As for the UVIS channel (see Section 5.4
), the IR channel flat-field reference files consist of two components. A high signal-to-noise component created on ground accounts for pixel-to-pixel variations in the QE (P-flat). A second low frequency component (L-flat) derived from on-orbit observations accounts for differences between the ground based and in orbit optical paths.
During spring 2008, flat-field images for the IR channel were produced in the laboratory (see WFC3 ISR 2008-28
) during the third and last thermal vacuum campaign (TV3) using the CASTLE Optical Stimulus (OS) system. The CASTLE is an HST simulator designed to deliver an OTA-like external beam to WFC3. It can provide either point-source or flat-field illumination in either monochromatic or broadband mode.
To test the large-scale uniformity of the IR channel detector response, as provided by the TV3 ground-based flat fields, in-flight observations of the globular cluster Omega Centauri using multiple pointing dithered patterns were taken during SMOV4 (program 11453). By placing the same group of stars over different portions of the detector and measuring relative changes in brightness, low frequency spatial variations in the response of the detector were measured. Average photometric errors of +/-1.5% were found in the original IR ground-based flat fields (see WFC3 ISR 2009-39,
indicating differences between the CASTLE and in flight optical paths. To further investigate low frequency residuals in inflight sensitivity, additional observations of Omega Centauri were obtained during HST Cycles 17 and 18 at multiple dither positions and with different roll angles (Programs 11928 and 12340). By observing the same stars at different locations of the detector and measuring relative differences in brightness, local variations in response was computed. The same methodology and software developed for the ACS L-flats (ACS ISR 03-10) was used for this work. For this application, the IR detector was divided into 16x16 grid and a unique solution was calculated for each grid point, representing the deviations from unity. Low frequency L-flats were derived for the F098M, F110W, F125W, F139M, and F160W filters. Results indicate L-flat variations with an rms of ~1.1% with a peak-to-peak of +/-2.5%. While these results strongly suggest that ground based flat fields need to be corrected for residual low frequency structure, the signal-to-noise in the L-flat solutions was not sufficient to derive high quality corrections over the whole detector. Instead, we use actual sky images to derive the L-flat solutions
L-flat corrections to the ground based IR flat-field images were calculated by combining calibration and GO IR data taken between September 2009 and December 2010. Details of this procedure is given in WFC3 ISR 2011-11
. In short, all observations taken with the F098M, F105W, F110W, F125W, F140W, and F160W filters with an exposure time in excess of 300s were flat fielded using the ground based flat field and then combined filter per filter after masking out objects. This resulted in a high quality sky image for the F160W filter, while noisier images were derived for F098M and F125W. There were not sufficient input images to derive sky images with adequate signal-to-noise in the remaining three filters. Comparing the resulting sky image between filters showed no clear indication of any color dependence of the low frequency structures. Therefore a final gray sky image was constructed using all available data (about 2000 datasets). Top left panel of Figure 7.8
shows the gray sky image. There are significant low frequency structures indicating differences between the CASTLE and inflight optical paths.
Images showing the difference between CASTLE and in-flight illumination patterns were also derived from observations of the moonlit Earth limb in HST program 11917. An image in the F105W filter, after having been flat fielded using the ground based flat-field image, is shown in the top right panel of Figure 7.8
. Both the shape and the amplitude of the low-frequency structures are very similar to the image derived from sky observations. To compare the sky image with results obtained from Omega Centauri, we bin the sky image to a 16x16 grid, the same resolution used for the L-flat derived from the star cluster observations. The bottom left panel of Figure 7.8
shows the binned sky image, while the bottom right panel shows the L-flat in the F160W filter derived from stellar observation. The same low frequency structures are present in both images, showing consistency between results. The lower signal-to-noise in the stellar derived L-flat is apparent in the image.
shows small dark spots (“IR blobs”) across the IR field of view (also seen in the top panels of Figure 7.8
). These are regions of lower sensitivity by about 10-15% (see WFC3 ISR 2010-06
and Section 7.4
). The blobs are physically located on the Channel Select Mechanism (CSM) mirror. The blobs are small, with a radii of about 10-15 pixels, and affect about 1.2% of the IR pixels.