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#1
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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 |
#2
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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 |
#4
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[fitsbits] frequency definitions
Well, not specifically about dual sideband data but about radio spectral
line data in general. The best description of the relevant keywords that I've found is in the NRAO AIPS++ documentation at http://aips2.nrao.edu/docs/notes/236/node14.html. However, the definitions are not entirely unambiguous. That's why I posted my interpretation, to see if anyone disagreed. Regards Tom David Berry wrote: 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 _______________________________________________ fitsbits mailing list http://listmgr.cv.nrao.edu/mailman/listinfo/fitsbits |
#5
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[fitsbits] frequency definitions
Since I'm the author of that part of that note, I should probably chime
in here. As mentioned elsewhere, the primary WCS papers are referenced in that previous reply and cited below. The SDFITS note in aips++ predates all of that. SDFITS has a long history, which I won't go into here. It remains primarily a locally implemented standard with a few version of "local" that aren't quite in agreement for lack of effort primarily on my part to try and reach a real consensus and, most importantly, exchange data between "local" implementations. Prior to the WCS papers, lots of people obviously described a frequency-like axis. So you're going to find that historically there are keywords and columns for which you'll just have to talk to the folks that wrote the FITS file to be sure you go the interpretation right - if there is any ambiguity in what you find there as compared to the WCS papers. For your specific questions ... CRVALx - I'd say "yes" OBSFREQ - 'probably'. I think "center of the IF" is possibly ambiguous enough that you might want to double check with the author/writer RESTFREQ - no. I'd interpret that as equivalent to the "RESTFRQ" value described in the WCS papers. It's the rest frequency of the spectral feature of interest - i.e. the frequency that that feature has when observed at rest with respect to the thing producing that feature. It's the lab-measured line frequency. It may or may not correspond to the center of the IF, depending on how the IF was set up and if doppler tracking was being done during the observations. It should only be necessary/useful if you want to convert from a frequency to a velocity (I'm 99% sure of that statement, but I'm sure someone here will correct me if that isn't true) - or rather, this quantity isn't necessary to recover the frequency at each pixel nor is it necessary to convert from a frequency in one reference frame to a frequency in a different reference frame assuming that TTYPEx is a frequency axis. Then you ask" What is the difference between FREQ-OBS and FREQREST?". But I'm not sure what you mean by "FREQ-OBS". You should an equation earlier (badly formatted in my copy, but I think I understand what you meant) showing the difference between OBSFREQ and RESTFREQ, so I don't think you mean FREQ-OBS = OBSFREQ. SDFITS doesn't use FREQ-OBS so I think you'll need to ask the creator of that FITS file what they meant. Unless I missed something in a quick scan of the relevent WCS paper, I don't see it defined there. Cheers, Bob Tom Kuiper wrote: Well, not specifically about dual sideband data but about radio spectral line data in general. The best description of the relevant keywords that I've found is in the NRAO AIPS++ documentation at http://aips2.nrao.edu/docs/notes/236/node14.html. However, the definitions are not entirely unambiguous. That's why I posted my interpretation, to see if anyone disagreed. Regards Tom David Berry wrote: 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 _______________________________________________ fitsbits mailing list http://listmgr.cv.nrao.edu/mailman/listinfo/fitsbits ------------------------------------------------------------------------ _______________________________________________ fitsbits mailing list http://listmgr.cv.nrao.edu/mailman/listinfo/fitsbits |
#6
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[fitsbits] frequency definitions
Bob Garwood wrote:
Then you ask" What is the difference between FREQ-OBS and FREQREST?". But I'm not sure what you mean by "FREQ-OBS". You should an equation earlier (badly formatted in my copy, but I think I understand what you meant) showing the difference between OBSFREQ and RESTFREQ, so I don't think you mean FREQ-OBS = OBSFREQ. SDFITS doesn't use FREQ-OBS so I think you'll need to ask the creator of that FITS file what they meant. Unless I missed something in a quick scan of the relevent WCS paper, I don't see it defined there. I don't think that is quite what I meant to ask. FREQ-OBS is a possible value for TTYPEx. It refers to some kind of reference frame. I assume it means this is a frequency axis defined in the frame of the observer. OBSFREQ is a keyword and I confess that it's not clear to me why it is needed if the same information, in some form, is required to define one of the axes. Best regards Tom |
#7
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[fitsbits] frequency definitions
On Wed, May 06, 2009 at 10:50:44AM -0700, Tom Kuiper wrote:
Bob Garwood wrote: Then you ask" What is the difference between FREQ-OBS and FREQREST?". But I'm not sure what you mean by "FREQ-OBS". You should an equation earlier (badly formatted in my copy, but I think I understand what you meant) showing the difference between OBSFREQ and RESTFREQ, so I don't think you mean FREQ-OBS = OBSFREQ. SDFITS doesn't use FREQ-OBS so I think you'll need to ask the creator of that FITS file what they meant. Unless I missed something in a quick scan of the relevent WCS paper, I don't see it defined there. I don't think that is quite what I meant to ask. FREQ-OBS is a possible value for TTYPEx. It refers to some kind of reference frame. I assume it means this is a frequency axis defined in the frame of the observer. OBSFREQ is a keyword and I confess that it's not clear to me why it is As Bob suggested, it's an older word that likely means the same thing. FREQ-OBS is defined in paper III (I think) to mean that the frequencies are as seen at the telescope, uncorrected for (for example) reference frame. You can have FREQ-other stuff as well in TTYPEx. SDFITS is quite a bit older and didn't have all that flexibility, so it had single keywords like OBSFREQ. Since FITS readers often want one thing or another, we often just put them both in if it's needed. 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. Cheers, -Mike -- Dr. Michael C. Nolan, Observatory Director +1 787 878 2612x212 Fax: +1 787 878 1861 Arecibo Observatory, HC 3 Box 53995, Arecibo, Puerto Rico 00612 USA |
#8
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[fitsbits] frequency definitions
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 |
#9
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[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. -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 _______________________________________________ fitsbits mailing list http://listmgr.cv.nrao.edu/mailman/listinfo/fitsbits |
#10
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[fitsbits] frequency definitions
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. but ... your 2 sidebands will have different frequencies. And my interpretation is that the _rest_ frequency is in lab context, or in the rest frame of the _source_. It doesn't matter if you put in an extra keyword or 3 to satisfy the requirements of one or the other data reduction package, redundant as they may be, as long as they are consistent. To chime in on the question asked about the radial velocities: that's why you should specify the frame. A radial velocity without specified frame doesn't have much meaning unless it's rather large. Aloha, Maren Purves (these days at UKIRT, but done my share of radio and submm) |
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