The MultiDrizzle Handbook
WFC3 has a CCD detector, "UVIS", and an infrared detector, "IR". The UVIS detector has a plate scale ~0.04 arcsec/pixel, and the point spread function (PSF) has FWHM 1.6 to 2.3 pixels, depending on wavelength. The IR detector provides coarser sampling of the PSF, with a plate scale of 0.13 arcsec/pixel and FWHM of 1.0 to 1.2 pixels, so IR imaging will benefit even more than UVIS imaging from combining dithered exposures. Both detectors have substantial non-linear distortion, so a dither step will have different sizes in pixels at different locations on the detectors.
WFC3/UVIS images will be similar to ACS/WFC images, as can be seen in the table of SIAF file information in "Summary of Detector Plate Scales" under "Detector Plate Scales and Geometric Distortion". These detectors comprise two rectangular CCD chips which together form a square. They have roughly comparable fields of view and plate scales. Both have strong linear and non-linear distortion, as explained in "Summary of Detector Plate Scales". The linear distortion is characterized by an angle of 86 degrees between the projected x and y axes of the WFC3/UVIS detector on the sky, and 85 degrees between the projected x and y axes of the ACS/WFC detector. Note that the on-axis location of WFC3 in the telescope does not ensure that either the linear or non-linear distortion will be small, since the optical path within the instrument is the determining factor.
The WFC3/IR detector is unique among the HST imaging instruments in having a rectangular projection on the sky to first order. At 1100 nm, it has a percentage of PSF flux in the peak pixel comparable to that of the NICMOS NIC3 detector during optimal focus runs, but has a much larger field of view and more uniform sensitivity across pixels. WFC3/IR data is taken in specified timing sequences of non-destructive reads, and the data reduction pipeline performs cosmic ray rejection using an up-the-ramp fitting process over the frames in the sequence. cosmic rays identified in the pipeline reductions for IR sequences and cr-split UVIS exposures will be flagged with DQI value 8192, while those identified by MultiDrizzle will be flagged with 4096.
Examples of using MultiDrizzle to process WFC3 images will be made available after WFC3 data have been obtained in orbit following Servicing Mission 4.
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