Results of WFC3 Thermal Vacuum Testing: IR Channel Subarray Photometry

The photometric accuracy of WFC3’s IR Channel was examined during thermal vacuum testing in September October, 2004. The measured signal levels from various incident flux levels and sample sequences were compared in order to determine the relative accuracy of FPA64. Significant deviations in measured versus expected flux levels were observed at some illumination levels. Introduction The purpose of this SMS, performed during the September October 2004 thermal vacuum testing campaign, was to test the photometric accuracy of WFC3’s IR channel over a range of incident fluxes. By imaging point sources with various known fluxes, and comparing the detected signals to the real fluxes, we were able to test the relative accuracy of FPA64. Data The data used for this analysis were part of the IR22 test. These files were composed of RAPID and SPARS10 ramps of various lengths. Details are presented in Table 1. All ramps were taken at the nominal gain value of 2.5 e-/ADU. At each of 5 Optical Stimulus (OS) flux levels, a series of 4 point source ramps were taken, followed by a set of dark current ramps. At the end of the test, a pair of point source Copyright© 2005 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved. Instrument Science Report WFC3 2006-01 files were taken at a 6th OS flux level. For each set of 4 point source ramps, the illuminating fiber was moved between each file, such that the point source fell in a different quadrant in each ramp. The penultimate set of point sources, with incident flux values of 5,179,000 photons/ sec, displayed saturated pixels in all reads of the ramp files. Without an accurate measurement of the detected flux, these files were not useful for examining the photometric accuracy of FPA64, and were ignored. Similarly, the point sources in ramps 27 and 28 displayed saturated pixels in the final read of the ramps, leaving only two reads with good signal measurements. This translated into a single good measurement of flux rates for each ramp, limiting the accuracy of subsequent data analysis. Ramp Number Sample Sequence Array Size (pixels) # Reads per Ramp Exposure Time (sec) Total Flux (Photons/sec) Point Source or Dark # of Files 0 RAPID 1024 15 67.0 NA Dark 1 1-5 RAPID 64 15 0.9 NA Dark 5 6-10 SPARS10 64 15 78.7 NA Dark 5 11-14 SPARS10 64 15 78.7 11,146* Point Source 4 15 RAPID 1024 15 67.0 NA Dark 1 16-20 RAPID 64 15 0.9 NA Dark 5 21-25 SPARS10 64 15 78.7 NA Dark 5 26-29 SPARS10 64 3 11.3 83,691 Point Source 4 30 RAPID 1024 15 67.0 NA Dark 1 31-35 RAPID 64 15 0.9 NA Dark 5 36-40 SPARS10 64 15 78.7 NA Dark 5 41-44 RAPID 64 15 0.9 88,864 Point Source 4 45 RAPID 1024 15 67.0 NA Dark 1 46-50 RAPID 64 15 0.9 NA Dark 5 51-55 SPARS10 64 15 78.7 NA Dark 5 56-59 RAPID 64 15 0.9 766,460 Point Source 4 60 RAPID 1024 15 67.0 NA Dark 1 61-65 RAPID 64 15 0.9 NA Dark 5 66-70 SPARS10 64 15 78.7 NA Dark 5 71-74 RAPID 64 8 0.485 5,179,800 Point Source 4 75 RAPID 1024 15 67.0 NA Dark 1 76-80 RAPID 64 15 0.9 NA Dark 5