TSR 2012-02: Definition of a Headerlet and Its Role in Updating WCS Information

Abstract:: Authors: Warren Hack, Nadezhda Dencheva Date: 22 Oct 2012

A headerlet is a self-consistent representation of a single WCS solution for a single exposure complete with all distortion information. FITS is the data storage format currently supported. It has no observational data which makes it relatively small and light to distribute. It is, essentially, a mechanism for encapsulating WCS information which can later be used to update the WCS of a science file and allows improved astrometric solutions to be stored and passed around easily. The HST archive is expected to start accepting headerlets for HST data soon. However the implementaiton is not HST specific and FITS WCS standard, as well as all WCS conventions implemented in pywcs are supported. This report describes the format and contents of a headerlet along with the software implementation and methods for creating headerlets and using them to update the WCS of a science observation.

Contents:

• Introduction
• Source Image
• Headerlet Definition
  • SIPWCS - A New FITS Extension
  • Headerlet File Structure
  • Headerlet Primary Header
• Working With Headerlets
  • Headerlet HDU - A New Type of FITS Extension
  • Examples
• Appendix 1: Headerlet API
  • apply_headerlet_as_alternate
  • apply_headerlet_as_primary
  • archive_as_headerlet
  • attach_headerlet
  • create_headerlet
  • delete_headerlet
  • extract_headerlet
  • print_summary
  • restore_all_with_distname
  • restore_from_headerlet
  • write_headerlet