------------
3.2.2.3.20 Histogram Telemetry

The Back End software produces "Histogram Mode" telemetry when the Front End Processor is in "Histogram Mode," as indicated by the "Front End Selection Algorithm."

Table 18 describes the content of the CCD data telemetry and Table 19 describes the exposure record telemetry when in this mode. Zero or more data packets, followed by one Histogram Record packet are sent for each active CCD.

TABLE 18. Histogram Data Content (Continued)

------------------------------------------------------------------------------------------------------
Req     Field           Description                                                                     
------------------------------------------------------------------------------------------------------
 18-1   Packet Synch    This field marks the start of the ACIS packet within the telemetry stream.      
 18-2   Packet Word     This field indicates the number of 32-bit words in the packet.                  
        Length                                                                                          
 18-3   Format Tag      This field indicates the type of data contained in the telemetry packet, in     
                        this case a Timed Exposure Histogram data packet.                               
 18-4   Packet          This field contains the packet sequence number.                                 
        Sequence Num                                                                                    
        ber                                                                                             
 18-5   CCD/FEP Iden    These identify which CCD and FEP produced the histogram.                        
        tifiers                                                                                         
 18-6   Data Packet     Since data from different CCDs can be interleaved, and also mixed with          
        Number          other kinds of packets, this field sequentially numbers the data packet for a   
                        given CCD exposure.                                                             
 18-7   Output Node     This identifies from which output node the histogram was formed.                
        Identifier                                                                                      
 18-8   Starting Bin    This item is the index of the first histogram bin contained within the          
        Number          packet.                                                                         
 18-9   Array of histo  This consists of an array of adjacent histogram bins. Each bin contains a       
        gram counts     count of the number of pixels from the specified output node whose pulse        
                        height corresponded to the bin index.                                           
                                                                                                        
------------------------------------------------------------------------------------------------------

TABLE 19. Histogram Record Content (Continued)

---------------------------------------------------------------------------------------------------------
Req      Field               Description                                                                   
---------------------------------------------------------------------------------------------------------
 19-1    Packet Synch        This field marks the start of the ACIS packet within the telemetry stream.    
 19-2    Packet Word         This field indicates the number of 32-bit words in the packet.                
         Length                                                                                            
 19-3    Format Tag          This field indicates the type of data contained in the telemetry packet, in   
                             this case a Timed Exposure Histogram Record packet.                           
 19-4    Packet Sequence     This field contains the packet sequence number.                               
         Number                                                                                            
 19-5    Science Run Start   This identifies the start time of the science run using the ACIS time-        
         Time                stamp latched by the hardware at the start of the run.                        
 19-6    Timed Exposure      This item is a copy of the Timed Exposure Parameter Block Identifier          
         Parameter Block     used to configure the current run.                                            
         Identifier                                                                                        
 19-7    CCD/FEP Identi      These identify which CCD and FEP produced the histogram.                      
         fiers                                                                                             
 19-8    First Exposure      This uniquely identifies the first exposure acquired into the histogram       
         Number              during the run. This field must allow the ground to uniquely identify over    
                             130,000 exposures during a single run (2 second exposures for 72 hours).      
 19-9    Last Exposure       This uniquely identifies the last exposure acquired into the histogram        
         Number              during the run. This field must allow the ground to uniquely identify over    
                             130,000 exposures during a single run (2 second exposures for 72 hours).      
 19-10   Output Node         This identifies from which output node the histogram was formed.              
         Identifier                                                                                        
 19-11   Number of Expo      This identifies the total number of exposures acquired in the histogram.      
         sures Processed     Since one or more exposures may be dropped or ignored, this may be dif        
                             ferent than the "Exposure Number" listed above.                               
 19-12   Overclock Values    These contain the minimum, maximum, mean and variance of the over             
                             clock values computed while the histogram was been formed.                    
                                                                                                           
---------------------------------------------------------------------------------------------------------
3.2.2.3.21 Faint Mode Event Telemetry

The Back End software produces Faint Mode event telemetry when the "Event List Packing Code" field of the Timed Exposure Parameter Block indicates a "Faint" packing mode. This mode applies only when the Front End Processor is in "Event-finding mode," as indicated by the "Front End Selection Algorithm." In this mode, sets of distinct events are telemetered as 3x3 arrays of pixels. Table 20 illustrates the content of the Faint-Mode event list data packet, and Table 21 illustrates the content of an Exposure Record used with Faint-Mode.

