Howto create FITS images/spectra to be VO-compatible

FITS format is a popular output used in astronomy for spectra and images. FITS recommendations when applied enable to make a clean mapping between FITS header and ObsCore DM.

Mapping FITS with ObsCore

Some Obscore items can be assigned by provider (i.e.: obsid, access_estimated_size, ..) and other can be detected in FITS using different keywords.
We can classify them into 3 categories (ordered by relevance):

1. WCS keywords
   Available for different topics (position, electromagnetics band..). WCS is a serialisation based on FITS keywords that make a bijection between a simple set of values and the true values (see the official document: https://fits.gsfc.nasa.gov/fits_wcs.html )
   An automated workflow requires all WCS keywords :
   i.e.: CRVAL, CTYPE, CRPIX, CDELT ,CUNIT, [CDi_j]
2. NASA standards and extended keywords list
   See https://fits.gsfc.nasa.gov/
3. Multipsec: The IRAF/NOAO Spectral World Coordinate Systems
4. Other keywords in usage
   Other keywords used FITS, not described by NASA

Proposed keywords grouped by topics

Validator tools

Tools like the CDS fits validator (based on Saada) checks the VO-compatibility according to WCS and a keyword list based on the FITS extended keywords.

https://cdsarc.cds.unistra.fr/vizier.submit/fitsvalidator.html

FITS (Nasa) recomendations

• FITS recommendation based on keywords :

  • Standard list of keyword : https://heasarc.gsfc.nasa.gov/docs/fcg/standard_dict.html

  • Extended list of keywords: https://heasarc.gsfc.nasa.gov/docs/fcg/common_dict.html

• FITS WCS

  • https://fits.gsfc.nasa.gov/fits_wcs.html

Note: NASA FITS url: https://fits.gsfc.nasa.gov/fits_dictionary.html , HEASARC URL : https://heasarc.gsfc.nasa.gov/docs/fcg/ (idem than FITS)