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DrizzlePac 2012 Handbook
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The DrizzlePac Handbook > Chapter 1: Introduction to AstroDrizzle and DrizzlePac > 1.2 What's new in AstroDrizzle?

1.2
1.2.1
While AstroDrizzle maintains much of the same algorithmic base as MultiDrizzle, this new code has undergone a number of substantial internal changes. Core routines have been re-coded in C and Python, written in a modular fashion for easier maintenance and updates.
All user interaction is performed using Python, either as command lines and/or using the TEAL (Task Editor and Launcher) Graphical User Interface (GUI) which is structured for easier use in setting task parameters.
1.2.2
In MultiDrizzle, non-linear distortions in an image are expressed as a set of polynomial function coefficients that are distinct from the image world coordinate system (WCS). These coefficients, unique for different types of observing modes, are stored in a reference file called the IDCTAB (Instrument Distortion Coefficients TABle). The Drizzle program uses this reference file to extract coefficient values that correspond to the images being processed.
In contrast, AstroDrizzle incorporates linear distortion corrections and higher order polynomial distortion corrections directly into the WCS in flt.fits headers, using the Simple Image Polynomial (SIP) convention (Shupe, et. al, 2005). This convention has, for some time, been in use for describing the geometry of Spitzer Space Telescope images, and it is expected to become a FITS standard in the near future. Representing image distortion corrections using the SIP convention will improve the handling of image combination and astrometric information.
When AstroDrizzle becomes part of the on-the-fly reprocessing (OTFR) calibration system, users will notice some changes in calibrated (flt.fits) ACS and WFC3 images1 retrieved from the HST Archive: distortion information is now directly incorporated into the image headers using FITS standard conventions. This new feature makes it easier for users to access distortion information for use in their own code. Publicly available software such as ds9, that can interpret these FITS conventions, are now able to convert between pixel positions and sky coordinates. For instance, when a flt.fits2 image from the HST Archive is displayed using ds9, the astrometric Right Ascension (R.A.) and Declination (Dec.) positions from ds9 show “undistorted” positions using the full polynomial distortion corrections.
1.2.3
ACS data have additional non-polynomial distortion corrections3.
In the previous pipeline, MultiDrizzle obtained small scale distortion corrections from reference files identified by the header keyword DGEOFILE (Differential GEOmetric distortion). These reference images, as large as 256 MB, are similar in size to flt.fits images. DGEOFILE images were created by interpolating tables that contained residual distortion corrections. For instance, an ACS/WFC DGEOFILE reference image was created by interpolating a set of 32x64-entry tables representing residual distortion corrections into a full-size image, then combining that image data with the detector column width corrections. (More about this in Section 3.2.)
The new OPUS pipeline with AstroDrizzle inserts the non-polynomial distortion corrections, in tabular form, directly in the flt.fits file as FITS extensions. (multidrizzle can still be run using these new-format flt.fits images, and the output will remain the same as before. However, the software still requires IDCTAB and DGEOFILE reference files.)
As a result, ACS data files from the current OPUS pipeline have several new FITS extensions. The D2IMARR extension, only required for ACS/WFC, contains information on the geometry of the detector not described by the image SIP coefficients.
The WCSDVARR FITS extensions hold tabular data describing small-scale distortion effects due to optics. (There are two such extensions per detector: one for corrections to the x-axis, the other for y-axis corrections. Therefore, a WFC flt.fits image has four WCSDVARR extensions while HRC and SBC flt.fits images each have two WCSDVARR extensions.)
1.2.4
The DrizzlePac package contains tasks that provide enhanced handling of astrometric information. These include,
Updating image WCS information with a user-supplied catalog as a reference: for instance, in comparing a catalog of object positions derived from a HST image with a user-supplied astrometric catalog, the software can update the HST image WCS to bring its coordinate system into agreement with the user's astrometric catalog. This astrometric catalog could be a set of positions, perhaps created from another AstroDrizzle-processed HST image, or it could be derived from a standard catalog (such as 2MASS or Kepler).
Corrected astrometric information can be fully captured in small FITS files called headerlets These headerlets can be sent to collaborators working on the same data, allowing just the image's astrometry component to be sent instead of the whole image. In addition, users will be able to attach several solutions to an image, for instance, one solution that best matches the 2MASS catalog and another that better agrees with UCAC3.

1
WFPC2 and NICMOS images, processed with MultiDrizzle, are in a static archive and will not undergo further pipeline processing.

2
flt.fits images from the HST Archive were processed by AstroDrizzle as part of the OPUS pipeline. While the image data remains unchanged, header keyword values and data quality information is updated.

3
Non-polynomial distortion corrections are currently not needed for WFC3 data.


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