TABLE 20. Faint Mode Exposure Event Data (Continued)

---------------------------------------------------------------------------------------------------------
Req     Field                Description                                                                   
---------------------------------------------------------------------------------------------------------
 20-1   Packet Synch         This field marks the start of the ACIS packet within the telemetry            
                             stream.                                                                       
 20-2   Packet Word          This field indicates the number of 32-bit words in the packet.                
        Length                                                                                             
 20-3   Format Tag           This field indicates the type of data contained in the telemetry packet, in   
                             this case a Timed Exposure Faint-Mode data packet.                            
 20-4   Packet Sequence      This field is used to sequentially number all ACIS telemetry packets.         
        Number                                                                                             
 20-5   CCD/FEP Identi       These identify which CCD and FEP produced the event                           
        fiers                                                                                              
 20-6   Data Packet Num      Since data from different CCDs can be interleaved, and also mixed with        
        ber                  other kinds of packets, this field sequentially numbers the data packet       
                             for a given CCD exposure.                                                     
                                                                                                           
        The remainder of the packet contains zero or more of the following:                                
 20-7   Position Identifier  This identifies the row and column of the center pixel of the event           
 20-8   Uncorrected          These items are the measured (uncorrected) pulse heights of the 9 pixels      
        Pulse Heights.       of the event's 3x3 array.                                                     
                                                                                                           
---------------------------------------------------------------------------------------------------------

TABLE 21. Faint Mode Exposure Record Content (Continued)

---------------------------------------------------------------------------------------------------------
Req      Field               Description                                                                   
---------------------------------------------------------------------------------------------------------
 21-1    Packet Synch        This field marks the start of the ACIS packet within the telemetry            
                             stream.                                                                       
 21-2    Packet Word         This field indicates the number of 32-bit words in the packet.                
         Length                                                                                            
 21-3    Format Tag          This field indicates the type of data contained in the telemetry packet, in   
                             this case, a Faint-Mode Exposure Record packet.                               
 21-4    Packet Sequence     This field is used to sequentially number all ACIS telemetry packets.         
         Number                                                                                            
 21-5    Science Run Start   This identifies the start time of the science run, as specified by the DEA    
         Time                latched ACIS time-stamp.                                                      
 21-6    Timed Exposure      This item is a copy of the Timed Exposure Parameter Block Identifier          
         Parameter Block     used to configure the current run.                                            
         Identifier                                                                                        
 21-7    Window Block        This item is a copy of the Window List Parameter Block Identifier used        
         Identifier          to configure the current run. If no Window List was specified, this field     
                             will be zero.                                                                 
 21-8    Bias Start Time     This identifies the start time of the most recently computed bias map,        
                             relative to when the DEA interface latched the ACIS time-stamp at the         
                             start of the bias computation.                                                
 21-9    Bias Parameter      This item is a copy of the Parameter Block Identifier used to configure       
         Block Identifier    the most recent bias map computation.                                         
 21-10   CCD/FEP Identi      These identify which CCD and FEP produced the exposure                        
         fiers                                                                                             
 21-11   FEP Timestamp       This is the value of the FEP time-stamp counter when the exposure             
                             arrived from the CCD.                                                         
 21-12   Exposure Number     This uniquely identifies the clocked exposure during the run. This field      
                             must allow the ground to uniquely identify over 130,000 exposures dur         
                             ing a single run (2 second exposures for 72 hours).                           
 21-13   Number of           This specifies the number of events being telemetered in the exposure.        
         Events Teleme       This field must accommodate at least 64K events.                              
         tered                                                                                             
 21-14   Number of Pixels    These specify the total number of pixels from a CCD whose pulse               
         above threshold     heights were above their respective spatial thresholds during the expo        
                             sure.                                                                         
 21-15   Number of           These specify the number of events which were discarded due to their          
         Events Discarded    amplitude. This field must accommodate the total number of pixels             
         by Amplitude        within the CCDs.                                                              
 21-16   Number of           These specify the number of events which were discarded due to their          
         Events Discarded    "grade."                                                                      
         by Grade                                                                                          
 21-17   Number of           These specify the number of events which were discarded by the pro            
         Events Discarded    cessing windows.                                                              
         by Window                                                                                         
 21-18   Overclock Levels    These identify the overclock levels used for the thresholding during the      
                             exposure which produced the events.                                           
 21-19   Bias Map Parity     This specifies the total number of pixels disabled due to parity hits.        
         Hit Count                                                                                         
---------------------------------------------------------------------------------------------------------
3.2.2.3.22 Faint-Bias Mode Telemetry

