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

Tue Jul 7 16:46:06 EDT 2026

It’s a weakness of SIP that it doesn’t support the alternative-WCS convention, and this has led to real problems in deciding what Astropy should do with it when there are alternative WCSes defined.

For cases where the convention actually supports the alternative-WCS “a” convention (as in the question in this thread) I think it’s very clear that the consistent intent of the design of FITS was to have each of the 27 possible WCSes be independent of the others.

There are cases, such as for SPHEREx, where you absolutely don’t want optical distortions to “leak” into other WCSes.  In SPHEREx, we use a FITS-standard (though rather exotic) 2D-lookup-table WCS to document the (intentionally) varying spectral bandpass across the field of view.  That bandpass is tied to the physical pixels, not to the sky, created by linear-variable filters immediately in front of the detectors, so including optical distortions from the primary spatial WCS in the spectral WCS would be a serious error.

We have to leave it up to projects/missions to choose which WCSes to apply distortions to; if that means they have to repeat distortion models for multiple “a” values, well, within the limitations of FITS: so be it.

Best wishes,

Gregory Dubois-Felsmann
Caltech/IPAC
NSF/DOE Vera C. Rubin Observatory Science Platform science lead, Data Engineering lead
NASA SPHEREx mission pipeline system designer

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Gregory Dubois-Felsmann, Ph.D. | Senior Staff Scientist | Caltech/IPAC
Science Platform Scientist, Vera C. Rubin Observatory
Pipeline System Designer, NASA SPHEREx mission
Mail Code MR 100-22 | Pasadena, CA 91125-2200 | gpdf at ipac.caltech.edu

________________________________
From: fitsbits <fitsbits-bounces at listmgr.nrao.edu> on behalf of Thompson, William T. (GSFC-671.0)[ADNET SYSTEMS INC] via fitsbits <fitsbits at listmgr.nrao.edu>
Sent: Tuesday, 07 July 2026 12:43:25
To: fitsbits at listmgr.nrao.edu <fitsbits at listmgr.nrao.edu>
Subject: [fitsbits] Question about distortions when multiple coordinate systems are applied {External}

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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