WFC3 Flat Fields

Elena Sabbi,, & John MacKenty,

A key step in calibrating imaging data from Wide Field Camera 3 (WFC3) is correction for variations in sensitivity as a function both of the spectral energy distribution of the incoming light, and of position within the field of view. In a task generically known to astronomers as “flat fielding,” the raw data must be corrected for differences in the detectors’ pixel-to-pixel response, and for structures introduced by the illumination patterns within the instrument. Normally, flat-fielding corrects these variations by normalizing the data to produce a uniform illumination of the detector with light through the entire optical path of the telescope.

In the spring and summer of 2008, prior to the installation of WFC3 in Hubble, the WFC3 team conducted an extensive calibration program, which created high signal-to-noise flat fields for all the WFC3 filters. For both ultraviolet–visible (UVIS) CCDs (Sabbi et al. 2008), and the infrared (IR) mercury-cadmium-telluride array (Bushouse 2008), the flat fields were taken in a thermal-vacuum chamber at the Goddard Space Flight Center, using a simulator of the Hubble optics that emulated sky illumination on the detector.

Soon after WFC3’s installation, as expected, the tests performed showed that the ground-based flat fields do not fully remove the low- and intermediate-frequency structures (Sabbi 2009, Hilbert et al. 2009). Residuals of the order of five percent remained, due to differences between the ground illumination simulator and the actual Hubble telescope.


Strategies for the creation of improved in-flight flat fields

Building upon the experience with prior Hubble imaging instruments, the WFC3 team followed three strategies to refine the flat-field calibrations in flight. First, internal lamps were used to measure the high spatial frequency (pixel-to-pixel) variations in flat fields, as compared with measurements with the same lamps during ground tests. These measurements demonstrate that the high spatial frequencies have remained very stable (better than 1 percent) over the nearly two years since WFC3 was first calibration in flight.

Second, observations of the rich globular cluster Omega Centauri at multiple pointings established a set of low spatial frequency corrections to the ground flat fields in ten UVIS and five IR filters. In this process, the WFC3 team used the same software and methodology designed for ACS to remove the residual structures (Mack et al. 2002; van der Marel 2003).

In the third strategy, we observed external sources with nearly uniform illumination. Unlike at ground-based observatories, where the inside of the dome or the twilight sky provides uniform illumination, this operation is more difficult for Hubble. In visible light, the sunlit Earth can be a suitable target with narrow filters, but streaks due to clouds and ocean glints pose significant limitations. In broader UVIS filters, and in the infrared, we have had some success using the moonlit, dark Earth, and we expect more results over the next year. At present, these “Earth flats” serve to validate the low spatial frequency corrections derived by other means, while we explore the longer term potential.