calibration results by item shutter AUTHORS : A.R. Martel, M. Mutchler, G. Hartig
PURPOSE :
Measure the WFC and HRC shutters' accuracy and shading effects over a large range of exposure times with external broad-band QTH flat field exposures.
DATA :
Flat fields were acquired with RAS/HOMS during the BATC campaign in June 2000 to determine the importance of shading and to measure deviations in count rates at different exposure times. The images were acquired with the non-flight build detectors HRC#1 and WFC#3 and the illumination was provided by the QTH lamp reflected off the Spectralon diffuser at the RAS source plane. The dataset was acquired with the two SMSs JRHW30A and JRHW30B.
METHOD :
A bias frame is subtracted from all the flat fields. To find evidence of shading, the short exposure flats were normalized and divided by the long exposure flats (4.0 sec). Experiments were made to determine the optimal binning that yields the highest contrast between streaks and background. In general, we found that binnings greater than 20x20 can potentially wash out important features so our analysis was restricted to smaller binnings. Count rates were simply derived by dividing the median counts (DN/pix) by the exposure time. The data did not permit a measurement of the absolute accuracy of the shutter.
RESULTS :
The shutter consists of two blades (or sides). It rotates in one direction only so two consecutive exposures are alternately shaded by Side 1 and Side 2. There are two shutter properties that require calibration : (1) accuracy : Does the shutter remain open exactly for the commanded integration time ? Are the deviations between the "effective" and commanded exposure times a function of the exposure time and shutter blade ? What is the necessary correction in the count rates ? (2) shading : does the finite time of passage of the shutter over the chip leave residual streaks or features on the image ? At what exposure times does shading become important and to what level ?
These two properties must be calibrated as a function of the shutter side (or blade) and of the commanded exposure time. Both are tested by comparing short (<1 sec) and long exposure flat fields since any deviations will be relatively more important and easily detectable at the shortest exposures. No differences are expected between the shutter blades.
1. WFC
In Figure 1, we show the normalized WFC flat-field ratio of the 0.5 sec and 4.0 sec images displayed between levels of 0.99 and 1.01 for a 1x1 binning. A hint of diagonal streaking/shading is observed at a level of ~0.2% in the corners and bottom of the field. Unfortunately, changes in the bias level throughout the test prevent detection of streaks at the field center.
For both datasets, the count rates (or fluxes) (DN/sec/pix) are not constant for either shutter blade at exposures of 4 sec or less (see Figure 2). The fluxes peak at ~1 sec exposures and then slowly level off. The maximum fluctuation at the peak is ~3%. No rise or drop in the flux are observed at the shortest exposure time of 0.5 sec. This behavior may be due to (a) fluctuations in the illumination or (b) absolute inaccuracies in the integration times. The results of the HRC shading test (see below) suggest that the count rates from the illumination slowly drift downward through the procedure, opposite to what is observed for the WFC, so option (a) is unlikely. Inaccuracies in the shutter integration times below 1 sec are most probable.
2. HRC
No clear evidence of diagonal streaking/shading is observed in the shortest exposures (0.1 sec) relative to the longest exposures (4 sec) (see Figure 3).
For all datasets, the fluxes (DN/sec/pix) are constant for both shutter blades for exposure times of 0.2 sec and greater (see Figure 4). A sharp rise of 3.5% in the flux is observed at the shortest exposure time of 0.1 sec. A downward drift in flux is observed throughout the test i.e. the average fluxes of set 4 are lower than set 1 by 1.2%. This behavior is likely due to a gradual decrease in the flat field illumination over the 1.2 hour course of the SMS execution.
CEI SPECIFICATIONS :
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CONCLUSION :
Evidence of streaking/shading is seen only in the WFC data at very low levels (~0.2%). Inaccuracies in the shutter timing appear to be important at integration times of 1 sec or less for the WFC and at the shortest allowed integration time of 0.1 sec for the HRC.
Future improvements will include monitoring the light throughput with an off-axis photodiode, a larger number of flat fields on both shutter sides above exposure times of 0.7 sec, and a separate procedure to measure the shutter accuracy only (by including long exposures of 100 sec).
