Stability and Accuracy of the WFC3 UVIS Shutter

Data taken during ambient and thermal vacuum testing have been used to examine shutter shading effects, shutter repeatability and to determine accurate exposure times for WFC3’s UVIS channel. Long wavelength illumination resulted in observable fringing effects, somewhat hindering the search for the shutter shading effects. From these data, no shutter shading effects are seen down to the ~0.5% level Shutter accuracy was measured by comparing the countrates of long vs. short exposures. Ambient data showed exposure times which deviated from nominal by up to 7.7%. (0.5 sec exposures) Thermal vacuum data were much better, showing deviations of less than 2% at all exposure times. For datasets taken at a single exposure time, both ambient and thermal vacuum data show variations in exposure time of less than 1.5% in all cases except for one. (i.e. 5.3 msec variation in a set of 0.5 sec exposures) Nevertheless, this level of variability is larger than that specified by the CEI spec. Searches for a shutter side dependence revealed no correlation between shutter side and accuracy. Introduction WFC3’s UVIS shutter is composed of a rotating disk with two quadrants left open, and two opaque quadrants. The shutter is capable of supporting exposures of 0.5 seconds, as well as 0.7 seconds through 60 minutes, with a 90o rotation used to begin and end an exposure. More details of the shutter can be found in OP-01. (Baggett, W. 2003) The purpose of this study was to examine several characteristics of the UVIS shutter, including the true amount of time the detectors are exposed to light for a given commanded exposure time. Also, the repeatability of the shutter movement was examined by Copyright© 1999 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved. Instrument Science Report WFC3 2004-14 taking multiple images with the same commanded exposure time. Finally, the consistency of the shutter speed was analyzed by looking for shutter shading effects in a combination of long and short exposures. If the shutter opened and closed with a variable speed, it would block certain portions of the detector longer than other portions, leaving behind a large spatial scale variation in flux on the detector. Two contract end item specifications (CEI specs) are relevant to this study. First, exposure times are to be repeatable to at least 0.01 second. Secondly, there should be a uniform exposure time to at least 0.01 second across the detector for a given image. Data Data files used in this study include 50 images taken in ambient conditions as well as 50 images taken during thermal vacuum testing. Both datasets are comprised of flat fields illuminated by the XE lamp, and observed through the F600LP filter. Exposure times ranged from 0.5 second to 30 seconds. The details of the data are given in Table 1. Multiple images were taken at each exposure time, in order to reduce the noise. All images were taken in 3x3 binning mode. Data Reduction Prior to analysis, several basic data reduction steps were performed. Row-by-row bias level correction on each image was performed using physical overscan reference pixels. The median of the reference pixels in each row was calculated. A line was then fit to these medians. The best-fit line value in each row was subtracted from the science pixels in that row. Corrections for the images observed through the neutral density filters were then made. The throughput of the neutral density filters varied with wavelength, making the correction of our broadband observations more difficult. In order to determine the attenuation factor of the neutral density filters on these observations, images at one exposure time were taken both with and without the neutral density filters in place. For the ambient data, 1.0 second images were taken both with and without the ND1 filter in place. Similarly, 4.0 second images with both the ND1 and ND2 filters were taken. In this way, the counts in the 1.0 second files with the ND1 filter could be adjusted until they matched the counts in the 1.0 second files without the neutral density filter. Once the correct adjustment was found, all images taken through the ND1 filter were adjusted by the same amount. The same strategy was used for the 4.0 second files with the ND1 and ND2 filters. Corrections were made based on the mean counts in one quadrant of the detector. The results are listed in Table 2. Errors in the countrate and neutral density attenuation correction are apparent in Figures 2 and 3, where the mean countrates in the two groups of 0.8 second or 4.0 second files differ. These errors could be due to the neutral density fil-