[fitsbits] changing WCS entries when transposing an image - how?
Doug Tody
tody at tucana.tuc.noao.edu
Tue Mar 12 20:48:43 EST 2002
Hi Michal -
All of the IMAGES tasks in IRAF support world coordinate systems and
will update the WCS if you do something like tranpose an image. The
problem is that the DSS images have a non-standard header (of course,
we don't have a standard for FITS WCS yet, so lets just say the headers
do not follow current conventions).
You can use the "makewcs" task in STSDAS (stsdas.analysis.gasp) to "fix"
the image headers so that IRAF tasks can properly interpret the headers.
The first part of the help page is copied below.
Cheers,
- Doug Tody
NAME
makewcs -- Produce WCS keywords for a dss scanned image plate
header.
USAGE
makewcs input
DESCRIPTION
The 'makewcs' task will create the World Coordinate System (WCS)
keywords in the image header of the Digital Scan Survey subimages.
These subimages contain the keywords CNPIX1 and CNPIX2 indicating
the lower left corner of the subimage with respect to the lower
left corner of the original image. Early versions of these subimage
headers had the keywords CRPIX1 and CRPIX2; the program will rename
these keywords to the new ones CNPIX1 and CNPIX2.
Headers for the original scanned images lack WCS keywords---namely,
the CD, CRPIX, and CRVAL keywords. The task then takes the plate
solution from the image header and calculates the CRVAL keyword
values for the center of the subimage. The CD values are calculated
by solving for a grid of 100 equally spaced points in the image.
[...]
---
On 12 Mar 2002, Michal Szymanski wrote:
> FITS files coming from DSS have the complete information regarding the
> transformation between pixel coords and RA/Dec, contained in their
> headers.
>
> What is the simplest way to transpose such an image *together* with its
> WCS header? I mean simple operations like cutting a subimage, shifting
> or rotating the image.
>
> I've tried IRAF's "imtranspose" but it seems to blindly copy all related
> header entries *as is*, so the resulting image has a completely broken
> transformation installed.
>
> any hints would be appreciated,
>
> regards, Michal.
>
> --
> Michal Szymanski (msz at astrouw.edu.pl)
> Warsaw University Observatory, Warszawa, POLAND
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