|
|
Thread Tools | Display Modes |
#1
|
|||
|
|||
[fitsbits] Plate Center on POSS2 Plates
Hi,
I've just retrieved a fits file from http://archive.stsci.edu/cgi-bin/dss_form and would like to know the coordinates which the center pixel of this image corresponds to. I notice that there are comments "Plate center RA" and "plate center DEC" next to keywords PLTRAH,PLTRAM,PLTRAS etc. but for the most part, these seem not to correspond to the center of the actual fits image since they are ~ few minutes different from the epoch 2000 coordinates and the star of interest lies fairly close to the center (~ tens of pixels from center). Can someone explain this to me? Thanks, KL |
#2
|
|||
|
|||
[fitsbits] Plate Center on POSS2 Plates
On Wed 2004-07-28T12:49:49 -0700, K. L. Tah hath writ:
I've just retrieved a fits file from http://archive.stsci.edu/cgi-bin/dss_form and would like to know the coordinates which the center pixel of this image corresponds to. I notice that there are comments "Plate center RA" and "plate center DEC" next to keywords PLTRAH,PLTRAM,PLTRAS etc. but for the most part, these seem not to correspond to the center of the actual fits image since they are ~ few minutes different from the epoch 2000 coordinates and the star of interest lies fairly close to the center (~ tens of pixels from center). Can someone explain this to me? Aside from documentation in the published papers one thing to try would be to get the image viewer named "ds9" from http://hea-www.harvard.edu/saord/ds9/ This should provide world coordinates for any pixel in an image. The underlying software which accomplishes the coordinate interpretation is also available http://tdc-www.harvard.edu/software/wcstools/ -- Steve Allen UCO/Lick Observatory Santa Cruz, CA 95064 Voice: +1 831 459 3046 http://www.ucolick.org/~sla PGP: 1024/E46978C5 F6 78 D1 10 62 94 8F 2E 49 89 0E FE 26 B4 14 93 |
#3
|
|||
|
|||
[fitsbits] Plate Center on POSS2 Plates
On Wed 2004-07-28T12:49:49 -0700, K. L. Tah hath writ:
I've just retrieved a fits file from http://archive.stsci.edu/cgi-bin/dss_form and would like to know the coordinates which the center pixel of this image corresponds to. I notice that there are comments "Plate center RA" and "plate center DEC" next to keywords PLTRAH,PLTRAM,PLTRAS etc. but for the most part, these seem not to correspond to the center of the actual fits image since they are ~ few minutes different from the epoch 2000 coordinates and the star of interest lies fairly close to the center (~ tens of pixels from center). Can someone explain this to me? Aside from documentation in the published papers one thing to try would be to get the image viewer named "ds9" from http://hea-www.harvard.edu/saord/ds9/ This should provide world coordinates for any pixel in an image. The underlying software which accomplishes the coordinate interpretation is also available http://tdc-www.harvard.edu/software/wcstools/ -- Steve Allen UCO/Lick Observatory Santa Cruz, CA 95064 Voice: +1 831 459 3046 http://www.ucolick.org/~sla PGP: 1024/E46978C5 F6 78 D1 10 62 94 8F 2E 49 89 0E FE 26 B4 14 93 |
#4
|
|||
|
|||
[fitsbits] Plate Center on POSS2 Plates
Those keywords presumably give the coordinate of the center of the
wide-field photographic plate, which could be degrees away from the center of the small image that you retrieved. The approximate coordinates of the center of your image appear to be given by the OBJCTRA and OBJCTDEC keywords. The exact coordinates of any pixel in the image can be calculated from a complicated polynomial expression whose coefficients are given by the PPnn, AMDXn, and AMDYn header keywords, but a much simplier way to get the coordinates is to display the image using a program such as 'ds9' that does the calculation for you. Bill Pence K. L. Tah wrote: Hi, I've just retrieved a fits file from http://archive.stsci.edu/cgi-bin/dss_form and would like to know the coordinates which the center pixel of this image corresponds to. I notice that there are comments "Plate center RA" and "plate center DEC" next to keywords PLTRAH,PLTRAM,PLTRAS etc. but for the most part, these seem not to correspond to the center of the actual fits image since they are ~ few minutes different from the epoch 2000 coordinates and the star of interest lies fairly close to the center (~ tens of pixels from center). Can someone explain this to me? Thanks, KL -- __________________________________________________ __________________ Dr. William Pence NASA/GSFC Code 662 HEASARC +1-301-286-4599 (voice) Greenbelt MD 20771 +1-301-286-1684 (fax) |
#5
|
|||
|
|||
[fitsbits] Plate Center on POSS2 Plates
Those keywords presumably give the coordinate of the center of the
