[fitsbits] Question about Simple Image Polynomial convention {External} {External}

[Shupe, David L.]

Hi Bill,

I’m currently on a cruise ship on the way to Alaska so I’m going by memory. I believe we originated the SIP convention before the alternative WCSes specified by letters were adopted. I know the original 2005 ADASS proceedings did not address it. As I recall, our later writeup for the FITS registry of conventions did not address it either. I wonder if SPHEREx’s usage of SIP would provide guidance. I would guess that the SIP coefficients without letter suffixes would apply to any SIP WCSes in the header.

David L Shupe, PhD | Scientist | Caltech/IPAC
________________________________
From: Thompson, William T. (GSFC-671.0)[ADNET SYSTEMS INC] <william.t.thompson at nasa.gov>
Sent: Friday, May 22, 2026 10:00 AM
To: FITS List <fitsbits at nrao.edu>
Subject: [fitsbits] Question about Simple Image Polynomial convention {External}

Folks:

I have a question about the Simple Image Polynomial (SIP) convention.  The question has to do with a matter that is not addressed in the convention write-up: what happens when there are multiple coordinate systems embedded within a file?  I’ve been presented with an application where the primary coordinate system does not use the SIP convention, but one of the alternative coordinate systems does.  For example, we have these keywords for the primary coordinate system:

CTYPE1  = 'HPLN-TAN'
CTYPE2  = 'HPLT-TAN'

while an alternative coordinate system has:

CTYPE1B = 'RA---TAN-SIP’
CTYPE2B = ‘DEC-TAN-SIP’

My implementation of the SIP system in SolarSoft assumes that the SIP keywords should end in “B” to associate them with this coordinate system, but the actual SIP keywords do not, e.g.

A_1_0   =            -0.006446 / SIP distortion coefficient

rather than “A_1_0B”.  I’ve been told that astropy applies these keywords to the “B” coordinate system, so is this how the convention should be interpreted?

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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