TSR 2012-01: Distortion Correction in HST FITS Files

Abstract: Authors: Warren Hack, Nadezhda Dencheva, Andy Fruchter, Perry Greenfield Date: 31 Oct 2012

A convention for storing distortion information in HST images was developed and implemented in two software packages - PyWCS and STWCS. These changes allow the development of a WCS based version of Multidrizzle, released now as AstroDrizzle, and image alignment software, released as tweakreg. The distribution of WCS solutions is discussed.

Contents:

• Introduction
• Problems Introduced by the HST/ACS Distortion
• SIP Convention
  • Velocity Aberration Correction
  • Time-Dependent Distortion
• FITS Distortion Proposal
• Non-polynomial Residual Correction
  • NPOLFILE reference File Format
• Detector To Image Correction
  • Detector To Image Reference File
• Merging Of The Conventions
• Updating the FITS File
  • Creating the D2IMARR extension
  • Creating the WCSDVARR Extension
• Summary
• Appendix 1 - Sample ACS/WFC Image
  • FITS File extensions
  • Primary Header
  • SCI Header Keywords
  • D2IMARR Header
  • WCSDVARR Header
• Appendix 2 - NPOLFILE Example
  • FITS File Extensions
  • Primary Header
  • Data Extension Header
• Appendix 3 - D2IMFILE Example
  • FITS File Extensions
  • PRIMARY Header
  • Data Extension Header