On 4/2/12 12:19 AM, Hal Murray wrote:
robert8...@yahoo.co.uk said:
Have a look in "The Art of Electronics" by Horrowitz& Hill (if you don't
hav a copy, you should! or try the local libary). It has a nice circuit for
this (Actually a telescope drive IIRC) type of application.
Rats. I can't find my copy.
I think the trick is that it needs to drive it at sidereal time so it's off a
bit from 60 Hz. I forget what they start with.
What do serious (optical) telescopes use for a time base? What did Hubble
use? What did they use for the Palomar Sky Survey?
These days.. a quartz oscillator, and/or, a closed loop control based on
tracking a guide star.
What did Hubble use up on Mt. Wilson? A mechanical clock
"One final accessory remained to be built, the clock drive that would
regulate the speed at which the telescope would sweep across the sky,
tracing the arcs followed by the stars during the night. The telescope
is like a giant grandfather clock, with the tube moved by the force of a
falling weight. The clock drive mechanism keeps that rate constant and
allows for minor adjustments. However, this clock had to be considerably
more massive than any timepiece. A 2- ton falling weight drives the
machine, while over 1,000 pounds of bronze parts were cast for the
mechanism, and almost 3,000 pounds of iron. The force is then
transmitted to the drive gear on the polar axis of the telescope, a
precision gear like that of a fine Swiss watch, but 17 feet across! "
http://www.mtwilson.edu/Simmons4.php
I think it has been updated to some sort of servo scheme based on a
clock that runs on sidereal time. A friend was up there about 10 years
ago for something and mentioned that a lot of the original drive
electronics is still in use. Carbon fiber filaments were mentioned.
John Strong's book on making stuff ("procedures in experimental
physics") probably has a lot of the details (like how they put the
reflective coating on the mirror). That book is a fascinating look at
state of the art in the early part of the 20th century (you want to make
your own Geiger-Muller tubes... it's in there). Every physics/lab
tinkerer should have a copy (it's cheap in paperback from Lindsay books
(http://www.lindsaybks.com/... don't know if they still have it), or
probably Amazon, too)(just checked amazon.. $35 for used??? what are
they thinking)
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