Re: ATM Seeing double...looks like the main mirror :/

[The Silent Observer <silent1@ix.netcom.com>]

ChrisR wrote:
> 
> Donald, Chuck,
> 
> Ok, I can try rotating the mirror in the cell.  The cell was made by me (so it
> 
> could be a problem ...) though the mirror was placed on the dobs of silicone
> and the silicone then had to conform to the mirror so stresses should be
> minimal.  But as you say, maybe its the problem.


It's not very likely the cell is the problem with silicone attachments,
but I've heard reports of it happening.  Worth the effort to check it,
since you can do that in a couple hours work over a couple days.  OTOH,
see below.

> Donald,
> the mirror is not commercial and is made by a glass technician I know.  Cost
> me about $20 US equivalent so an ok risk if something was wrong with it.  He
> said that it had been tested using the Ronchii method, but no Foucault test
> was performed. Because detailed testing was not done I am perhaps more
> pessimestic than if the mirror was $500 commercial job.  Perhaps I ought to
> look into making a Foucault tester, though can it be used on an already
> aluminised mirror?

Yes, a Foucault test can be done on an aluminized mirror; in fact, the
brighter reflection makes it easier to align and lets you use a much
weaker light source (or finer slit or smaller pinhole) and still see the
shadows well.

I built my Foucault, a rather crude one, in a couple hours over a week
of spare time, using a pair of single edge blades, a bright LED, a 1/4"
x 20 threaded rod, and some scraps of plywood.

> The technician still has the original fine grinding lap and pitch
> lap........which is giving me ideas, though I don't have much experience fine
> grinding or figuring.  This mirror was also ground and polished on a machine
> as opposed to by hand if this makes any difference.

It's very likely that if the mirror, rather than the cell, is at fault,
you'll need to return to 320 or so, possibly even 220.  The level of
error you have in order to make a distinct double image rather than
astigmatic blurring is probably more than can be polished out with
reasonable effort.  Worth noting, though, that if the mirror was
improperly bedded while being machine ground and polished, the
variations in the bed will have printed through to the mirror surface. 
You'll want to be sure to properly float the mirror while regrinding to
a figure of rotation.

A machine could make a difference; if polishing or smoothing was done
MOT and the mirror wasn't rotating in the machine as it should for some
period of time, it would have gotten a big trough ground or polished in
-- and that would only show on a Ronchi or even Foucault if the mirror
was rotated in the test stand and tested on more than one axis; the
symptom is a different ROC on different axes.

-- 
It is well to approach all wonders and miracles by gradual stages or
degrees . . .
                                                           Fritz Leiber

Donald Qualls, aka The Silent Observer           NAR # 70141-SR Insured
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Opinions expressed are my own -- take them for what they're worth
and don't expect them to be perfect.