Subject: Re: Dual point-of-symmetry distortion model

From: Steve Allen <[email-masked]>
Date: Tue, 14 Dec 1999 00:41:43 -0800

On Tue 1999-12-14T16:54:01 +1100, Mark Calabretta hath writ:

> At ADASS you showed us a plot of the distortions expected from your optics
> and I offered to see how well the polynomial currently proposed for the TAN
> projection would cope with it.  Could you send me the code you used to
> generate that plot please, or at least some expression of the distortion
> that I can play with.

The PS file showing the picture is at

http://www.ucolick.org/~sla/deimos/swpdr/distort.ps

with the source code being
http://www.ucolick.org/~sla/deimos/swpdr/distort.fig
this also needing
http://www.ucolick.org/~sla/deimos/swpdr/disinc.eps

For cross checks and reference, see the specs at
http://www.ucolick.org/~loen/Deimos/deimos_specs.html

These are 8kx4k (where 1k = 1024) CCDs with 15 micrometer pixels.

The distortion vector field is plotted with the camera center
subtracted out, but recall that in practice the imaging region is
actually divided into 4 CCDs, and the imaging mode does not illuminate
the entirety of those CCDs.

So, for purposes of the WCS there will be a separate WCS for each of
these 4 CCDs, and along the vertical direction only the central most
3000 (approximately) pixels are relevant.  This means that the vector
field distortions which are important for WCS should be zeroed at a
point about 1500 pixels up each CCD.  For the purposes of a test
calculation the symmetry means that there are only 2 cases to examine,
and the outer CCDs appear to have the worst distortions.

These distortions are being re-examined with an aim for producing a more
accurate form, but the form actually used in the plot is visible in
mail stored at

http://www.ucolick.org/~sla/deimos/swpdr/distort.mail

If these expressions prove too cumbersome, I'll have a hack at
reproducing them in a more palatable form.

--
Steve Allen          UCO/Lick Observatory       Santa Cruz, CA 95064
[email-masked]      Voice: +1 831 459 3046     http://www.ucolick.org/~sla
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