The Back End software produces Faint Bias Mode event telemetry when the "Event List Packing Code" field of the Timed Exposure Parameter Block indicates a "Faint-Bias" packing mode. This mode applies only when the Front End Processor is in "Event-finding mode," as indicated by the "Front End Selection Algorithm." In this mode, sets of distinct events are telemetered as 3x3 arrays of pixels along with the corresponding pixel bias values. Table 22 illustrates the content of the Faint-Bias Mode event list data packet, and Table 23 illustrates the content of a Faint-Bias Mode Exposure Record.

TABLE 22. Faint-Bias Mode Exposure Data Content (Continued)

----------------------------------------------------------------------------------------------------
Req     Field                      Description                                                        
----------------------------------------------------------------------------------------------------
 22-1   Packet Synch               This field marks the start of the ACIS packet within the telem     
                                   etry stream.                                                       
 22-2   Packet Word Length         This field indicates the number of 32-bit words in the packet.     
 22-3   Format Tag                 This field indicates the type of data contained in the telemetry   
                                   packet, in this case a Timed Exposure Faint-Bias Mode data         
                                   packet.                                                            
 22-4   Packet Sequence Number     This field is used to sequentially number all ACIS telemetry       
                                   packets.                                                           
 22-5   CCD/FEP Identifiers        These identify which CCD and FEP produced the event                
 22-6   Data Packet Number         Since data from different CCDs can be interleaved, and also        
                                   mixed with other kinds of packets, this field sequentially num     
                                   bers the data packet for a given CCD exposure.                     
                                                                                                      
        The remainder of the packet contains zero or more of the following:                           
 22-7   Position Identifier        This identifies the row and column of the center pixel of the      
                                   event                                                              
 22-8   Uncorrected Pulse Heights  These items are the measured (uncorrected) pulse heights of        
                                   the 9 pixels of the event's 3x3 array.                             
 22-9   Pixel Bias Map Values      These are the computed pixel bias values associated with the       
                                   event's 9 pixels.                                                  
                                                                                                      
----------------------------------------------------------------------------------------------------

TABLE 23. Faint-Bias Mode Exposure Record Content (Continued)

-------------------------------------------------------------------------------------------------------
Req      Field                      Description                                                          
-------------------------------------------------------------------------------------------------------
 23-1    Packet Synch               This field marks the start of the ACIS packet within the telem       
                                    etry stream.                                                         
 23-2    Packet Word Length         This field indicates the number of 32-bit words in the packet.       
 23-3    Format Tag                 This field indicates the type of data contained in the telemetry     
                                    packet, in this case, a Faint-Bias Mode Exposure Record              
                                    packet.                                                              
 23-4    Packet Sequence Number     This field is used to sequentially number all ACIS telemetry         
                                    packets.                                                             
 23-5    Science Run Start Time     This identifies the start time of the science run, as specified by   
                                    the DEA latched ACIS time-stamp.                                     
 23-6    Timed Exposure Parameter   This item is a copy of the Timed Exposure Parameter Block            
         Block Identifier           Identifier used to configure the current run.                        
 23-7    Window Block Identifier    This item is a copy of the Window List Parameter Block Iden          
                                    tifier used to configure the current run. If no Window List was      
                                    specified, this field will be zero.                                  
 23-8    Bias Start Time            This identifies the start time of the most recently computed         
                                    bias map, relative to when the DEA interface latched the             
                                    ACIS time-stamp at the start of the bias computation.                
 23-9    Bias Parameter Block Iden  This item is a copy of the Parameter Block Identifier used to        
         tifier                     configure the most recent bias map computation.                      
 23-10   CCD/FEP Identifiers        These identify which CCD and FEP produced the exposure               
 23-11   FEP Timestamp              This is the value of the FEP time-stamp counter when the             
                                    exposure arrived from the CCD.                                       
 23-12   Exposure Number            This uniquely identifies the clocked exposure during the run.        
                                    This field must allow the ground to uniquely identify over           
                                    130,000 exposures during a single run (2 second exposures            
                                    for 72 hours).                                                       
 23-13   Number of Events Teleme    This specifies the number of events being telemetered in the         
         tered                      exposure. This field must accommodate at least 64K events.           
 23-14   Number of Pixels above     These specify the total number of pixels from a CCD whose            
         threshold                  pulse heights were above their respective spatial thresholds         
                                    during the exposure.                                                 
 23-15   Number of Events Dis       These specifies the number of events which were discarded            
         carded by Amplitude        due to their amplitude. This field must accommodate the total        
                                    number of pixels within the CCDs.                                    
 23-16   Number of Events Dis       These specify the number of events which were discarded due          
         carded by Grade            to their "grade."                                                    
 23-17   Number of Events Dis       These specify the number of events which were discarded by           
         carded by Window           the processing windows.                                              
 23-18   Overclock Levels           These identify the overclock levels used for the thresholding        
                                    during the exposure which produced the events.                       
 23-19   Bias Map Parity Hit Count  This specifies the total number of pixels disabled due to parity     
                                    hits in the corresponding bias map location.                         
 23-20   Initial Overclock          This is the initial overclock added to the pixel bias map val        
                                    ues. (NOTE: This should have the same value for the length           
                                    of the entire run).                                                  
                                                                                                         
