[fitsbits] Question about CROTA

Folks:

I have a question about the CROTAi keywords. I know that the WCS papers
deprecate their use, but they're still extremely popular.

We've always been in the habit of putting CROTAi keywords in for both spatial
coordinate axes, with exactly the same value for each. That seems the most
reasonable interpretation of the original FITS documentation.

Calabretta and Greisen (2002) describe the AIPS convention, which only pays
attention to the CROTAi attached to the latitude axis. I've had it expressed to
me that the AIPS convention makes it illegal to also put the CROTAi keyword for
the longitude axis, but I don't see anything in Calabretta and Greisen to that
effect. Is it true that it's illegal to replicate the rotation angle in the
longitude axis? As far as I can tell, any CROTAi keyword attached to axes other
than the latitude axis is ignored.

If so, that could disrupt a large body of FITS handling software within the
solar community. We don't use AIPS, and this aspect of the "AIPS convention",
if true, is taking us by surprise.

Bill Thompson


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William Thompson
NASA Goddard Space Flight Center
Code 612.1
Greenbelt, MD 20771
USA

301-286-2040