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[fitsbits] Comments on image distortion conventions
Regarding the proposed Simple Imaging Polynomial (SIP) convention for
representing non-linear image distortions: 1. The document should specify default values for any coefficients that are absent from the header, but might be expected based upon the value of A_ORDER or B_ORDER. Presumably either the values would be taken to be zero or the convention should require that all keywords be present. Similarly, are A_ORDER and B_ORDER restricted to be non-negative? 2. It is unfortunate that the transformation goes directly from raw array pixels to world coordinates, without defining an intermediate pixel coordinate system. This approach carries some liabilities, one of which follows: 3. The convention would appear to mis-use the CTYPEi reserved keyword in a way that at least technically violates the FITS standard. Quoting from the definition of this term in Sect. 8.2 of the FITS V3.0 Standard: "All non-linear coordinate system names _must_ be expressed in "4-3" form: the first four characters specify the coordinate type, the fifth character is a hyphen, and the remaning three characters specify an algorithm code for computing the world coordinate value...." -Dick Shaw |
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