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[fitsbits] Plate Center on POSS2 Plates



 
 
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  #1  
Old July 28th 04, 08:49 PM
K. L. Tah
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Default [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  
Old July 28th 04, 09:32 PM
Steve Allen
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Posts: n/a
Default [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  
Old July 28th 04, 09:32 PM
Steve Allen
external usenet poster
 
Posts: n/a
Default [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  
Old July 28th 04, 09:40 PM
William Pence
external usenet poster
 
Posts: n/a
Default [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  
Old July 28th 04, 09:40 PM
William Pence
external usenet poster
 
Posts: n/a
Default [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  
Old July 29th 04, 12:59 AM
Mark Calabretta
external usenet poster
 
Posts: n/a
Default [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  
Old July 29th 04, 12:59 AM
Mark Calabretta
external usenet poster
 
Posts: n/a
Default [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  
Old July 29th 04, 06:10 PM
Doug Mink
external usenet poster
 
Posts: n/a
Default [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  
Old July 29th 04, 06:10 PM
Doug Mink
external usenet poster
 
Posts: n/a
Default [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

 




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