[fitswcs] Polynomial Correction Function & Alternative-WCS

[Don Wells]

Don Wells writes:
 >      Polynomial Correction Function, Alternative-WCS Descriptions
 > 		    DRAFT-for-comments 1998-05-06

 > ..  The polynomial coefficients
 > will be specified by keywords of the form:
 > 
 >     CPn  = <E-format-value> / coeff of polynomial term n
       ^^
       WC

I think 'WC' might be a better choice for the prefix.

 > .. It appears that this proposed function could replace the functionality
 > of CRVALi, CDi_j, CDELTi and PCiiijjj. The CDi_j (partial derivatives
 > at the reference point) notation is equivalent to the vector-matrix
 > product of the CDELTi and PCiiijjj.  For the NAXIS=2 case, the CRVALi
 > and CDi_j can be expressed as:
 > 
 >     CRVAL1 --> CP1000
       ^^^^^^^^^^^^^^^^^
 >     CD1_1  --> CP1001
 >     CD1_2  --> CP1002
 >     CRVAL2 --> CP2000
       ^^^^^^^^^^^^^^^^^
 >     CD2_1  --> CP2001
 >     CD2_2  --> CP2002

I suspect that in the celestial coordinate case CRVALi must be carried
to the celestial coordinate step of the calculation; probably I should
withdraw my CRVAL1 --> CP1000 suggestion. I still think that the CDi_j
keywords can be replaced by WCn (CPn).

-Don
-- 
  Donald C. Wells         Associate Scientist         dwells at nrao.edu
		    http://www.cv.nrao.edu/~dwells
  National Radio Astronomy Observatory                +1-804-296-0277
  520 Edgemont Road,   Charlottesville, Virginia       22903-2475 USA