Next: 3.3.4 Telemetry Formats Up: 3.3 On-board data processing Previous: 3.3.2 Timed Exposure Mode
An observer uses Continuous Clocking Science Mode to acquire, process, and telemeter high time-resolution, 1-dimensional images from the CCD array. This mode provides data as a time-series of events which occurred along the columns of a CCD, acting like a strip-chart recorder, where each CCD column is analogous to a single pen.
To execute a Continuous Clocking Science Run, the observer sends a ``Start Continuous Clocking Run'' 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 Continuous Run'' command to ACIS, which then stops the run and telemeters a Science Run Report. If the observer requires parameters not already provided by the ACIS parameter library, the observer must issue a ``Load Continuous Clocking Parameter Block'' command to load the desired parameters for the run. Table 3.28 shows the contents of that block. If the loaded parameter block requires custom event-processing windows to select and process events based on certain column positions, the observer must also issue a ``Load 1-D Window List'' command to place the desired window settings into the instrument. The parameter blocks to define these 1-D window settings are described in Tables 3.29 and 3.30.
|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 combination of an array's 6 CCDs.|
|Number of Rows to Sum||Number of rows to sum prior to clocking out the output registers. Ranges from 1 to 1024.|
|Number of Columns to Sum||Number of pixels to sum at the Output Node prior to each transfer to the DPA. Ranges from 1 to 1024.|
|Output Register Clocking Mode||Controls the clocking mode of all of the CCD's Serial Output Shift Registers, either: Full-Mode, AC-Mode, BD-Mode or Diagnostic-Mode.|
|Number of Overclock Sums||Number of extra summed pixels to clock out of the Serial Output Shift Registers. Continuous Clocking Mode uses these pixel sums to determine the bias introduced in the CCD's pixel pulse height by the analog electronics. Ranges from 0 to 32, in 2 pixel sum 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||Control the minimum pulse-height above bias a pixel sum should have to be considered an event candidate. Each item applies to a specific CCD. 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 and whose pulse height is greater than or equal to the (possibly summed) pixel to its left, and greater than the (possibly summed) pixel to its right (3x1 local maxima).|
|Bias Row Count||How many (possibly summed) rows over which to compute the overclock level and modify the threshold level. Currently, this value is in units of 1024 rows.|
|Threshold Overrides||Fixed threshold value to use for each CCD. Values between 0 and 4095, inclusive, specify the threshold to use. Values 4096 and greater disable the override feature for the corresponding CCD.|
|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 1-D Window Collection||Specifies a set of 1-dimensional event selection and processing windows to use for the Science Run. If no windows are specified, Continuous Clocking Mode shall process all events from the CCDs using processing parameters specified in this block.|
|Split Threshold||Threshold used when grading events. Ranges from 0 to 4095.|
|Ignore Bad Column Map Flag||Controls whether or not to discard events on the basis of the CCDs list of bad pixels.|
|Grading Selection Bit Map||Selects which event grades to accept. Accepted events whose grade is in this list shall be telemetered.|
|Pulse Height Lower Bound||Minimum amplitude of accepted events. Events whose amplitude is less than this value shall not be telemetered. Range is governed by the minimum and maximum event amplitudes.|
|Pulse Height Upper Bound||Maximum amplitude of accepted events. Events whose amplitude is greater than this value shall not be telemetered. Range is governed by the minimum and maximum event amplitudes.|
|Event List Packing Code||Format to use when telemetering events. Currently, the following formats are supported: Raw, 3x1, Graded.|
|CCD Identifier||CCD to which the window applies.|
|Window Column||Position of the window in terms of the left-most column within the CCD's Image Array. Position is independent of any summing column parameters in the Continuous Clocking Parameter Block.|
|Window Width||Number of columns (un-summed) in the window.|
|Selection Code||Selection rules used for all events whose position is within the limits of the window.|
|Selection Argument(s)||Any selection arguments needed by the code.|
|Local Amplitude Range||Lower and upper amplitude range for candidate events. Events outside this range are rejected. Value is not affected by any row or column summation.|
|Window Count||Number of 1-D Windows contained in the Window List. Ranges from 0 to 36|
|Windows [Window Count]||Array of 1-D windows. Content is described in Table 3.29. The number of windows is `Window Count'.|
The overclock processing for this mode is almost identical to that for Timed Exposure Science Mode, except the number of clocks and exposures are specified via the Continuous Clocking Parameter Block. The FEP computes the CCD threshold according to the following formula:
|Tn = On + Tn0 + Bi,||(20)|
The FEP contains a Pixel Threshold Bitmap. This bitmap indicates which pixels in the acquired CCD data have pulse heights greater than the threshold. When the ``Front End Selection Mode'' field of the Continuous Clocking Parameter Block indicates the 3x1 processing mode, the FEP software will scan this bitmap for each exposure. Any pixel marked as above its threshold will be counted in an ``above threshold'' counter and be tested as an event. When the selection mode specifies that all pixel data should be sent by the FEP (Raw Mode), the threshold map is ignored and all pixel data are sent from the FEP to the Back End.
The 3x1 processing mode indicates that the FEP should select events from candidates indicated by the hardware Pixel Threshold Map. If a pixel is indicated in this map, the Front End software compares the uncorrected pulse height of the pixel to each pixel on either side of the candidate. If the candidate's pulse height is greater than or equal to the pixel immediately preceding it in the row, and greater than the pixel immediately following it in the row, the FEP forwards the three pixels to the BEP for further selection and processing.
To obtain the corrected pulse height of a (possibly summed) pixel, the ACIS software subtracts the scaled overclock level from the measured pulse height. The corrected pulse height of a 3x1 group of pixels is computed by summing the corrected pulse heights of all three pixels, which are greater than the ``Split Threshold'' as specified in the Continuous Clocking Parameter Block. The BEP compares the overall corrected pulse height of the 3x1 events with the ``Pulse Height Lower Bound'' and ``Pulse Height Upper Bound,'' specified in the parameter block. If the value is less than the lower bound or greater than the upper bound, the software increments the event rejection counter and discards the event. If the value is within range, the event is accepted 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 the ``Front End Selection Mode'' field of the Continuous Clocking Parameter Block indicates a 3x1 processing mode and the ``Grading Selection Bitmap'' does not indicate that all possible grades are to be accepted, the BEP software computes the grade on an event and compares the computed grade to the list of desired grades. If the computed grade is not in the list, the event is counted and discarded. If the event's grade is in the list, the event is accepted for further selection and processing. If Raw Mode is set or if the ``Grading Selection Bitmap'' accepts all possible grades, the software accepts the data for further selection and processing.
The BEP software uses 1-D windows to select events and pixels to telemeter. If an event's center pixel or an individual pixel's position is within the bounds of a 1-D window, it is processed according to the window's selection code. 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 column is within the bounds of the window, the event/pixel is processed by the window. The window boundaries are expressed independently of the number of summed columns. Therefore, the pixel position, relative to a window, is really a range of positions. If this range intersects the window, it is considered a match, and the event is selected by the window. If the position is outside the bounds of the window, the next window is checked. This continues until either a match is found or 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 is, by default, accepted for processing.
As with the Timed Exposure data, the 1x3 event data are telemetered as raw, non-bias subtracted data. In Continuous Clocking the bias frames are all so small (only 1x1024 vs. 1024x1024 for Timed Exposure), that the entire Bias is always telemetered before the start of normal data collection. There is no equivalent to FaintBias mode for Continuous Clocking data.