[Author Prev][Author Next][Thread Prev][Thread Next][Author Index][Thread Index]
Re: [ATM] PLOP, new developments in mirror support,state of the art edge support, etc
Jay,
Small mirrors usually have a thicness ratio such that
they do not deform easily. Thus testing them in a
tower is not effective. They will not show any
deformation either way.
However, I do have a mirror that will deform. It is a
16 x 1 inch F5. That means the tester at ROC must be
13 feet away. My basement has 7.5 foot ceilings. The
mirror is in its cell and on the floor tilted up. My
tester is on a tripod on top of a table. I have to
stand on a chair to adjust it. I can't get my head
behind it but I have a web cam setup. The bottom line
is since the mirror center is about 1 foot off the
floor and the tester optics are about 1 foot from the
ceiling the largest angle I can get from horizontal is
about 20 degrees. I never use my scope at the shallow
of an angle. But I tried testing it anyway because it
is the best I can do.
This is what I have found so far. You cannot measure
small astig with the Foucault test. You can see small
astig using a ronchii setup and the correct technique.
I have measured the mirror both at the horizontal and
at 20 deg in the mirror cell with +-45 deg lateral
support. I have also measure it horizontal on the
mirror stand with the same lateral support. Most of
the testing was done with my Robo Foucault tester. So
I can't get any astig measurements from that.
The ronchi technique I have tried is to adjust the
screen so that there is only one line on the mirror
and in the center. Then you watch the line as you
move through and past ROC. If the line tends to
rotate there is astig. With my setup up I can't keep
everything aligned well enough to keep the line in the
center. Moving the tester just .0001 is enough to
decenter it. It almost moves that much when I breath
on it. This is hard to do even when everything is
horizontal and easy to reach. When it was up at the
ceiling and tilted down it was next to impossible. I
tried anyway and could see none.
I have tried other techniques to measure astig while
it was horizontal on the test stand. I can not see
any. So I'm not sure if it is my test setup or the
mirror has none. It is very likely the mirror will
show astig in the horizontal setup. But I haven't
been able to measure it.
Thus, I find it hard to do the experiment you have
suggested.
I have started working with an interferometer and hope
to have some positive results but so far I have been
having problems that are common to that form of
testing.
Hopefully I will have better data someday. I have
been working on this problem since June of this year.
So far I have faild.
Dale Eason
--- Jay Kirkland <jaykirk2@compusmart.ab.ca> wrote:
> Okay, Mark, so testing mirrors to see the
> various effects of
> various mirror cells would be difficult (the
> testing pit) but still not
> impossible, especially with smaller mirrors. (I have
> to admit, I would
> not want to be the one to suspend any kind of
> testing device 14 feet
> directly above my mirror.) But wouln't it also be
> possible to test the
> mirror/cell in a tilted config? That should still
> tell us how well our
> mirror supports are doing while pointing halfway up
> the sky.
> Actually, I'm a big believer in PLOP (I'm just
> accepting it as a
> matter of faith, much the same way I'm accepting
> Dark Matter and String
> Theory) and in its degrees of reasonable tolerance
> in designing cells,
> and (despite much opinion to the contrary) in
> reasonable tolerances for
> error in the actual cell manufacture (otherwise how
> do you explain that
> anyone can see anything out of their scopes - let
> alone some of the
> stunning images seen and/or recorded by Obsession
> users and even John
> Dobson's own creaky scopes - and by "creaky" there I
> mean mechanical
> tolerances measured probably in inches, if at all).
> I just can't
> believe that no one who's crazy enough and curious
> enough to figure and
> test their own optics hasn't tried out, tested, and
> measured different
> mirror cell designs for their own scopes.
> But I can't believe that with all the debate
> going on for at least
> the last year on the group that no one is testing
> and measuring the
> effects of different forms of edge support on their
> mirrors. That can be
> (has to be?) done with the mirror on edge for at
> least much of the
> experiment. I mean, anyone who has tested their
> mirror during figuring
> already has the setup to test edge support
> variables.
> Mel Bartels mentioned those books a few times
> before and I did look
> their prices up on the 'net but ran away. I guess I
> really better head
> over to the university library and see if they have
> them.
>
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
>
__________________________________
Do you Yahoo!?
Yahoo! Mail - You care about security. So do we.
http://promotions.yahoo.com/new_mail
_______________________________________________
ATM mailing list http://www.atmlist.net/