wide-field photographic plate, which could be degrees away from the center of the small image that you retrieved. The approximate coordinates of the center of your image appear to be given by the OBJCTRA and OBJCTDEC keywords. The exact coordinates of any pixel in the image can be calculated from a complicated polynomial expression whose coefficients are given by the PPnn, AMDXn, and AMDYn header keywords, but a much simplier way to get the coordinates is to display the image using a program such as 'ds9' that does the calculation for you. Bill Pence K. L. Tah wrote: Hi, I've just retrieved a fits file from http://archive.stsci.edu/cgi-bin/dss_form and would like to know the coordinates which the center pixel of this image corresponds to. I notice that there are comments "Plate center RA" and "plate center DEC" next to keywords PLTRAH,PLTRAM,PLTRAS etc. but for the most part, these seem not to correspond to the center of the actual fits image since they are ~ few minutes different from the epoch 2000 coordinates and the star of interest lies fairly close to the center (~ tens of pixels from center). Can someone explain this to me? Thanks, KL -- __________________________________________________ __________________ Dr. William Pence NASA/GSFC Code 662 HEASARC +1-301-286-4599 (voice) Greenbelt MD 20771 +1-301-286-1684 (fax) |
#6
|
|||
|
|||
[fitsbits] Plate Center on POSS2 Plates
On Wed 2004/07/28 12:49:49 MST, "K. L. Tah" wrote in a message to: part, these seem not to correspond to the center of the actual fits image since they are ~ few minutes different from the epoch 2000 coordinates and the star of interest lies fairly close to the center (~ tens of pixels from center). Can someone explain this to me? Sect. 5.2 of FITS WCS Paper IV (draft) discusses this anomaly in some detail. http://www.atnf.csiro.au/~mcalabre/WCS, text after Eq. (30) and again after Eq. (42). Mark Calabretta ATNF |
#7
|
|||
|
|||
[fitsbits] Plate Center on POSS2 Plates
On Wed 2004/07/28 12:49:49 MST, "K. L. Tah" wrote in a message to: part, these seem not to correspond to the center of the actual fits image since they are ~ few minutes different from the epoch 2000 coordinates and the star of interest lies fairly close to the center (~ tens of pixels from center). Can someone explain this to me? Sect. 5.2 of FITS WCS Paper IV (draft) discusses this anomaly in some detail. http://www.atnf.csiro.au/~mcalabre/WCS, text after Eq. (30) and again after Eq. (42). Mark Calabretta ATNF |
#8
|
|||
|
|||
[fitsbits] Plate Center on POSS2 Plates
I've just retrieved a fits file from
http://archive.stsci.edu/cgi-bin/dss_form and would like to know the coordinates which the center pixel of this image corresponds to. I notice that there are comments "Plate center RA" and "plate center DEC" next to keywords PLTRAH,PLTRAM,PLTRAS etc. but for the most part, these seem not to correspond to the center of the actual fits image since they are ~ few minutes different from the epoch 2000 coordinates and the star of interest lies fairly close to the center (~ tens of pixels from center). Can someone explain this to me? The PLT plate center in the FITS header is the center of the photgraphic plate which was scanned to create the image from which your image was extracted. There are other parameters which record the offset of your image from that center. My WCSTools package, at http://tdc-www.harvard.edu/software/wcstools , can decode the DSS headers (as well as FITS standard WCS) and return the sky position of any pixel. There is a program in the package, imsize, which returns the coordinates of the center of the FITS image based on the world coordinate system defined in its header. The xy2sky and sky2xy convert between image pixels and sky coordinates. -Doug Mink Telescope Data Center Smithsonian Astrophysical Observatory Cambridge, Massachusetts USA |
#9
|
|||
|
|||
[fitsbits] Plate Center on POSS2 Plates
I've just retrieved a fits file from
http://archive.stsci.edu/cgi-bin/dss_form and would like to know the coordinates which the center pixel of this image corresponds to. I notice that there are comments "Plate center RA" and "plate center DEC" next to keywords PLTRAH,PLTRAM,PLTRAS etc. but for the most part, these seem not to correspond to the center of the actual fits image since they are ~ few minutes different from the epoch 2000 coordinates and the star of interest lies fairly close to the center (~ tens of pixels from center). Can someone explain this to me? The PLT plate center in the FITS header is the center of the photgraphic plate which was scanned to create the image from which your image was extracted. There are other parameters which record the offset of your image from that center. My WCSTools package, at http://tdc-www.harvard.edu/software/wcstools , can decode the DSS headers (as well as FITS standard WCS) and return the sky position of any pixel. There is a program in the package, imsize, which returns the coordinates of the center of the FITS image based on the world coordinate system defined in its header. The xy2sky and sky2xy convert between image pixels and sky coordinates. -Doug Mink Telescope Data Center Smithsonian Astrophysical Observatory Cambridge, Massachusetts USA |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Unofficial Space Shuttle Launch Guide | Steven S. Pietrobon | Space Shuttle | 0 | August 5th 04 01:36 AM |
[fitsbits] Plate Center on POSS2 Plates | K. L. Tah | FITS | 0 | July 28th 04 04:03 AM |
Unofficial Space Shuttle Launch Guide | Steven S. Pietrobon | Space Shuttle | 0 | April 2nd 04 12:01 AM |
Unofficial Space Shuttle Launch Guide | Steven S. Pietrobon | Space Shuttle | 0 | February 2nd 04 03:33 AM |
Unofficial Space Shuttle Launch Guide | Steven S. Pietrobon | Space Shuttle | 0 | September 12th 03 01:37 AM |