ACA quadrant gains measured from dark current calibration data
Introduction
We use ACA dark current calibration data from the calibration of 2004:084 to examine the differences in gains in each of the ACA primary CCD quadrants. We measured the dark current histograms for each of the ACA CCD quadrants, which are shown in the following figure. Assuming that the CCD dark current, in electrons/second, is the same for all quadrants, then gain differences will be revealed in dark current histograms shown in units of ADC counts.
Data
The measured dark current histograms for each CCD quadrant are shown in the following figure. These histograms are constructed directly from the "imd.fits" image of the ACA CCD, which is the difference between the median-filtered 4.992 second and 9.984 second exposure images ("im1.fits" and "im2.fits" respectively).
The following table shows various measurements from each histogram which characterize the level of dark current in the quadrant.
| Quadrant | Mean (ADU counts) | Median (ADU counts) |
Peak (ADU counts) | Mean-near-Peak (ADU counts) |
|---|---|---|---|---|
| A | 22.54 | 9.0 | 8.75 | 8.77 |
| B | 23.55 | 10.0 | 9.39 | 9.38 |
| C | 22.96 | 10.0 | 9.00 | 8.99 |
| D | 23.70 | 10.0 | 9.40 | 9.37 |
Comments
- It can be seen from the plotted histograms that there is little evidence of significant gain differences among the four quadrants.
- Guide star magnitude changes of about 0.08 have been seen in CCD quadrant boundary crossing tests used to measure CTI effects in the CCD. If caused by gain differences, this would imply about 8% gain changes, which would correspond to about 0.7 counts shift in the peak position of a quadrant dark current histogram. This shift would be difficult to see in the plotted histograms. However, the tabular data show historam peak position differences up to about 0.7 counts.
- This indicates that gain differences are a possible contributor to the magnitude variations seen in the CCD quadrant boundary crossing tests that have been used to determine the effects of CTI in the ACA CCD.
Analysis directory
The files for this analysis are located in /proj/sot/ska/acacal/quad_gains/Aspect Information main page
Comments or questions: Aspect Help
Last modified:12/27/13