[fitsbits] WCS coordinates for North/South Pole composite image?

[Steve Allen]

On Thu 2003-03-13T14:28:20 -0500, Wayne Landsman hath writ:
> I have an image displaying the entire sky in a Zenithal Equal Area (ZEA)
> projection, with the left side of the image centered on the North pole,
> and the right side centered on the South pole.     The situation is very
> similar to that described in Section 7.4.2 of Calabretta & Greisen
> (2002, A&A, 395, 1077) who derive the WCS info for the North pole image
> and the (different) WCS info for the South pole image.     However, I
> want to view the two poles side by side -- is there any way to write WCS
> coordinates that would work for the composite image?

You can split the original image into two separate IMAGE extensions in
a single FITS file, and give the correct ZEA-style CTYPE to each one
as the primary WCS.

Then you could attach alternate WCS information of the sort that is
used by CCD mosaic image display programs, and tell such an image
display to use the alternate WCS when painting the picture on the
screen.

The result will be that the image is tiled into two adjacent regions
on the screen, and the image display cursor coordinate may be selected
from the primary and/or secondary WCS.

I do not know whether ds9 or the NOAO mscdisplay programs currently
grok the ZEA CTYPE, but they can do the tiling.  Unfortunately the
tiling is currently accomplished by using rather obscure non-WCS
keywords.

See my page on mosaic displays for more info.
http://www.ucolick.org/~sla/fits/mosaic/

--
Steve Allen          UCO/Lick Observatory       Santa Cruz, CA 95064
sla at ucolick.org      Voice: +1 831 459 3046     http://www.ucolick.org/~sla
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