pointing accuracy of modern observatory telescopes

What is the best pointing accuracy achieved in modern observatory
telescopes?

The pointing accuracy of HST is, according to one source, one
milliarcsec.*
Is this correct?
And is this a major factor in the increased resolution achieved with
HST?

(Other issues include......Was HST built of non magnetic materials to
avoid effects from the earth's magnetosphere? Does one have to
compensate for the gravitational effects of the earth, moon, & sun? The
solar wind?.....but these aren't what I need to know)

Thanks, Peter Abrahams

*G. Fritz Benedict. Hubble's True First Light. STSCI Newsletter 21:1
(Winter 2004) p15.

=============================
Peter Abrahams telscope.at.europa.dot.com
The history of the telescope and the binocular:
http://home.europa.com/~telscope/binotele.htm

Peter Abrahams telscope.at.europa.dot.com wrote:
What is the best pointing accuracy achieved in modern observatory
telescopes?

All-sky, most manage several arcseconds these days (the Anglo-Australina
telescope folks pride themelves on having reached almost 1" some years
ago). Within a small region such as offsetting from a reference star
a few arcminutes away, some manage 0.1" relative positioning. Once there,
tracking can be good to rather better than 0.1" (for IR work with adaptive
optics, perhaps much better) using nearby guide stars.

The pointing accuracy of HST is, according to one source, one
milliarcsec.*
Is this correct?
And is this a major factor in the increased resolution achieved with
HST?

The _pointing_ accuracy of HST is about 0.5 arcsecond, based on how exactly
in comes back to the same position many months apart. Its _tracking_ accuracy
is a few milliarcseconds - and indeed if it were worse than about 10,
that would start to degrade its highest-resolution imaging modes.
(Pixel size for the Advanced Camera in visible light is about 50
milliarcseconds, gets down to 25 in the UV). Tracking tells how precisely
it stays pointed at the same inertial coordinates during a single
observation.

(Other issues include......Was HST built of non magnetic materials to
avoid effects from the earth's magnetosphere? Does one have to
compensate for the gravitational effects of the earth, moon, & sun? The
solar wind?.....but these aren't what I need to know)

Thanks, Peter Abrahams

It relies on the Earth's magnetosphere to dump excess spin built up
during maneuvers ("unloading the reaction wheels"), but that is
done slowly. Another astronomical spacecraft, the Far-Ultraviolet
Spectroscopic Explorer, was reconfigured after gyro failures to
use the Earth's magnetic field and electrical currents for fine tracking
at the few arcsecond level. Incredibly, it tracks more
smoothly with the remaining gyros turned off, owing to getting rid of
the residual vibrations.

Bill Keel