-------------------------------------------------------------------------------------------------------
3.2.2.3.23 Graded Event Telemetry

The Back End software produces Graded event telemetry when the "Event List Packing Code" field of the Timed Exposure Parameter Block indicates a "Graded" packing mode. This mode applies only when the Front End Processor is in "3x3 Mode," as indicated by the "Front End Selection Algorithm." In this mode, sets of distinct events are telemetered using reduced event amplitude information and event "Grade" (see Section 3.2.2.3.16 on page 66). The content of an Exposure Record in this mode is identical to that used by Faint Mode Telemetry (see Table 21). The event data content, however, is different. Table 24 illustrates the content of the event lists telemetered when in this mode.

TABLE 24. Graded Event Telemetry Content (Continued)

----------------------------------------------------------------------------------------------------------
Req      Item                 Description                                                                   
----------------------------------------------------------------------------------------------------------
 24-1    Packet Synch         This field marks the start of the ACIS packet within the telemetry            
                              stream.                                                                       
 24-2    Packet Word          This field indicates the number of 32-bit words in the packet.                
         Length                                                                                             
 24-3    Format Tag           This field indicates the type of data contained in the telemetry packet, in   
                              this case a Timed Exposure Graded Event Mode data packet.                     
 24-4    Packet Sequence      This field is used to sequentially number all ACIS telemetry packets.         
         Number                                                                                             
 24-5    CCD/FEP Identi       These identify which CCD and FEP produced the event                           
         fiers                                                                                              
 24-6    Data Packet Num      Since data from different CCDs can be interleaved, and also mixed with        
         ber                  other kinds of packets, this field sequentially numbers the data packet       
                              for a given CCD exposure.                                                     
                                                                                                            
         The remainder of the packet contains zero or more of the following:                                
 24-7    Position Identifier  This identifies the row and column of the center pixel of the event           
 24-8    Event Amplitude      This identifies the amplitude of the event (see Section 3.2.2.3.14 on         
                              page 65).                                                                     
 24-9    Grade Code           This identifies the Grade Code of the event. The computation of an            
                              event's Grade Code is described by Section 3.2.2.3.16 on page 66.             
 24-10   Corner Pulse         This is the mean, rounded to the nearest integer, of the corrected pulse-     
         Height Mean          heights of those corner pixels in the 3x3 event array which are below         
                              their corresponding split threshold in two's complement.                      
----------------------------------------------------------------------------------------------------------
3.2.2.3.24 Pixel Bias Map Telemetry

When configured to re-compute and telemeter the bias maps, the instrument software shall trickle the pixel bias maps from each of the active Front End Processors, into the telemetry stream. The algorithm used shall ensure a 10% minimum utilization of the telemetry stream. The algorithm is permitted to consume under-utilized telemetry. The pixel bias map values may be compressed in a number of rows using a compression scheme which utilizes patchable, pre-defined CCD-specific compression tables.

