[fitsbits] frequency definitions

[Tom Kuiper]

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