[fitsbits] frequency definitions

[Bob Garwood]

I think I'd avoid that usage and invent another keyword for that 
purpose.  It's a useful thing to make note of - i.e. that's the 
frequency we're using to do the updates necessary for dopper tracking, 
but as you say, it would generate a not particularly useful velocity 
scale and readers who are unfamiliar with the CSO and that use will be 
likely to make that mistake, which I think is a bad thing.

And I'm sorry for mis-interpreting you use of "FREQ-OBS".  In 
retrospect, that's a CTYPEn value and Arnold's reply is useful.  In the 
SDFITS convention, OBSFREQ is explicitly defined to be in the observers 
frame of reference independent of the CTYPEn value.

-Bob

Tom Kuiper wrote:
> Mike Nolan wrote:
>   
>> 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.
>>   
>>     
> We had a situation at CSO  recently in which we wanted two specific 
> lines (one in each sideband) well positioned in the spectrometer.  We 
> calculated a frequency that would do that.  That was the frequency which 
> was Doppler-shifted to tune the receiver.  I would call that FREQREST, 
> i.e., the frequency we wanted at the center of the band in the source 
> rest frame.  However, I agree that this makes a velocity scale hard to 
> interpret.
>
> Regards
>
> Tom
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