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Old May 6th 09, 08:10 PM posted to sci.astro.fits
Bob Garwood
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Posts: 8
Default [fitsbits] frequency definitions

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.


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


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