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IR Alpha Flats

WFC3/IR channel low-frequency flat field correction, alpha release

UPDATED: December 8, 2010

Variations in detector properties, such as the pixel thickness and non-uniform doping can cause differences in the pixel-to-pixel response of the detector that affect the accuracy of astronomical data. Additional wavelength-dependent low-order structures are introduced by the system illumination pattern and variations in the filter response. These variations are usually removed by normalizing the astronomical data to a uniform illumination of the detector with light passing through the entire optical path of the telescope. This process is commonly called flatfielding, and as a result the collected flux of a source should not depend on its position on the detector.

During the spring of 2008, in the third and last thermal vacuum test (TV3), the Wide Field Camera 3 (WFC3) team carried out an intense ground-based campaign to, among other purposes, create flatfields that will be used for the reduction of all WFC3 on-orbit data. Ground-based flatfields have been obtained by simulating the sky illumination of the UVIS CCDs (Sabbi et al., ISR 2008-46) a nd the IR array (Bushouse, ISR 2008-28) using the optical stimulus (CASTLE). These flat-fields should include both the high frequency pixel-to-pixel (P-flat) and low frequency (L-flat) structures. After WFC3 was installed on Hubble in Servicing Mission 4 (SM4), tests performed during the Servicing Mission Observatory Verification (SMOV) in proposals CAL-11452 and CAL-11453 indicate that the ground-based flatfields do not fully remove the low-frequency structures. These results were confirmed during in the Cycle 17 calibration programs CAL-11911 and CAL-11928.

Here we present an alpha release of the new IR flatfields obtained from a combination of the existing ground-based flat-fields (LP-flats) and a new low-frequency correction (L-flat) derived from combining a large number of long exposure science images after masking out objects, creating an equivalent of sky image. As a first step, sky images were produced in multiple filters. Investigating the wave-length dependence of the sky images showed no significant deviation between filters, with a variation of less than 1%. Therefore, a single gray L-flat correction was calculated and applied to all filters. In total over 2000 images were combined, all with exposure times >300s.

Independent measurements of the star cluster Omega Centauri in different broad-band filters confirms the results from the sky residual both when it comes to the spatial features and the amplitude of the L-flat corrections, as well as the lack of a significant dependence on wave-length.

The uncertainty in photometry due to the flat-field correction has an rms of 0.7% over the whole detector, with a maximum peak-to-peak range of -2.0/+1.9%. The uncertainties are typically larger at the edges of the detector. For the central part of the detector [129:896,129:896] (i.e., excluding a region around the detector edges with thickness 1/8 of the detector size), the uncertainty has an rms of 0.5%, with peak to peak range of -1.5/+1.6%. For the edge region of the detector only (i.e., a frame with 128 pixel thickness), which includes the "wagon-wheel" feature, the rms is 0.8%, with peak-to-peak -2.0/+1.9%. The introduced uncertainty in photometry due to the flatfielding therefore typically has an rms of less than 0.01 mag, at the edges of the detector the errors may reach 0.02 mag. In addition, the uncertainty in the wave-length dependence of the skyimage correction should also be less than 0.01 mag.

The new LP-flatfields are:
flat_f098m_lpflt.fits
flat_f105w_lpflt.fits
flat_f110w_lpflt.fits
flat_f125w_lpflt.fits
flat_f126n_lpflt.fits
flat_f127m_lpflt.fits
flat_f128n_lpflt.fits
flat_f130n_lpflt.fits
flat_f132n_lpflt.fits
flat_f139m_lpflt.fits
flat_f140w_lpflt.fits
flat_f153m_lpflt.fits
flat_f160w_lpflt.fits
flat_f164n_lpflt.fits
flat_f167n_lpflt.fits

These LP-flats replaces the previously released alpha-version from May 25, 2010.

The new flat-fields were ingested into CDBS in December 7, 10.31pm. Data retrieved after this date are processed using the new reference files. (Note that the file names in CDBS differs from the names used on this webpage.) WFC3/IR users interested in applying these new flat-fields to older data can either re-retrieve the data from the archive, or re-run the standard calibration pipeline CALWF3 on the IR raw data. Before running CALWF3, the header keyword PFLTFILE must be updated in the raw images to the new flat-field name, while DFLTFILE and LFLTFILE should remain unchanged (= N/A):

>hedit my_raw.fits[0] PFLTFILE flat_filter_lpflt.fits

>calwf3 my_raw.fits

Alternately, users who do not wish to reprocess their data with CALWF3 can apply the L-flat correction only to their calibrated data products in the following way:

>copy my_flt.fits my_copy_flt.fits

>imcalc my_flt.fits[1],wfc3_IR_lflt.fits my_flt_corr.fits “im1/im2”

>imcopy my_flt_corr.fits[1] my_flt.fits[1,overwrite+]

The new L-flatfield is:
wfc3_IR_lflt.fits