Space Telescope Science Institute
WFC3 Data Handbook 2.1 May 2011
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WFC3 Data Handbook > Chapter 5: WFC3-UVIS Error Sources > 5.4 Flat Fields

During spring 2008, flat-field images for the UVIS channel were produced at the Goddard Space Flight Center (see WFC3 ISR 2008-46) during the third and last thermal vacuum campaign (TV3) using the CASTLE Optical Stimulus (OS) system. The CASTLE is an HST simulator capable of delivering OTA-like, external, monochromatic point-source and broad-band full field illumination.
During TV3, CASTLE flat fields were acquired only in the standard CCD readout configuration of four amplifiers (ABCD), gain=1.5 and binning=1x1. Flat fields with the OS Xenon lamp were taken with the detector at its nominal operating temperature of -82C. A subset of ultraviolet (UV) flat fields was also acquired at a warmer temperature (-49C) using the deuterium lamp to achieve higher count rates.
A total signal of about 75,000 electrons per pixel was required for each flat field to avoid degrading the intrinsic pixel-to-pixel rms response of <1%. The flats are normalized to 1.0 over the chip 1 section [1031:1133,328:429]. This region was selected to avoid the small dark rings (“droplets”) that are spread across the UVIS field of view. The droplets are likely shadows of mineral residue caused by a condensation event that occurred before TV3 (see WFC3 ISR 2008-10).
In the case of full-frame filters, the chip 2 images are divided by the chip 1 average value, in order to preserve the overall sensitivity difference between the two CCD chips across the gap that separates the two independent pieces of the UVIS channel detector. Because of the different response of the various quadrants of each QUAD filter, each quadrant was normalized to a level of 1.0, with respect to the median value in a 100x100 pixel box of that quadrant (for more details see WFC3 ISR 2008-46).
The large-scale uniformity of the UVIS channel detector response, as provided by the TV3 ground-based flats, can be improved in-flight by using multiple pointing dithered patterns of a stellar field. Because of the broad wavelength range covered by the UVIS filters, we used the globular cluster Omega Centauri, which hosts thousands of bright red giant branch (RGB) stars, and at the same time shows a spectacular blue horizontal branch (HB). 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 each detector have been measured. Average photometric errors of +/-1.5% to +/-4.5% have been found in the original UVIS ground-based flat fields (see WFC3 ISR 2009-19). The derived L-flats are based on a 3rd-order polynomial fit and are shown in Figure 5.3, where white indicates that the photometry produced using the ground-based flats is too faint with respect to the true stellar magnitude, and black indicates that the photometry is too bright. There is a gradient in the L-flat correction along the diagonal of the detector, which corresponds to the axis of maximum geometric distortion.
L-flats were determined from in-flight observations using filters F225W, F275W, F336W, F390W, F438W, F555W, F606W, F775W, F814W, and F850LP. The L-flat correction for the remaining filters was derived by using a linear interpolation as a function of wavelength. The pivot wavelength of each filter was used for the interpolation, where the resulting L-flat is equal to the weighted average of the L-flat for the two filters nearest in wavelength. For a discussion of the mathematical algorithm used to derive the L-flats, refer to ACS ISR 03-10.
The L-flat calibration program revisited the same target two times during Cycle 17, so that observations are obtained at two different orientations, due to the roll of the telescope. Differential photometry of stars falling on different portions of the detector as the telescope rolls provides an independent test of the absolute sensitivity dependence with time for the full suite of the UVIS channel filters. Initial testing indicates that the photometric response for a given star is now the same to ~1% for any position in the field of view for filters which were observed during the in-flight L-flat campaign. Further observations of Omega Centauri are planned for Cycle 18. Once the analysis of additional stellar observations is complete, new flat fields will be delivered to the pipeline and the errors are expected to be less then <1%.
Figure 5.3: UVIS L-flat corrections required for the ground-based P-flat. From top to bottom, left to right the corrections for the filters F225W, F275W, F336W, F438W, F555W, F606W, F775W, F814W and F850LP are shown. Peak to peak corrections range from -4 to +7% for the F225W filter to -1 to +3% for the filter F775W.
At this time the calibration pipeline is using the ground-based P-flats only.
The low-frequency variations in sensitivity over the detector field of view were derived in-flight from dithered stellar observations of the globular cluster Omega Centauri.
These observations allowed us to model the L-flat corrections, which can be applied to the corresponding P-flats. The resulting corrected flat fields (LP-flats) are available on the WFC3 Web site. Observers can download these LP-flats and run the CALWF3 program on their own computer to apply the corrected flats.
Figure 5.4 shows the corrected UVIS channel ground-based flats for several broadband filters. Note that the gap between the top and bottom halves of the UVIS channel detector is not shown here. The dark structure (“happy bunny”) visible in the lower right quadrant is due to variations in chip thickness (see WFC3 ISR 2010-05) and is dependent on wavelength.
The small dark rings (“droplets”) spread across the UVIS field of view are shadows of mineral residue caused by a condensation event that occurred before TV3 (see WFC3 ISR 2008-10). About 1/3 of the droplets moved in a coherent way during the launch (see WFC3 ISR 2009-27).
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 must be multiplied by the effective pixel area map (see Section 7.2.1). This correction is accounted for in pipeline processing by MultiDrizzle (see Section 4.2), which uses the geometric distortion solution to correct all pixels to equal areas. In the drizzled images, photometry is correct for both point and extended sources.
Observations of the bright Earth will be acquired during Cycle 18 to provide a uniform flat-field source for the complete OTA optical complement and incorporate both the low frequency L-flat and the high frequency pixel-to-pixel P-flat response.
To summarize, the pipeline flats were created by correcting the pixel-to-pixel flats by low-frequency corrections derived from dithered stellar observations. For the most used modes, the flats are accurate to better than 1% across the detector. Additional verification of the UVIS pipeline flats will be provided by earth flats, confirming also the derived L-flats and setting limits to their dependence on the color of the source spectrum.
Figure 5.4: UVIS LP-flats for the filters F225W, F336W, F606W, and F814W (from left to right and from top to bottom).

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