|Date:||June 20, 1997|
|To:||ACIS Science Team|
|Re:||Comparison of XRCF Flat Field High Speed Tap and Telemetry Data|
By comparing the flight software telemetry with the high speed tap data it can be shown that the x-ray event information telemetered by the flight software preserve the spectral, time, and position information from the raw data frames. Both the pixel by pixel and the frame by frame bias variations are accounted for in the telemetry.
During flat field calibration operations at the XRCF in May of 1997, telemetry data and high speed tap data were taken simultaneously for a number of sources for all ten of the flight devices.
The standard procedure was to begin a science run, then use the high speed tap to collect raw data frames for each of the chips being operated.
After some processing, the event list generated from the raw high speed tap data frames can be directly compared with the events telemetered by the flight software.
Both of the event lists in the comparison contain events from 755 frames. The average number of events per frame in the 56 rows of the comparison is 56.6 events per frame. The total number of events in the filtered telemetry event list is 42738. There are 42768 events in the filtered high speed tap event list.
After subtracting the average bias and delta-overclocks, the event amplitudes in the two event lists can be directly compared. The spectra for grades 0, 2, 3, 4, and 6 for both the high speed tap and telemetry event lists are shown below.
|Telemetry||High Speed Tap|
The peak and sigma fit to the data for each of the four quadrants is the same for each of the event lists.
The total number of events in each frame is shown below for both the telemetry event list and the high speed tap event list as well as the difference between the two.
|Telemetry||High Speed Tap||Difference|
Subtracting the event list images for the two files shows that there are 124 pixels for which the total number of events is different.
Ninety-six of the differences are due to split events which, because to differences in the biases or processing order, have a difference in the position of one row or column.
The following is an example of an event found in the exposure taken at 22:45.558 GMT (frame 502 for the high speed tap, frame 5075 for the telemetry). Due to the 3 ADU difference in the bias levels, the peak of this split event was found at two different positions.
|Telemetry Event||High Speed Tap Event||Bias Difference|
The remaining events which don't appear in both lists are events which are, due to the variations in the bias, either below the threshold, or are no longer the local maximum.
The data set chosen for this comparison is TRW ID I-IAS-EA-2.017. The chip is s4 (aka. w457c4). The anode is silicon. The data set was captured on 09 May 1997 at 22:12 GMT.
This set was chosen because there were no dropped high speed tap frames, there were no bad high speed tap frames, and the telemetry event list is currently available in Cambridge.
The telemetry event list is from Science Run 72, which began on 09 May 1997 at 17:48 GMT as TRW ID I-IAS-SG-2.008.
The parameter block used was "xfaint_win_s.te". "xfaint_win_s.te" is full frame, faint telemetry, trickle bias, with a window.
The window block used was "xsik56b50.2d". For the s4 chip, this has two windows. The first window begins at row 9 (counting from 0) and is 28 rows high. The second window begins at row 972 and is 28 rows high. All 1024 columns were telemetered.
The first event for the science run was on 09 May 1997 at 18:03 GMT. The last event for the science run was on 09 May 1997 at 23:49 GMT.
An ERV event list and fits format average bias frame for this science run were created from the ACIS telemetry using PSCI. For a description of PSCI and the ERV format see here.
The ERV format event list was converted to ACIS RV format using an IDL program which combines the fits format average bias file and the ERV format event list. For a description of the ACIS RV format see this.
To directly compare the telemetry event list with the high speed tap event list, events which the telemetry includes but the high speed tap does were removed. There are three types of events that the high speed tap event list does not include; events on quadrant boundaries, events taken before (22:17 GMT) or after (23:00 GMT) the high speed tap data were captured, and events in frames that the high speed tap dropped.
The high speed tap event list used was taken as TRW ID I-IAS-EA-2.017, which began on 09 May 1997 at 22:12 GMT. The first event was at 22:17 and the last event was at 23:00 GMT.
An RV format event list was created from the fits format bias frames (taken with the gate valve shut, immediately prior to the data) and the fits format data frames using "xrcf_find_events".
To directly compare the high speed tap event list to the telemetry event list the high speed tap times and coordinates were converted to telemetry times and coordinates, and events not included in the telemetry event list were removed.
The times had to be converted because the telemetry times are given in seconds from the beginning of 1997 while the high speed tap times are given in seconds from the beginning of 1994.
The coordinates needed to be converted because the telemetry row and column coordinates are given in chip coordinates while the high speed tap coordinates are given in readout coordinates.
There are two types of events that the high speed tap event list includes that the telemetry event list does not; events that occur outside of the telemetry window, and events in frames that the telemetry dropped.