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[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 |
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[fitsbits] frequency definitions
On Mon, 4 May 2009, Tom Kuiper wrote:
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. Not a spectral WCS expert, but the standard and official references are : - http://fits.gsfc.nasa.gov/fits_standard.html the latest release of the FITS standard, in particular chapter 8 - the full WCS paper III, either as published or http://www.atnf.csiro.au/people/mcalabre/WCS/scs.pdf (the other WCS papers are all listed in http://www.atnf.csiro.au/people/mcalabre/WCS/index.html -- ---------------------------------------------------------------------- is a newsreading account used by more persons to avoid unwanted spam. Any mail returning to this address will be rejected. Users can disclose their e-mail address in the article if they wish so. |
#3
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[fitsbits] frequency definitions
It looks like Tom's query was about dual sideband data, which is not
covered by paper III. OBSFREQ, etc, are not in the standard. There are probably various different schemes in use out there. For instance JCMT and CLASS software use a system based on a keyword called IMAGFREQ, which holds the image sideband rest frequency (with RESTFREQ holding the observed sideband rest frequency). David 2009/5/5 LC's No-Spam Newsreading account : On Mon, 4 May 2009, Tom Kuiper wrote: 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. Not a spectral WCS expert, but the standard and official references are : - http://fits.gsfc.nasa.gov/fits_standard.html * the latest release of the FITS standard, in particular chapter 8 - the full WCS paper III, either as published or * http://www.atnf.csiro.au/people/mcalabre/WCS/scs.pdf * (the other WCS papers are all listed in * http://www.atnf.csiro.au/people/mcalabre/WCS/index.html -- ---------------------------------------------------------------------- is a newsreading account used by more persons to avoid unwanted spam. Any mail returning to this address will be rejected. Users can disclose their e-mail address in the article if they wish so. _______________________________________________ fitsbits mailing list http://listmgr.cv.nrao.edu/mailman/listinfo/fitsbits |
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