From: Robert A. Knop Jr. (robert.a.knop@vanderbilt.edu)
Date: Mon Apr 15 2002 - 10:38:08 PDT
> We also continued to be plagued by astrometry and acquisition
> problems. We can't tell for sure if we have reliable offsets and
> offset stars (the Subaru team using their astrometry did not get all the
> same offsets as on our latest finding charts). We also don't find
> complete closed-loop reproducability for our multi-star fits of the
> guider field (or ESI direct image) to the finding chart FITS image.
This has all been explained by E-mails from Alex Conley-- go back and
take a look at them if you haven't yet. The basic pro blem is that the
very deep searches are nowadays saturating any star which is bright
enough to be used as an offset star by Keck and friends. As such, it is
*very* important to get a "short" (1-2 minute) exposure of each deep
field in addition to the deep search exposures. Those have enough
well-detected objects in common that an astrometric solution can be
determined between the shallow and deep images; the positions of the
offset stars can be measured from the shallow images.
fchartng already has a way of handling this, allowing you to choose your
offset stars from an image different fromthe image where the candidateis
(and/or which is being used for the finding chart image). But it
requires the right input image to work. (I.e., we've come up against
this before.)
Given that the offset stars are saturated, I'd be worried about *both*
the Japanese and Berkeley offsets. I'm not surprised they come up with
different offsets, but it becomes much harder for anybody to come up
with reliable offsets to saturated stars.
-Rob
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