[fitswcs] Re: State of the discussion

[Don Wells]

Doug Mink writes:
 > Don Wells wrote:
 > >  ..If WFPC2 uses two axis points (which I suspect), then our
 > >    distortion keywords will need a convention to express the second
 > >    point relative to the tangent point defined by CRPIXi. The
 > >    [fitswcs] subscribers who are associated with re-imaging cameras
 > >    should declare their requirements..
 > ..  My feeling
 > is that we should try the polnomial distortion first on images from the
 > instruments which may present problems and proced to a more complicated
 > model only if the data requires it.

Yes. I hope that examples of re-imaging camera imagery will be made
available by various observatories for tests.

				 -=-

There is another question which I forgot to mention yesterday:

4) Should special Schmidt plate terms be added to the DSS terms? Pat
   Wallace tells me that he has constructed a routine which he calls
   UNBEND for the geometry of the UKST [UK Schmidt Telescope]: "..I
   used data from 10 plates, on each of which several hundred Tycho
   stars were measured.  The straight ARC fit (actually tangent plane
   plus a cubic distortion coefficient of -1/3) typically gave 0.33
   arcsec RMS; with the UNBEND corrections, the RMS dropped to 0.30
   arcsec RMS.  The improvement, about 0.14 arcsec RMS, although
   superficially unimpressive, is concentrated at the plate edges.
   This is where the pre-UNBEND error vectors are seen to lie in
   obvious "swirling" patterns; UNBEND eliminates these systematic
   effects while also reducing the residuals at the plate edges to the
   same level as those in the rest of the plate.."

-Don