[fitsbits] frequency definitions

[Mike Nolan]

On Wed, May 06, 2009 at 10:50:44AM -0700, Tom Kuiper wrote:
> 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 

As Bob suggested, it's an older word that likely means the same thing. FREQ-OBS
is defined in paper III (I think) to mean that the frequencies are as seen
at the telescope, uncorrected for (for example) reference frame. You can have
FREQ-other stuff as well in TTYPEx. SDFITS is quite a bit older and didn't have
all that flexibility, so it had single keywords like OBSFREQ.

Since FITS readers often want one thing or another, we often just put them
both in if it's needed.

FREQREST should be the line frequency, but some writers put it in as the
center of the band to make some piece of software happy: Some readers need it,
but if you're doing a raw search, it doesn't make a lot of sense.

Cheers,

-Mike

-- 
Dr. Michael C. Nolan, Observatory Director
+1 787 878 2612x212 Fax: +1 787 878 1861 nolan at naic.edu
Arecibo Observatory, HC 3 Box 53995, Arecibo, Puerto Rico 00612 USA