[fitsbits] WCS for HEALpix and HTM data.

[Tom McGlynn]

One thing that has come up recently in a couple of contexts
is dealing with image data that is stored using a pixelization
that is not the standard rectangular grid.  The HEALPix format
is very popular for images in the CMB community while HTM is being used
for a lot of very large catalog data and one can easily imagine
HTM images.

Has anyone begun working on WCS specifications for these?  The
underlying pixelizations are inherently 1-dimensional, but I
can imagine something like:

NAXIS    = 2
NAXIS1   = NumberOfPixels used
NAXIS2   = 1

CRVAL1   = 0.    // Reference pixel at center of coordinates (like CAR or AIT)
CRVAL2   = 0.

CDELT1   = Only certain discrete values are valid here depending
            on how finely the sphere has been divided.
CDELT2   = Same as CDELT1 (or just ignored)

CRTYP1   = RA---HTM (or RA---HPX)
CRTYP2   = DEC--HTM "

CRPIX1   = The pixelizations give pixels over the entire sky as a linear stream.
            If only part of the sky is given, then CRPIX1 gives the offset of the
            first pixel that the user is including.
CRPIX2   = 1  // Not used.


This will not be trivial for display programs, becuase the region of the sky that
is included when something less than a whose sky image is given will never be
rectangular and may not even be contiguous.  I've not been a subscriber to the
WCS mailing list.  Has there been discussion there?

	Regards,
	Tom McGlynn