The Foreign FITS Encodings

As mentioned previously (§16.1), there are a number of conventions currently in use for storing WCS information in FITS headers, which we call encodings. Here, we are concerned with those encodings defined by software systems other than AST, which we term foreign encodings.

Currently, AST supports six foreign encodings, which may be selected by setting the EncodingEncoding attribute of a FitsChanFitsChan to one of the following (character string) values:

DSS

This encoding stores WCS information using the convention developed at the Space Telescope Science Institute for the Digitised Sky Survey (DSS) astrometric plate calibrations. DSS images which use this convention are widely available and it is understood by a number of important and well-established astronomy applications.

However, the calibration model used (based on a polynomial fit) is not easily applicable to other types of data and creating the polynomial coefficients needed to calibrate your own images can prove difficult. For this reason, the DSS encoding is probably best viewed as a “read-only” format. It is possible, however, to read in WCS information using this encoding and then to write it back out again, so long as only minor changes have been made.

FITS-WCS

This encoding is very important because it is based on a new FITS standard which should, for the first time, address the problem of celestial coordinate systems in a proper manner, by considerably extending the original FITS standard.

The conventions used are described in a series of papers by E.W. Greisen, M. Calabretta, et. al., often referred to as the “FITS-WCS papers”. They are described at http://fits.gsfc.nasa.gov/fits_wcs.html. Now that the first two papers in this series have been agreed, this encoding should be understood by any FITS-WCS compliant software and it is likely to be adopted widely for FITS data in future. For details of the coverage of these conventions provided by the FitsChan class, see Appendix G.

FITS-IRAF

This encoding is based on the conventions described in the document “World Coordinate Systems Representations Within the FITS Format” by R.J. Hanisch and D.G. Wells, 1988. It is employed by the IRAF data analysis facility, so its use will facilitate data exchange with IRAF. This encoding is in effect a sub-set of the current FITS-WCS encoding.

FITS-PC

This encoding is based on a previous version of the proposed new FITS WCS standard which used PCjjjjiii and CDELTj keywords to describe axis rotation and scaling. Versions of AST prior to V1.5 used this scheme for the FITS-WCS encoding. As of V1.5, FITS-WCS uses CDi_j keywords instead. The FITS-PC encoding is included in AST V1.5 only to allow FITS-WCS data created with previous versions to be read. It should not, in general, be used to create new data sets.

FITS-AIPS

This encoding is based on the conventions described in the document “Non-linear Coordinate Systems in AIPS” by Eric W. Greisen (revised 9th September, 1994). It is currently employed by the AIPS data analysis facility, so its use will facilitate data exchange with AIPS. This encoding uses CROTAi and CDELTi keywords to describe axis rotation and scaling.

FITS-AIPS++

Encodes coordinate system information in FITS header cards using the conventions used by the AIPS++ project. This is an extension of FITS-AIPS which includes some of the features of FITS-PC and FITS-IRAF.

For more detail about the above encodings, see the description of the Encoding attribute in Appendix C.