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DrizzlePac 2012 Handbook
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The DrizzlePac Handbook > Chapter 3: Astrometric Information in the Header > 3.1 Introduction

3.1
A major enhancement in AstroDrizzle software is its handling of astrometric information. In the pipeline, AstroDrizzle uses the Simple Image Polynomial (SIP) convention (Shupe, et. al, 2005). Pipeline image headers contain keywords, some that are new, that describe two types of distortion corrections: linear distortion corrections that are incorporated into the CD matrix, and non-linear distortion corrections expressed as simple image polynomial (SIP) quadratic functions. For the latter, polynomial distortion coefficients are stored in the IDCTAB reference file. These are the only two types of geometric distortion corrections required for WFC3, STIS, and NICMOS data.
A third class of distortions, called residual or non-polynomial distortions, cannot be expressed as functions. Corrections for non-polynomial distortions come in the form of look-up tables that are stored in the image as FITS extensions. Therefore, it is no longer necessary to use the DGEOFILE reference file in AstroDrizzle processing, as was done in running MultiDrizzle. There are also several new header keywords associated with these look-up tables.
WFPC2 data is only available from the static Archive, where it was processed by MultiDrizzle. Users who wish to reprocess that data using AstroDrizzle only require the relevant distortion correction files for their images so that AstroDrizzle can use them to directly update the images. (Please see the example in Chapter 7 for more information.)
All three distortion correction operations for ACS, and the linear and polynomial distortion corrections for WFC3, are performed in the pipeline by AstroDrizzle and associated tasks. This chapter describes the new distortion correction features in more detail.

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