Subject: wrt Sidney's requirements

From: Dave Monet

Submitted: Fri, 11 Oct 2002 05:30:48 -0700

Message number: 14 (previous: 13, next: 15 up: Index)

Sorry for my delay in responding to Sidney's request for Requirements that
flow from astrometric accuracy.  My old, comfortable workstation was swapped
for a new, gee-whiz Linux box, and I am showing my age.  Resistance is
futile: I must learn to point-and-click, and to retro-engineer that which
is intuitive to the plastic-pocket-protector software folks.  All that pain
and agony suffered in the basement of 950 N. Cherry 25 years ago is lost.
I must have missed the funeral announcement for the Unix command line.

In my (humble) opinion, the utter failure of USNO folks (and a few others)
to derive engineering requirements from astrometric accuracy was the
tombstone of the FAME satellite.  We (self included) never did figure out how
many milliarcseconds of astrometric dispersion could be traced to a 0.01 change
in the Strehl ratio, for example.  Indeed, I still do not know how to
do this calculation.  The final death throes of FAME included attempts to
limit the asymmetric Zernike's of a fixed Strehl ratio, and the opticians
laughed at us for the obvious reason.  I wasted a year worrying about
diffraction limited polychromatic PSFs derived from Zemax analysis and their
sub-pixel phase contribution to the systematic astrometric error.  Those were
the Dark Years.

I look forward to working with the NOAO and other folks worrying the
engineering side of LSST.  The old adage that you can't get good
astrometry from crummy images is still true, but the impact of "good"
and "crummy" are TBD.  At FAME, the internal version was "don't do anything
that we can't model", and that is not overly far from what will happen
in the astrometric pipeline.  The only premise that comes to mind is that
we need to have as few free parameters to describe the metrics of a single
CCD at any given time, and the fewer terms in the temporal and pointing
behavior of the positions of the CCDs in the focal plane, the better.

As noted before, I am trying to sign up to worrying about (and probably
simulating) such problems in the near future.  The fall-back position
is that we will do as good a job as we can given whatever telescope
and detector system is delivered.  The challenge is to come up with finite
Requirements that could make it better.  Sign me up!

-Dave Monet is dgm@nofs.navy.mil

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