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Old May 4th 09, 06:38 PM posted to sci.astro.fits
Tom Kuiper[_2_]
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Posts: 7
Default [fitsbits] frequency definitions

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.

CRVALx - the value corresponding to the pixel CRPIXx in frame
defined by CTYPEx.
OBSFREQ - the actual frequency to which the receiver is tuned
corresponding to the center of the IF.
RESTFREQ - the receiver frequency (corresponding to the center of the
IF) in the reference frame defined in TTYPEx.

( Vdop )
OBSFREQ = RESTFREQ * ( 1 - ------)
( c )

where Vdop is the velocity of the reference frame with respect to the
observer.

If we assume that CRPIXx is the pixel corresponding to the center of the
IF, then
CRVALx = RESTFREQ if CTYPEx = FREQ-LSR,
CRVALx = OBSFREQ if CTYPEx = FREQ-OBS

What is the difference between FREQ-OBS and FREQREST?

Thanks and regards

Tom