Space Telescope Science Institute
WFC3 Data Handbook 2.1 May 2011
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WFC3 Data Handbook > Chapter 6: WFC3-IR Error Sources > 6.7 Flat-Fielding

As for the UVIS channel (see Section 5.2), the IR channel flat-field reference files currently in use in the WFC3 calibration pipeline were created at the Goddard Space Flight Center under thermal vacuum conditions using an external illumination source. These flat fields take into account the pixel-to-pixel variations in QE (P-flats). Low-frequency variations in the illumination pattern, however, are present in these flats. These variations can be removed using L-flat corrections, which have been derived from in-flight dithered observations of the globular cluster Omega Centauri.
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 and flat-field illumination in either monochromatic or broadband mode.
During TV3, CASTLE flat fields were acquired using the SPARS10 sample sequence for the readout mode, with varying numbers of readouts (samples) per exposure, chosen to obtain a signal of about 60,000 electrons per pixel in the final read. Flat fields with the OS tungsten lamp were taken with the detector at its nominal operating temperature.
The flats are 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”.
The large-scale uniformity of the IR channel detector response, as provided by the TV3 ground-based flats, has been improved in-flight by using multiple pointing dithered patterns of the globular cluster Omega Centauri. 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 have been measured. Average photometric errors of +/-1.5% have been found in the original IR ground-based flat fields (see WFC3 ISR 2009-39). The derived L-flats are based on a 3rd-order polynomial fit and are shown in Figure 6.7, 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.
L-flats were determined from in-flight observations using filters F098M, F110W, F125W, F139M, and F160W. 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 three times during Cycle 17, so that observations are obtained at three 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 IR 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 reduced to <1%.
Figure 6.7: IR L-Flats
IR L-flat corrections required for the ground-based P-flat. From left to right and top to bottom the corrections for the filters F098M, F110W, F125W, and F160W are shown. Peak to peak corrections range from -3 to +1%.
At this time the calibration pipeline is using the ground-based P-flats only. The derived L-flat corrections have been multiplied into the corresponding P-flats, to produce corrected hybrid LP-flats. The resulting LP-flats are available on the WFC3 Web site for downloading:
Figure 6.8 shows the corrected IR channel ground-based flats for several broadband filters. The 18 small dark spots (“IR blobs”) across the IR field of view are regions of lower sensitivity (by about 10-15%, see WFC3 ISR 2010-06). 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.
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 7.2.3). 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.
The numerous survey programs that the IR channel is executing are being used to build high signal-to-noise sky flats. These data consist mostly of sparsely populated images with relatively uniform sky that can be stacked to further quantify the low frequency and high frequency flat-filed components.
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 IR 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 6.8: IR LP-Flats
From from left to right and top to bottom, IR flat fields for the filters F098M, F110W, F125W, and F160W.
On-orbit monitoring of the IR flat field response is done using an internal flat-field lamp. The only flat fields obtained using an external source were obtained during the ground testing of the instrument. Neither of these two types of flat fields are true observations of the illumination of the IR detector in real operating conditions, where the instrument is being operated in orbit and the incoming light propagates through the entire optical system consisting of HST and WFC3. One commonly used method to obtain realistic flat fields consists of observing a large uniform source that is external to the observatory. In our case, we currently do not have the ability to do this, while we are investigating using observing the Earth bright limb for this purpose, and we have instead assembled “super sky” type flat fields. These were generated by combining WFC3-IR observations of sparse fields after masking out astronomical sources. We have so far been able to assemble relatively high signal to noise Sky flats for both the F125W and F160W filters. The F160W sky flat is shown in Figure 6.9. The sky flats show  a +/- 3% field variation of the flat field that is currently uncorrected by the current ground based pipeline flat fields. Both the F125W and the F160W IR Sky Flat have larger pixel to pixel variation than the current pipeline flat fields, which is a direct consequence of their lower signal-to-noise levels but both show the identical large scale structure. The F125W and the F160W sky flats differ by less than 1%, indicative that this correction may not have a large color component. Sky Flats will continue to be assembled as more WFC3-IR data becomes available and will be made available to users wishing to use them.
Figure 6.9: IR Sky Flat
Preliminary IR sky flat in the F160W filter.

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