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[fitsbits] New DUMP FITS extension
On Thu 2007/08/16 11:54:16 -1000, Maren Purves wrote in a message to: Doug Tody and copied to: William Pence , FITSBITS Where do you want to draw the line? The interpolation test may help to draw the line. Simply put, an array is an "image" if it makes sense to interpolate adjacent array elements. While a collection of 1-D spectra packed into a 2-D array does pass this test on one axis it fails on the other so fails it overall. Many historical FITS "images" are also not images according to this test. For example the conventional axis types of COMPLEX, STOKES and CUBEFACE, and the ones proposed for colour - RGB, CMYK and HSB. Conventional axes were a useful storage mechanism in the days when there was no legitimate alternative (and once FITS...), especially since the WCS for the true image axes imprints itself on the conventional axis (except for CUBEFACE). One characteristic of the existing conventional axes is that they are all short, of lengths 2, 3, 4 or 6. They are also all defined once and for all in the literature, and straightforwardly so. However, a new conventional axis type for 1-D spectra packed into a 2-D array could be as long as the number of fibres in your IFU. Its associated WCS, e.g. the celestial coordinates of each fibre, would have to be recorded in header cards for each point on the axis. There is no standard mechanism for doing this. However, there is a much better alternative now. That is to store the spectra as bintable image arrays with a static (i.e. not using the Greenbank convention), column-specific WCS. Celestial coordinates can easily be stored in a separate column or two. Mark Calabretta |
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