[fitsbits] Question about CROTA

[William Thompson]

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


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

301-286-2040
William.T.Thompson.1 at gsfc.nasa.gov