From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: Constant Frequency at LOW temperature
Date: Sun, 03 Nov 2002 19:17:54 GMT
NNTP-Posting-Date: Sun, 03 Nov 2002 11:17:54 PST
James Meyer wrote:
> On Sun, 03 Nov 2002 10:32:50 GMT, "Arch-lab" wroth:
> >Feq. isn't pinned down yet since my client hasn't told me yet what the gear
> >ratios are that they are building. However, the application will use a
> >stepper. As far as accuracy, basically it will have to be able to keep up
> >with the sky (telescope tracking).
> In that case, you don't really need extremely precise frequencies.
> Since stars move at a different rate than planets, moons, and asteroids, you
> need a variable rate that has short term stability. Even stars will appear to
> move at different rates depending on whether they are overhead and seen through
> a minimum of atmosphere or closer to the horizon and seen through a much thicker
> section of air.
> A really good tracking system will be closed loop instead of open loop.
No. You start with an extremely good tracking system first. An essential
part of this is polar alignment (or for el-az knowing celestial north
accurately). Field rotation in polar mounts is a problem even when
alignment errors are small enough to track. El-az field derotation
becomes a nightmare in the face of axis errors. So you start with a
system as near perfect as it can be. Then you close the loop in the case
of exposures exceeding about 10 minutes. If you cannot track open loop
better than 100 milliarcseconds in 10 minutes, you have failed
miserably. Lunar and planetary rates have to be purely calculated since
there is no way to close the loop properly (nothing for an offset guider
to work on). Every successful telescope I have seen has had excellent
open loop tracking.
BTW, I used to work for Steward Observatory first on the joint
Steward/Smithsonian MMT (recently upgraded to single mirror), second on
their 2.3 meter telescope and then on a photon counting imager which we
used with open loop tracking on the 2.3 meter. More recently, I did
precision machining for a 26.5 inch telescope, consulted on servo system
issues for it, helped install the thing at its permanent site and did
major visual work in final alignment of the optics. This telescope
points to within 10 arcseconds to random objects, has lunar and
planetary rates and, given orbital parameters, can track comets. All of
this is open loop. The telescope is fitted with an offset guider but the
guider is not used except for photometry where filter attenuation
increases exposure times enormously. Unfortunately, the telescope is
sited in Colorado and rarely enjoys seeing below 2 arcseconds FWHM. It
is, however, excellent for photometry and there is a choice of two
cooled CCD cameras capable of supporting long exposures.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons email@example.com