Subject: WRT Astrometric Accuracy

From: dgm@nofs.navy.mil

Submitted: Wed, 9 Oct 2002 09:16:02 -0700

Message number: 9 (previous: 8, next: 10 up: Index)

Michael is correct that my note had an optimistic spin on the anticipated
astrometric accuracy.  Nick Kaiser has (often) reminded me that theory
predicts astrometric accuracy that is independent of aperture size so
long as D >> L_o, so this tosses out most of the historical data from
transit circles (~50mas on the best of nights but with a human observer,
and USNO's UCAC 8-inch astrograph of ~50mas).  I have been a spectator
to the SDSS astrometric reductions, but for mostly political reasons I
have not had a chance to really understand what is going in, particularly
in the removal of terms that might arise from scanning as differing from
staring (the LSST concept).  It is the case that the 2MASS folks are
getting 80-mas with respect to J2000 with their 6x1.2-sec integrations
1.3-m telescopes and large pixels.  I am trying to get colleagues to take
astrometric frames with reasonable combinations of aperture and field of
view, but getting 1 square degree is tough on 3+ meter apertures.  I am
also trying to twist John Tonry's arm to get astrometric data from OTCCDs.

The terra incognito is not whether we get 30, 50, or 80 mas with respect
to J2000, but (a) whether we can get astrometric solutions limited by
photon statistics in small areas (how good? how small?), and (b) whether
the nightly astrometric solutions will improve as SQRT(N_visit) or
whether there are other problems.  The former will enable the short-arc
astrometric solutions that would allow a census of all stars within 10pc
during the first year of operation, and the latter will determine the
overall quality of the end-of-life astrometric solution.

Now that USNO-B is mostly done, I am volunteering to lead/assist/slave
on these sorts of problems.  I think that I/we can make serious progress
in the next year.  I accept MIPR, Visa, MC, and PayPal.

-Dave

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