[ATM] Star testing an uncoated mirror? No luck.
baldjeff at comcast.net
Thu Oct 22 08:23:35 JST 2009
there are three positions your focuser could be in; right on, too far in, or
too far out. If it's right on, then maybe your scope's tail end is too
bright and overwhelming dim stars. If it's too far out, then you will be
able to focus on nearby false stars but not one at 'infinity'. If it's too
far in, then nothing will ever come to focus. You may want to establish
where your focuser is with respect to where it is supposed to be. The fact
that you have a red blur when you shine your laser in it tells me that you
aren't focusing, perhaps due to focuser position. Let us know what you
Jeff and Glenda Baldwin
----- Original Message -----
From: "Guy Brandenburg" <gfbrandenburg at yahoo.com>
To: "atmlist" <atm at atmlist.net>
Cc: <novac at his.com>; <capitalastronomers at yahoogroups.com>
Sent: Wednesday, October 21, 2009 3:57 PM
Subject: [ATM] Star testing an uncoated mirror? No luck.
We finally tried last night to use a quick-and-dirty all-purpose star-tester
on a unsilvered mirror that was being worked on and tested. It was an
adaptation, with modifications, of the universal star tester that was
recently featured in Sky & Telescope, and was worked on and modified by
myself and Bill B over a period of some weeks.
Sad to report, our first try was a complete and utter failure. Couldn't see
ANYTHING, no matter what we did. I felt like a fool. At our first location,
right next to the Community Center building where we have the telescope
making class, which is a sidewalk illuminated by numerous lights, all I
could see in the eyepiece was a blur; if I removed the eyepiece I got a nice
view of the plywood in back of the primary mirror.
To try to focus on something - ANYTHING - we hauled the contraption - which
looked like a combination of a stretcher and a coffin for the world's
tallest and skinniest man - to the darkest corner of the playground and
parking lot, and put it on some chairs, but it still wasn't very dark.
We collimated the parts inside the tester the best we could - the secondary
pointed right at the mirror, the secondary was nicely centered, but the
problem was that nothing at all was visible in the unsilvered primary.
We tried aiming it at a special flashlight with microscopic pinholes of
various sizes, suggested on this list. No dice - couldn't make it out, no
matter how far away, or close, or what angle, the flashlightr was.
We tried aiming it at a regular flashlight. No dice.
We tried aiming it at a weak (low batteries) red laser pointer. At best we
saw a very rough, and weak, reddish glow.
Didn't want to try a bright green laser - we thought it was too hazardous.
So we gave up for the night.
As some of us commented, it felt like trying to thread a needle, using a
thick rope, at a distance of 10 feet, while the needle was still hidden in
After we took it back inside, one person suggested we should have put the
bright green laser into the eyepiece hole and then seen where the light came
out. This sort of sounded good, but I wonder if an expanding lens
(bi-concave) might be needed to spread out the beam, or else it would get
intercepted by the secondary mirror after the second reflection.
Possible reasons and solutions: (Any other suggestions are welcome)
1. All of the bright lights in the area washed out the very weak reflections
(~4%) from the primary. Solution: Unlike the magazine article, a seriously
black and dark tube will be needed, which will make things larger and
bulkier. [A differnt solution, of using about a case of .22 Long Rifle
bullets, is ruled out of order.] Another solution: Wait until a clear night
with a moon about to set; and go out to a dark sky site; align via
reflections and moonshine onto the moon; wait for the moon to go down; then
finally point at the North Star. (Sort of eliminates the "quick and dirty"
part of the stare test, doesn't it?)
2. Alignment of the primary is impossible. Solution: Line up things with a
green laser pointed into the eyepiece hole, as described earlier.
BTW - how on earth does somebody do a star test on a mirror by using the
reflection off the eye of a crow, as was related to me by John Dobson? Or is
that just a myth, promoted by a mythical being?
Guy Brandenburg, Washington, DC
My home page on astronomy, mathematics, education:
or else http://tinyurl.com/r6fh2
"...to conclude that problems of international competitiveness can be solved
by educational reform, especially educational reform defined solely as
school reform, is not merely utopian and millennialist, it is at best a
foolish and at worst a crass effort to direct attention away from those
truly responsible for doing something about competitiveness and to lay the
burden instead on the schools." ---- Popular Education and its Discontents
by Lawrence Arthur Cremin
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