7.8 IR Flat Fields

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 from ground test data 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 on-orbit optical paths.

7.8.1 Ground Flats (P-flats)

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. Flat fields with the OS tungsten lamp were taken with the detector at its nominal operating temperature.

During TV3, CASTLE flat fields were acquired using the SPARS10 sample sequence, with varying numbers of readouts (samples) per exposure, chosen to obtain a signal of about 60,000 electrons per pixel in the final read. Four up-the-ramp exposures were taken per filter, providing a mean signal-to-noise ratio of ~500, yielding corrections down to a level of 0.2%.

7.8.2 On-orbit Test of the Ground Flats

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), due to differences between the CASTLE and in flight optical paths. To further investigate low frequency residuals in in-flight 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 were computed. The same methodology and software developed for the ACS L-flats( ACS ISR 03-10) was used for this work. For application to WFC3, the IR detector was divided into 16 × 16 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 show the L-flat variations have 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, deep images with small number of sources were used to derive the L-flat solutions.

7.8.3 On-orbit L-flats from Sky Observations.

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 are 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, and somewhat noisier sky images for F098M and F125W. There were insufficient input images to derive sky images with adequate signal-to-noise in the remaining three filters (F105W, F110W, F140W). 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 7.7 shows the gray sky image. As can be seen in the figure, there are significant low frequency structures due to differences between the CASTLE and inflight optical paths.

Figure 7.7: Top left: gray IR sky image. Top right: Earth flat taken in the F105W filter. Bottom left: 16 x 16 binned image of the gray IR sky image. Bottom right: Stellar L-flat from aperture photometry of Omega Centauri (F160W).

Images showing the difference between CASTLE and in-flight illumination patterns were also derived from observations of the moonlit Earth limb(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 7.7. 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 16 × 16 grid, the same resolution used for the L-flat derived from the star cluster observations. The bottom left panel of Figure 7.7 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, i.e. the results are consistent. The lower signal-to-noise in the stellar derived L-flat is apparent in the image.

7.8.4 Pipeline Flats

The current IR pipeline flat-field images were constructed by combining the ground based high frequency P-flats with the gray sky image (discussed in the previous section). Before combining images, the noise was removed from the gray sky image using Fourier filtering. This was done mainly to remove outlier pixels. The gray sky image therefore represents an L-flat correcting for low and mid-frequency structure. Each flat-field image was normalized to 1.0 over the image section [101:900,101:900], which excludes areas of the detector known to contain anomalies such as the "Death Star" and the "Wagon Wheel” (see Section 7.9.2). Figure 7.8 shows an example of a pipeline flat field (F160W).

The photometric accuracy using the pipeline flat fields is better than 0.5% (peak to peak variation of -1.5 to +1.6%) if avoiding the outermost 128 pixels of the edge of the detector. Within the “Wagon-Wheel” region and the edge of the detector, photometric accuracy is reduced to about 0.8% (peak to peak variation of -2.0 to +1.9%).
The calibration files based on combining the CASTLE ground flat fields and the gray on-orbit L-flat were ingested into the HST Data Archive on Dec 2010. Observers with WFC3 IR data retrieved from the archive prior to that date can re-request their data in order to have them processed using the latest flat-field calibration files.

Additional discussion and the latest information about IR flat fields can be found on the WFC3 website

Because of geometric distortion effects, the area of the sky seen by a given pixel is not constant; therefore, observations of a constant surface brightness object will have count rates per pixel that vary over the detector, even if every pixel has the same sensitivity. In order to produce images that appear uniform for uniform illumination, the observed flat fields include the effect of the variable pixel area across the field. A consequence of dividing by these flat fields is that two stars of equal brightness do not have the same total counts after the flat fielding step. Thus, point source photometry extracted from a flat-fielded image (flt) must be multiplied by the effective pixel area map. This correction is accounted for in pipeline processing by AstroDrizzle, which uses the geometric distortion solution to correct all pixels to equal areas. In the drizzled images (drz), photometry is correct for both point and extended sources.