Bob Garwood wrote:
Then you ask" What is the difference between FREQ-OBS and FREQREST?".
But I'm not sure what you mean by "FREQ-OBS". You should an equation
earlier (badly formatted in my copy, but I think I understand what you
meant) showing the difference between OBSFREQ and RESTFREQ, so I don't
think you mean FREQ-OBS = OBSFREQ. SDFITS doesn't use FREQ-OBS so I
think you'll need to ask the creator of that FITS file what they
meant. Unless I missed something in a quick scan of the relevent WCS
paper, I don't see it defined there.

I don't think that is quite what I meant to ask. FREQ-OBS is a possible
value for TTYPEx. It refers to some kind of reference frame. I assume
it means this is a frequency axis defined in the frame of the observer.
OBSFREQ is a keyword and I confess that it's not clear to me why it is
needed if the same information, in some form, is required to define one
of the axes.

Best regards

Tom