[fitsbits] Comments on image distortion conventions

[Dick Shaw]

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