[fitsbits] Question about distortions when multiple coordinate systems are applied {External}

[Thompson, William T. (GSFC-671.0)[ADNET SYSTEMS INC]]

Folks:

I’ve been presented with a question about the use of distortion parameters when a FITS header contains multiple coordinate systems.  Earlier I had asked about this issue when using the Simple Image Polynomial convention, but now I’m asking about distortions as described in the Calabretta et al. 2004 paper.

Specifically I’m asking about prior (DP) distortions, since subsequent (DQ) distortions are a different case entirely.  I’ve been told that astropy will automatically apply distortions encoded in DPj keywords when there are no DPja keywords for alternative coordinate system “a”.  I’ve looked through the Calabretta et al. 2004 paper, and cannot find anything that says that this substitution should be made.  Is this a convention that is widely adopted?

To me, it does make sense that any prior distortion parameters applicable to one coordinate system would be equally applicable to any alternate coordinate system.  The same is not necessarily true for subsequent distortion parameters, given the possibly different effects from the CRPIXja and CDi_ja values between the systems.

I’d appreciate any clarity that can be applied to this situation.  If that convention is widely used, then I should adopt it too.

Thank you,

Bill Thompson

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William Thompson
Senior Scientist
ADNET SYSTEMS, Inc.
NASA Goddard Space Flight Center, Code 671
Greenbelt, MD 20771
o: 301.286.2040
William.T.Thompson at nasa.gov<mailto:William.T.Thompson at nasa.gov>
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