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[Fwd: Hinode XRT Plate Scale]

-------- Original Message --------
Subject: Hinode XRT Plate Scale
Date: Mon, 23 Apr 2007 20:14:41 -0400
From: Paul Reid <preid@cfa.harvard.edu>
To: Leon Golub <lgolub@cfa.harvard.edu>
CC: Peter Cheimets <pcheimets@cfa.harvard.edu>,        "William A. Podgorski" 
<wpodgorski@cfa.harvard.edu>,        Jay Bookbinder 
<jbookbinder@cfa.harvard.edu>,        "Reid, Paul" <preid@cfa.harvard.edu>, 
    "Mark A. Weber" <mweber@head.cfa.harvard.edu>,        edeluca 

Hi Leon,

Using a flat focal plane, I believe there may be a slight field
dependence of the plate scale, but it is at a level comparable to or
less than the uncertainty in the focal length.

Using my raytrace code, for the Goodrich as-measured XRT, including the
final clear aperture, I raytraced on-axis, 10, 300, 310, 600, and 610
arc-sec field positions. I computed the local [field-dependent]
plate scale for a flat detector, located at the best on-axis focus, by
differencing image centroids and dividing by the delta field angle.  I
find the plate scale varies from 13.131 um/arc-sec on-axis to 13.133
um/arc-sec at 10 arc-min off-axis.  It is entirely possible that this
variation is due to computational inaccuracies. (The Goodrich plate
scale differs from mine by ~ 0.01 um/arc-sec.  There is, however, also
sensitivity to the exact mirror clear apertures, and these may account
for the difference between the two raytraces).   Goodrich also found
field dependent variations in the focal length, which would correspond
to a weakly field dependent plate scale.

Note also, these field dependent variations are small relative to the
equivalent variation from +/- 1 mm on an ~ 2707 mm focal length (i.e., ~
+/- 370 ppm, or +/- 0.005 um/arc-sec).

I believe the field dependence is a natural outgrowth of some higher
order effect occurring as we violate paraxial conditions - there is a
variation in plate scale even for the nominal mirror, although it is of
comparably small amplitude.  The effects, however, are quite small.

Does more need to be done on this?


Dr. Paul B. Reid
Harvard-Smithsonian Center for Astrophysics
60 Garden St.   M/S 04
Cambridge, MA 02138

617-495-7233 (voice)
617-495-7356 (fax)


Dr. Leon Golub; Harvard-Smithsonian Center for Astrophysics
60 Garden Street, Cambridge MA 02138
Ph: 617 495 7177; Fax: 617 496 7577
e-mail: lgolub@cfa.harvard.edu