One feature that we’ve added to the underlying SkyView code recently is the ability to create approximate projections. Several surveys in SkyView, the DSSn and NEAT include distortions in the projection plane that mean that the data cannot be accurately represented as a simple projection. However the standard FITS WCS of an image allows for a general affine transformation so even though we cannot do make an exact WCS we can use the affine transformation to build an approximate WCS for the image that is correct to first order at the center of the image. It’s relatively straightforward to compute the Jacobian of the distortion at the center of the user requested image and we simply need to add this to the existing WCS. For a DSSx the resulting WCS can be quite accurate over an image with a size of a thousand or more pixels. For NEAT data the fit is almost always good at least to several hundred pixels. Here I’m using very good to mean that the error is less than an arcsecond.
One use for this is to allow SkyView to return the raw pixels from a survey. Of course this limits the ability to mosaic, but users sometimes want to get the data exactly as taken. One concern about providing raw cutouts was that we couldn’t describe their geometry appropriately for some of the most commonly used surveys. We’ll we looking at providing a ‘Raw’ projection in SkyView over the next few months.