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.
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 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.
