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[fitsbits] CRPIX clarification
Rob Seaman wrote:
This discussion reminds me of the innumerable "which way is up?" questions in the IRAF mail over the years. The answer goes something like "the question is meaningless until you display the image". unless ... we had a rather interesting thing happening with one of our instruments that uses sub-arrays: Until very recently (this is for an instrument that's been in operation for about 10 years) the CRPIX etc. coordinates followed the array (array controller) numbering, which starts in the top right corner rather than in the bottom left. This works for almost everything, the one exception being if you use a full array flat field (or dark) for a subarray observation, because you'll get the wrong quadrant. Following this I implemented the "up" that the data reduction expects which is the opposite of what the array controller thinks, i.e. I had to flip the coordinate axes (rotate 180), but only for the header, not for the readout - independent of displaying. Maren (UKIRT) |
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