FITS WCS output

If requested (by the presence of the fits= specifier on the command line), ASTROM will write out the plate solution in a series of FITS files, containing headers conforming (largely) to the FITS WCS standards (known as `Paper I' and `Paper II') [1,2]. ASTROM will generally attempt more than one fit. The file names will start with the string given in the fits= parameter.

There is more than one way to encode the required WCS information, and which way is used depends on the value of the parameter wcsstyle= on the command line. The allowed values of this are qtan and xtan, and these are discussed now.

There is a standard for specifying world coordinate systems in FITS files [2], and ASTROM conforms to this. At present (May 2003) there is only an early draft standard for representing distortions, Representation of distortions in FITS world coordinate systems, Calabretta et al. (also known as `Paper IV'), available at Mark Calabretta's web pages [3]. Part of ASTROM's function is to determine and report such distortions, but since there is not yet any standardised way to do this, we have something of a problem.

This program does not attempt to produce output using the distortion model described in Paper IV; that seems premature. Instead, it describes distortions using the model described as `distorted gnomonic' (TAN) in the late draft versions of Paper II. If you specify wcsstyle=xtan (not recommended), then ASTROM emits FITS headers which fully conform to these drafts; if you specify wcsstyle=qtan, the FITS headers are essentially the same, but with the draft paper's PVj_m headers replaced by non-standard QVj_m. In this latter case, the headers are conformant with the final Paper II, but the distortion information is available to software which knows how to use it. You are strongly advised not to produce new FITS files using option wcsstyle=xtan, unless you are obliged to by old versions of software.

The `distorted gnomonic' TAN projection is not documented here (deliberately), and the drafts describing it are no longer readily available on the web. However, should you need to, you would probably be able to find a copy through Mark Calabretta's WCS web pages [3].

This is an interim solution. It's anyone's guess how long it will take for the FITS community to agree on a final version of Paper IV. Once that is finalised, however, it's quite possible that ASTROM's support for the above header styles will be removed.