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An observer uses Timed Exposure Science Mode to acquire, process and telemeter two-dimensional images from the CCD array. 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 and proceeds to execute the science run. 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. Figure 3.3 describes the pixel processing data flow through the DPA in Timed Exposure mode.
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. Table 3.25 describes the contents of the Timed Exposure Parameter Block, which the observer must specify to configure the Timed Exposure Mode. Since Timed Exposure Mode has the capability to select and process events in restricted 2-D windows, the selection and processing of events in specific windows must also be controlled, via a ``2-D Window Parameter Block.'' Table 3.26 describes the contents of this block, while Table 3.27 shows the information which must be supplied to specify a list of windows.
|CCD Array Selection||Which CCD array to use for the Science Run: Imaging or Spectroscopy|
|CCD Selections||Which CCDs to use within the selected CCD Array. Capable of specifying any of the 64 possible combinations of an array's 6 CCDs.|
|Sub-array Readout Start and End Row||An observer uses these items to shorten the time it takes to transfer an image from the CCDs to the Front End Processors by sacrificing some of the imaging area. The start and end rows specify which contiguous horizontal swath of CCD rows are transferred from the Framestore back to the FEPs. These values are CCD-specific.|
|Exposure Times||Controls the amount of time that the Image Area of each CCD is integrating data. Can range from 0.2 to 10 seconds in 0.1 second increments.|
|Output Register Clocking Mode||Controls the clocking mode of all of the CCD's Serial Output Shift Registers. Can specify either: Full-Mode, AC-Mode, BD-Mode or Diagnostic-Mode.|
|Number of Overclock Pixels||Specifies the number of extra pixels to clock out all of the Serial Output Shift Registers. Timed Exposure Mode uses these pixels to determine the bias introduced to the CCD's pixel pulse height by the analog electronics. Range is 0 to 32 pixels, in 2 pixel increments.|
|Video Chain Response||Controls the response of the Video Subsection in the Detector Electronics Assembly. Range is 1 or 4 electrons per Analog-to-Digital Unit (ADU).|
|Reference to an explicit DEA Load||Optional item which allows a maintainer to by-pass the on-board SRAM/PRAM synthesizer, and load these areas directly.|
|Threshold Set Points||Controls minimum pulse-height above bias a pixel should have to be considered as an event candidate. Each item applies to a specific CCD. These values can range from 0 to 4095.|
|Front End Selection Algorithm||Specifies whether or not the Front End Processor should supply all pixels to the Back End (Raw Mode), or only those 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).|
|Bias Exposure Count||Specifies how many exposures over which to compute the overclock level and to modify the spatial threshold levels.|
|Reference to Front End Processor Code||Allows a maintainer to override the standard code and data loaded into the Front End Processor, and provide a special load for the Science Run.|
|Reference to a 2-D Window Collection||Specifies a set of 2-dimensional event selection and processing windows to use for the Science Run. If no windows are specified, Timed Exposure Mode shall process all events from the CCDs using the processing parameters specified in this block.|
|Split Threshold Set Point||Threshold used when grading events. Range from 0 to 4095.|
|Ignore Bad Pixel Map Flag||Controls whether or not to discard events on the basis of the CCD's list of bad pixels and columns.|
|Grading Selection Bit Map||Selects which event grades to accept. Accepted events whose grades are indicated in this list shall be telemetered. Consists of 256 bits, where each bit corresponds to one of the possible grade codes.|
|Pulse Height Lower Bound||Selects the minimum total pulse height of accepted events. Events whose total pulse height (function of the 3x3 grid around the event) is less than this value shall not be telemetered. Range is governed by the minimum and maximum corrected event pulse heights.|
|Pulse Height Upper Bound||Selects the maximum total pulse height of accepted events. Events whose pulse height is greater than this value shall not be telemetered. Range is governed by the minimum and maximum corrected event pulse heights.|
|Event List Packing Code||Selects the format to use when sending events. Currently, the following formats are supported: Raw, Raw Histogram, 3x3, 5x5, and Graded.|
|CCD Identifier||Selects the CCD (out of all 10) to which the window applies.|
|Window Position||Specifies the window position in terms of the bottom-left pixel within the CCD's Imaging Section.|
|Window Width||Width of the window in terms of Imaging Section columns (1-1024).|
|Window Height||Height of the window in terms of Imaging Section rows (1-1024).|
|Selection Code||Code identifying selection rules of all events whose position is within the limits of the window.|
|Selection Argument(s)||Any selection arguments needed by the code|
|Local Pulse Height Range||Sets the lower and upper pulse heights of candidate events. Events whose pulse-height is outside this range would be rejected.|
|Window Count||Identifies the number of 2-D Windows contained in the Window List. Ranges from 0 to 36|
|Windows [Window Count]||Array of 2-D windows. The content of these windows is described in Table 3.26. The number of windows in the array is `Window Count'.|
If configured to do so, the FEP software on ACIS will be responsible for computing, reporting, and using the overclock bias of its CCD. Overclock data are generated by clocking the Serial Output Shift Registers after all of a row's pixel data have been clocked out. As the overclock data arrive from the CCDs, the Front End hardware writes the overclock data to a set of special buffers. The FEP software then averages the data contained in these buffers to produce an overclock bias of the CCD. ACIS uses the resulting overclock bias to control the pulse height lookup table for the next exposures and the spatial threshold values for the subsequent exposures. The FEP software recomputes and applies the overclock bias every set of exposures, as specified by the ``Bias Exposure Count'' field of the Timed Exposure Parameter Block.