TABLE 25. Pixel Bias Map Telemetry Content (Continued)

----------------------------------------------------------------------------------------------------------
Req      Item               Description                                                                     
----------------------------------------------------------------------------------------------------------
 25-1    Packet Synch       This field marks the start of the ACIS packet within the telemetry stream.      
 25-2    Packet Word        This field indicates the number of 32-bit words in the packet.                  
         Length                                                                                             
 25-3    Format Tag         This field indicates the type of data contained in the telemetry packet, in     
                            this case a Timed Exposure Pixel Bias Map packet.                               
 25-4    Packet Sequence    This field is used to sequentially number all ACIS telemetry packets.           
         Number                                                                                             
 25-5    Bias Start Time    This identifies the start time of the bias run, as specified by the DEA         
                            latched ACIS time-stamp.                                                        
 25-6    Timed Exposure     This item is a copy of the Timed Exposure Parameter Block Identifier used       
         Parameter Block    to configure the current run.                                                   
         Identifier                                                                                         
 25-7    CCD/FEP Identi     These identify which CCD and FEP produced the bias map                          
         fiers                                                                                              
 25-8    Bias Packet        Since bias data from different CCDs can be interleaved, and also mixed          
         Number             with other kinds of packets, this field sequentially numbers the data packet    
                            for a given CCD exposure.                                                       
 25-9    Initial Overclock  This specifies the initial overclock value used to correct subsequent expo      
                            sures for changes in average overclock.                                         
 25-10   Compression        These fields indicate whether or not the data has been compress, and if so,     
         Selection Flag     by which compression table.                                                     
         and Table Selec                                                                                    
         tion                                                                                               
 25-11   Row Identifier     This specifies which row of the pixel bias map is being sent first within the   
                            packet. Since the rows are being sent in reverse order, subsequent rows in      
                            the packet are those consecutively closer to the Framestore.                    
 25-12   Row Count          This specifies the number of rows being sent.                                   
 25-13   Pixels Per Row     This specifies the number of pixel bias values per row                          
                                                                                                            
         The remainder of the packet contains zero or more of the following:                                
 25-14   Compressed         This is a bit-packed array of compressed Bias map data, starting from           
         Bias Map Data      "Row Identifier" and encompassing "Row Count". In order to send the             
                            bias map rows near the most likely focal point first, the rows are sent in      
                            reverse order, starting with those farthest from the Framestore. The col        
                            umn order is NOT reversed.                                                      
                                                                                                            
----------------------------------------------------------------------------------------------------------
3.2.2.3.25 Pixel Bias Map Parity Error Telemetry

In the process of producing events during an exposure. the Front End Processors detect and forward pixel bias map parity errors to the Back End Processor. The Back End accumulates these errors into one or more parity error telemetry packets, and posts these packet to telemetry as they fill, or at the end of each exposure if a partially filled data packet remains. The content of these packets is illustrated in Table 26.

TABLE 26. Bias Map Parity Error Telemetry Content (Continued)

-------------------------------------------------------------------------------------------------
Req     Item                    Description                                                        
-------------------------------------------------------------------------------------------------
 26-1   Packet Synch            This field marks the start of the ACIS packet within the teleme    
                                try stream.                                                        
 26-2   Packet Word Length      This field indicates the number of 32-bit words in the packet.     
 26-3   Format Tag              This field indicates the type of data contained in the telemetry   
                                packet, in this case a Timed Exposure Parity Error data packet.    
 26-4   Packet Sequence Number  This field is used to sequentially number all ACIS telemetry       
                                packets.                                                           
 26-5   CCD/FEP Identifiers     These identify which CCD and FEP produced the event                
 26-6   Data Packet Number      Since data from different CCDs can be interleaved, and also        
                                mixed with other kinds of packets, this field sequentially num     
                                bers the parity error packet for a given CCD exposure.             
        The remainder of the packet contains zero or more of the following:                        
 26-7   Position Identifier     This identifies the row and column of the bias error.              
 26-8   Corrupted Value         This a copy of the corrupted pixel bias map value.                 
                                                                                                   
-------------------------------------------------------------------------------------------------
3.2.2.3.26 Diagnostic Features

The diagnostic extensions of this mode are described in more detail by Section 3.2.5 on page 97. The extensions specific to Timed Exposure Mode include:

 

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