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Peter, On Thu, 17 Aug 2006, David Berry wrote: > On Thu, 17 Aug 2006, Peter W. Draper wrote: > > > Hi Mark, > > > > as you suspected this is down to the size (on the sky) of the image. I > > just run astDistance to get some "radius" of the image in radians, but > > clearly at this scale there's a shorter way around the great circle > > between the two points I've chosen (bottom left and centre of image). > > > > David, is there a better way to size an image in world coordinates? > > Probably best to use an additional point mid way between the corner and > the centre of the image, then use astDistance twice (i.e. get the distance > from corner to mid-point, and then get the distance from mid-point to > centre) and just add the two distances together. Another point - in an all-sky image the bottom left pixel may well not have any associated world coords. And there is always a possibility that some pathological projection may result in the centre not having valid world coords. David