[fitswcs] Detector distortion correction representations in FITS

[Mark Calabretta]

On Mon 2000/01/31 15:38:29 +1100, Mark Calabretta wrote
in a message to: Richard Hook <rhook at eso.org>
and copied to: fitswcs at NRAO.EDU, sparks at stsci.edu, clampin at stsci.edu,
     koekemoer at stsci.edu, rfosbury at eso.org, jwalsh at eso.org, apasqual at eso.org

>However, doesn't this argue against you?  Perhaps I am misunderstanding but
>shouldn't you do your solution the other way around?  That is, shouldn't

OK, having thought about it some more I think I understand your problem
a bit better.  You're reading out lines from a CCD which is spatially
distorted in some way and you want to de-distort the pixel coordinates
before anything else.  (It would have helped a lot if you'd briefly
described your instrument - my background is in radioastronomy.)

However, my answer is basically unchanged for the reasons previously given.
Building a transformation matrix into TAN+poly itself seems the way to go,
possibly even adding one ahead of the polynomial as well as after it so
that the CD matrix could be used for scaling, etc. in the way you expect
and provide a good first-order approximation.  It would look like

   (i,j) -> (x,y)		...CD matrix

   (x,y) -> (i,j)		...pre-poly matrix   }
   (i,j) -> (I,J)		...polynomial        }  Augmented TAN+poly
   (I,J) -> (xi,eta)		...post-poly matrix  }

The pre-poly matrix would effectively undo the CD matrix; the post-poly
matrix would apply it again.  The combined effect is that of applying the
polynomial before the CD matrix.  Admittedly it's a little clumsy but it's
conceptually straightforward and should work and, most importantly, the
added complexity is confined to one projection. 

The pre-, and post-poly matrices would be built into the TAN projection
(or perhaps a new projection) using two sets of four PVj_ms parameters.

I thought about defining a new projection similar to the current TAN in
which the pre-, and post-poly matrices were implicitly constructed from CD
but decided it would probably be a bad idea from the FITS interpreter's
point-of-view - it would violate the encapsulation of different steps of
the algorithm chain by effectively subsuming CD into PV.  The same goes
for any global keyword which would change the order of operations.

Does that sound reasonable to you?

Does anyone else on this list have anything to say?

Cheers, Mark