[fitsbits] Cartesian coordinates question.

[Tom McGlynn]

Recently I had a user of SkyView who raised a concern
at the way SkyView creates images in the Cartesian
(CAR) projection.  This was more of a general usage
question than detailed issue with the new WCS papers
to I think this may be the appropriate forum to
discuss it.

When an image is requested in a Cartesian
projection, SkyView always sets the reference coordinate
to 0,0.  Thus for a patch of the
sky with RA (or longitude) between 180 and 360, the
X-coordinates of the center of the image, i.e.,
(NAXIS1/2-CRPIX1)*CDELT1 is negative.

Our user was concerned that SkyView was making it
unnecessarily difficult to use the image, since they couldn't
just use the X-value as the RA without checking
the range.  In fact, since the CAR projection is
cylindrically symmetric it is possible to use
a value for CRVAL1 such that for most smallish
images the RA within the image would always be
computed in the range 0<RA<360 even without
any range checking.

To my mind this is a mixing of the roles
of the CRVALs and CRPIXs with only limited benefits,
but if it's the kind of thing that other's have
done SkyView should probably adopt community practice.

Does anyone have any experience with how others
have addressed this issue?

	Regards,
	Tom McGlynn