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Cartesian coordinates question.
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 0RA360 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 |
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