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