[fitsbits] frequency definitions

[Bob Garwood]

Since I'm the author of that part of that note, I should probably chime 
in here.

As mentioned elsewhere, the primary WCS papers are referenced in that 
previous reply and cited below.  The SDFITS note in aips++ predates all 
of that.  SDFITS has a long history, which I won't go into here.  It 
remains primarily a locally implemented standard with a few version of 
"local" that aren't quite in agreement for lack of effort primarily on 
my part to try and reach a real consensus and, most importantly, 
exchange data between "local" implementations. 

Prior to the WCS papers, lots of people obviously described a 
frequency-like axis.  So you're going to find that historically there 
are keywords and columns for which you'll just have to talk to the folks 
that wrote the FITS file to be sure you go the interpretation right - if 
there is any ambiguity in what you find there as compared to the WCS papers.

For your specific questions ...

CRVALx - I'd say "yes"
OBSFREQ - 'probably'.  I think "center of the IF" is possibly ambiguous 
enough that you might want to double check with the author/writer
RESTFREQ - no.  I'd interpret that as equivalent to the "RESTFRQ" value 
described in the WCS papers.  It's the rest frequency of the spectral 
feature of interest - i.e. the frequency that that feature has when 
observed at rest with respect to the thing producing that feature.  It's 
the lab-measured line frequency.   It may or may not correspond to the 
center of the IF, depending on how the IF was set up and if doppler 
tracking was being done during the observations.  It should only be 
necessary/useful if you want to convert from a frequency to a velocity 
(I'm 99% sure of that statement, but I'm sure someone here will correct 
me if that isn't true) - or rather, this quantity isn't necessary to 
recover the frequency at each pixel nor is it necessary to convert from 
a frequency in one reference frame to a frequency in a different 
reference frame assuming that TTYPEx is a frequency axis.

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.

Cheers,
  Bob



Tom Kuiper wrote:
> Well, not specifically about dual sideband data but about radio 
> spectral line data in general.  The best description of the relevant 
> keywords that I've found is in the NRAO AIPS++ documentation at 
> http://aips2.nrao.edu/docs/notes/236/node14.html.  However, the 
> definitions are not entirely unambiguous.  That's why I posted my 
> interpretation, to see if anyone disagreed.
>
> Regards
>
> Tom
>
> David Berry wrote:
>> It looks like Tom's query was about dual sideband data, which is not
>> covered by paper III. OBSFREQ, etc, are not in the standard. There are
>> probably various different schemes in use out there. For instance JCMT
>> and CLASS software use a system based on a keyword called IMAGFREQ,
>> which holds the image sideband rest frequency (with RESTFREQ holding
>> the observed sideband rest frequency).
>>
>> David
>>
>>
>> 2009/5/5 LC's No-Spam Newsreading account <nospam at mi.iasf.cnr.it>:
>>   
>>> On Mon, 4 May 2009, Tom Kuiper wrote:
>>>
>>>     
>>>> I'd like to get it clear in my mind what the various frequency
>>>> definitions are.  Is the following right? If there is a place where
>>>> all this is clearly laid ou, please refer me to that.
>>>>       
>>> Not a spectral WCS expert, but the standard and official references
>>> are :
>>>
>>> - http://fits.gsfc.nasa.gov/fits_standard.html
>>>   the latest release of the FITS standard, in particular chapter 8
>>>
>>> - the full WCS paper III, either as published or
>>>   http://www.atnf.csiro.au/people/mcalabre/WCS/scs.pdf
>>>
>>>   (the other WCS papers are all listed in
>>>   http://www.atnf.csiro.au/people/mcalabre/WCS/index.html
>>>
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