IRAM Memo 2023-1 Review of the spatial projections support in GILDAS Compatibility with FITS

# IRAM Memo 2023-1
Review of the spatial projections support in GILDAS
Compatibility with FITS

S. Bardeau, J. Pety
1. IRAM (Grenoble)
2. Observatoire de Paris

Date: 22-nov-2023
version 1.0

Note: this is the on-line version of the Revised support for the SFL projection in GREG. A PDF version is also available.

Related information is available in:

- GREG: Grenoble Graphics

### Abstract:

This memo reviews the spatial projections implemented in GILDAS and their compatibility with the FITS conventions defined by the AIPS memos and by Calabretta & Greisen (2002). It focuses on the radio projection used at the IRAM 30m telescope, explains why its legacy GLS description is obsolete in FITS, and shows how the same data can be represented with the CG02-compliant SFL projection through a header translation only. It also summarizes the related corrections introduced in GILDAS to restore reliable FITS import and export for the affected projections.

Keywords: projection, FITS

Related documents: GreG documentation

- Contents
- Description
  - IRAM context
  - FITS formalism
  - Compatibility between FITS and GILDAS
    - Direct compatibility
    - Incompatibilities
    - The GLS/radio projection in details
    - The Hammer-Aitoff case
- GILDAS implementation
  - Gnomonic projection
  - Orthographic projection
  - Azimuthal projection
  - North Celestial Pole projection
  - Stereographic projection
  - Aitoff projection
  - Radio projection
    - Radio-projected cubes
    - 30m CLASS cubes
  - SFL projection
- New SFL implementation in GILDAS
  - From projection plane to celestial coordinates
  - From celestial to projection plane coordinates
  - Special case of SFL with reference on the Equator
- Consistency checks
  - Support for TAN / gnomonic projection
  - Support for GLS / radio projection
  - Support for SFL (properly shifted) projection
  - Support for SFL (no shift) projection
- About this document ...