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WFC3 Data Handbook 2.1 May 2011
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WFC3 Data Handbook > Chapter 4: WFC3 Images: Distortion Correction and MultiDrizzle > 4.2 MultiDrizzle: Distortion Correction and Dither Combination

4.2
4.2.1
Images produced by both WFC3 channels are affected by considerable geometric distortion, introduced by the tilt of the image surface with respect to the chief ray. This is compounded by non-linear terms that produce changes across the field of view in both plate scale and area subtended by the pixels. MultiDrizzle relies on the Image Distortion Correction Table (IDCTAB) reference file for a description of the WFC3 distortion model. (Other reference files to correct for filter dependence and time dependence will be added as on-orbit measurements are made.) Using the task “makewcs”, MultiDrizzle interprets the distortion model, updates the headers, and provides the drizzle task with the information needed to resample the data.
MultiDrizzle automatically produces images that are both astrometrically and photometrically accurate. The drizzling process removes the geometric distortion and leaves the sky flat, so photometry of any sources in “_drz” images is uniform across the image. This is not true of the calibrated flt data products, and therefore a field-dependent correction factor is needed to 1.) achieve uniformity in the measured counts of an object across the field, and 2.) match the output drizzled counts. This correction is called the Pixel Area Map and simply reflects the area of the pixels at the location of the source. By multiplying the flt images by the pixel area map, users will recover the same counts on flt and drz images.
The MultiDrizzle software was designed to provide a seamless, integrated approach to using the various tasks in the IRAF/STSDAS dither package to correct images for distortion and to register, clean, and optimally combine dithered observations. The algorithm, known as Variable-Pixel Linear Reconstruction (or informally as drizzle), was developed by Fruchter & Hook (2002) to combine under-sampled dithered images of the Hubble Deep Field and has now been implemented as part of standard processing by the HST calibration pipeline.
Drizzling can be used to combine both dithered and mosaicked exposures. Mosaicking is performed with the aim of increasing the area of sky covered, usually to provide a seamless joining of contiguous frames. Dithering is employed in imaging programs for several reasons, including:
4.2.2
All WFC3 data taken in orbit will be automatically corrected for geometric distortion with MultiDrizzle during OTFR pipeline processing. This correction was implemented in the OPUS pipeline for WFC3 on February 4, 2010, and the drizzled data products make use of the latest on-orbit distortion solutions and MultiDrizzle parameter tables, as given by the IDCTAB and MDRIZTAB reference files.
MultiDrizzle can process single as well as dithered exposures taken with both UVIS and IR detectors. Dithered exposures are combined through the use of association tables (see Section 2.1.2 for more information).
To understand the processing which took place in the pipeline, it can be helpful to inspect the MultiDrizzle parameter table or MDRIZTAB. Drizzled products obtained from the archive have been processed using a default set of parameters, as specified in the MDRIZTAB. These parameters work best for observations which were obtained as part of a pre-defined observing pattern and thus are `associated' in the pipeline via an association table (“_asn”).
For example, images which were obtained using a sub-pixel dither box pattern are usually aligned to better than 0.1 pixels and have highly accurate cosmic-ray flags. For images obtained in separate visits, the image alignment and cosmic-ray flagging must usually be fine-tuned through manual reprocessing (see Section 5.2.3 for details). When a sub-pixel dither pattern has been used, the final drizzle sampling can be fine-tuned to produce images with improved overall resolution. The pipeline uses coarse values for both the output pixel size (scale) and drizzling kernel (pixfrac). This speeds up processing of the pipeline and is sufficient to give the user a very good quick view of the field. However, when the user has subsampled dithered data, rerunning MultiDrizzle manually can be useful to derive a more optimal set of parameters.
The calibration steps for both UVIS and IR images performed by MultiDrizzle include the following:
Using the flt files produced with calwf3 as input, MultiDrizzle performs the geometric distortion correction on all individual images (dithered or not), carries out cosmic-ray rejection (if multiple images exist), and combines dithered images into a single output image with the drz file name suffix. For UVIS exposures, MultiDrizzle combines the data from both chips into a single image.
MultiDrizzle is built around the script PyDrizzle, which is capable of aligning images and correcting for geometric distortion, but does not remove cosmic rays. MultiDrizzle supersedes the crj processing done by calwf3 and uses the individual flt files directly as input, performing cosmic-ray rejection in the process of producing the final drizzled image. This has significant advantages in cases where small numbers of CR-SPLIT images were obtained at a small number of different dither positions, because MultiDrizzle will use all the information from all the flt files to produce the best cosmic-ray rejection. The resulting drizzled images should generally be useful for science as-is, although subsequent reprocessing off-line may be desirable in some cases to optimize the data for specific scientific applications.
4.2.3
Extensive documentation on the MultiDrizzle software is available via the MultiDrizzle Handbook (Fruchter & Sosey et al. 2009).
Download the MultiDrizzle Handbook from the following Web site:
http://www.stsci.edu/hst/hst_overview/documents
The handbook provides a description of the mathematical algorithms used by the software, a discussion of how the astrometric header information is used to align images, and an overview of the separate IRAF/STSDAS tasks and parameters linked together by the MultiDrizzle interface.

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