FITS Distortion Proposal

The current FITS Distortion Paper conventions [DistortionPaper] provide a mechanism for specifying either a lookup table or polynomial model for the distortion of each axis. The standard states in Section 2.1:

Note that the prior distortion functions, $\delta_p(p)$ , operate on pixel coordinates (i.e. $p$ rather than $p-r$ ), and that the independent variables of the distortion functions are the uncorrected pixel or intermediate pixel coordinates. That is, for example, we do not allow the possibility of

(1) $q'_{3} = q_{3} + \delta_{q_{3}}(q'_{1},q'_{2})$

The keywords used for describing these corrections use the syntax given in Table 2 of the FITS Distortion Paper. For our purposes, the keywords of interest are those related to lookup tables; namely,

CPDISja        string    2.4.1 distortion code new Prior distortion function type.
DPja           record    2.4.2 distortion parameter new Parameter for a prior distortion
                                 function, for use in an image header

This syntax only provides the option to specify one correction at a time for each axis of the image. This precludes being able to use this convention to specify both a lookup table and a polynomial model at the same time for the same axis. It does not state what should be done if the polynomial has been specified using a different convention, for example, the SIP convention. Thus, SIP and FITS Distortion Paper should not be seen as mutually exclusive. In fact, they may work together rather naturally since the SIP and FITS Distortion Paper conventions both assume the corrections will work on the input pixel and add to the output frame.

The sample header in Appendix 1: Headerlet API shows how these keywords get populated for an actual reference file; specifically, an NPOLFILE as described in the next section.