The FEP software computes the threshold for event recognition on a pixel-by-pixel basis. This threshold is used during event processing, in order to separate read noise from X-ray event candidates. The spatially varying threshold is computed for each pixel according to the following formula:
|Sij = Bij + Tn + On,||(18)|
The FEP contains a Pixel Threshold Bitmap. This indicates which pixels in the acquired CCD image have pulse heights greater than their respective spatial threshold registers. When the ``Front End Selection Mode'' field of the Timed Exposure Parameter Block indicates the 3x3 or 5x5 processing mode, the FEP software scans this bitmap for each exposure. Any pixel marked as above its threshold is counted in an ``above threshold'' counter and is tested as an event. When the FEP is in Raw or Histogram Mode, the threshold map is ignored and all pixel data are examined.
When the ``Front End Selection Mode'' field of the Timed Exposure Parameter Block indicates the Event-Finding Mode, the FEP selects events from candidates indicated by the hardware Pixel Threshold Map. If a pixel is indicated in this map, the FEP software compares the uncorrected pulse height of the pixel to the eight surrounding pixels, as shown in Figure 3.4. In order to meet the criteria for an event, the center pixel (black square) must have a 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 pulse height greater than or equal to the pixels in the subsequent row and the pixel immediately after it in its row (hatched squares). If the pixels meet the criteria, the FEP forwards the 3x3 group of pixels to the BEP for further selection and processing.
ACIS contains a list of bad pixels and columns from each CCD. These maps are edited using maintenance commands from the ground (see Sect. 3.2). The Bad Pixel Map and Bad Column Map entries contain lists of individual pixel locations or entire columns which will be excluded from the event telemetry, when the instrument is commanded to do so. When the ``Front End Selection Mode'' field of the Timed Exposure Parameter Block indicates the 3x3 or 5x5 processing mode and the ``Ignore Bad Pixel Map'' flag is de-asserted, the software tests any events generated by the Front End to ensure that the center pixel of the 3x3 event is not listed in the Bad Pixel Map. If the center pixel is in the list, the event is counted in a ``discarded event'' list and 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. When the selection mode specifies that all pixel data shall be sent by the FEP (Raw Mode) or the ``Ignore Bad Pixel Map'' flag is asserted, the Bad Pixel Map is ignored and the software accepts the pixel/event data for further selection and processing.
To obtain the corrected pulse height of a pixel, the ACIS Software subtracts the overclock level and bias offset from each pixel in the 3x3 group, to obtain a bias-corrected pulse height for each pixel. 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 3.5, and are as follows:
Note that, in faint-bias mode telemetry, the bias levels are sent along with the pixel neighborhood data. In graded mode the goal is to reduce the volume of telemetry per event, so this information is not sent.
While in the 3x3 processing mode, the BEP computes the overall corrected pulse height of 3x3 events received from the FEP and compares the result with the ``Pulse Height Lower Bound'' and ``Pulse Height Upper Bound'' specified in the Timed Exposure Parameter Block. If the value is less than the lower bound or greater than the upper bound, the software increments an event rejection counter and discards the event. If the value is within range, the software accepts the event for further selection and processing. If the ``Front End Selection Mode'' field indicates ``Raw Mode,'' the software will not apply a pulse-height range test and will accept all pixel data for further selection and processing.
When needed for event selection or telemetry, the BEP software computes the ``grade'' of a 3x3 or 5x5 pixel event. To accomplish this, the software compares the corrected pulse heights of the eight 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 eight 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. Figure 3.6 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. Since the center pixel has been selected on the basis of being a local maximum, it will always be the pixel with the highest signal. (It is shown using a darker shade in the diagram.) The diagram then shows the mapping of the pixels onto grading bits, and from grading bits onto the resulting Grade Code. Note that when in 5x5 mode only the central 3x3 region is used to define the grade, and these grade definitions are identical to the 3x3 mode.
A complete list of the ACIS grades with a diagram illustrating the split event threshold crossings is given in Appendix A. Please note that ACIS grades and ASCA grades do not use the same numbering convention. ACIS grades run from 0 to 255, while ASCA grades run from 0 to 7. ACIS grade 0 is identical to ASCA grade 0 (a single, isolated event without any neighboring split threshold crossings), but all other ASCA grades are sums of many ACIS grades. Also note that when ACIS operates in ``Graded Mode'' only the total reconstructed energy of the X-ray event is telemetered to the ground. The total energy is computed by summing all pixels which exceed the split event threshold. On ASCA the corresponding mode (`Bright Mode') sums pixels regardless of whether the split threshold is crossed. The result is that some ASCA grade 5 and 7 event total energy values can not be calculated from ACIS grade and energy information alone.
When in the 3x3 or 5x5 processing mode and the ``Grading Selection Bitmap'' does not indicate that all possible grades are to be accepted, the BEP software computes the grade of an event and compares the computed code to the list of desired grades. If the computed grade is not in the list, the discard counter is incremented and the event is ignored. If the event's grade is in the list, the event is accepted for further selection and processing. When in Raw Mode or if the ``Grading Selection Bitmap'' indicates that all possible grades are to be accepted, the software accepts the data for further selection and processing without needing to compute the grade of the event.
The BEP software can 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 it is 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 list becomes exhausted with the event/pixel not being picked up by any of the windows, the event/pixel by default is accepted for processing. This scheme acts like a default window which encompasses the entire CCD and whose property is to accept all events/pixels.
The final result of this processing is a 3x3 or 5x5 array of pixel values which are packaged into Science Data Format packets which are presented to the RCTU for telemetry to the ground. Please note that the values in the 3x3 array are telemetered in their original form (i.e. raw or non-bias subtracted). In order to properly interpret these data it is necessary to subtract the bias values on the ground. If the mode is Timed Exposure with Bias, the telemetry for each event will also contain a 3x3 array of the bias values for these same pixels.
The full expression for Bias Subtraction (in terms of packet field names as defined in the IPCL) is: