[fitsbits] frequency definitions

[Tom Kuiper]

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