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Re: [ATM] RE: Ball bearing for triangles

 > Bruce may be on vacation, so it may take some time to hear back from
> him...but I had contact with him a month or two ago, so he should still be
> around.

Bruce answered me tonight and pointed me toward the Mc master site

>
> You mention spherical aberration in your star test.  That is a radially
> symmetric mirror error.  With friction/stiction in your mirror cell
> pivots...I'd expect that they would 'stick' in random directions and
> magnitudes that overall would give an irregular astigmatism to your
> mirror...and every time you remove and replace the mirror on the
cell...you
> would get a different type/magnitude/orientation of irregular astigmatism
in
> the star test.  Is this what you see?  If yes, then friction/stiction may
be
> the culprit in your cell design.  If no, then maybe the spherical
aberration
> is in the mirror?

I think you're right Tom: I contated a french professional optician and he
told me exactly the same thing: he thinks a friction in the mirror cell
would produce astigmatism insteed of spherical aberration.
He thinks MAYBE my spherical abberation could come from a different
temperature between the front and the rear face of the mirror: Since my
mirror is very thin (40mm) and very fast (f/3.3), the center of the mirror
is only 28 mm thick. Since the front face of the miror is looking toward
space, with a 3K temperature, he thinks its face has always a lower
temperature than the other face (even if the miror has completely cooled
down).
With a so big difference of thickness between center and edge of the miror,
this difference in temp could act as a "bilame" (in french...) and produce
spherical aberation.
He mentionned me he has made a 1.5 m miror, with only 28 mm thikness and
this phenomena leads to 10 wave of spherical abberation (even if the miror
is perfect in the workshop). To get rid of this aberation, they put a small
amount of current in the aluminium reflective coating to control the
temperature of both side of the miror within 1/100°C...
To know if I could have a similar problem, he told me to mount the scope
WITHOUT removing my primary dust cover, wait 2-3 hours that the miror is in
total equilibrium (because when the miror isn't in equilibrium, it could
also lead to spherical aberation: see Suiter's book), the remove the dust
cover and make quickly a star test to see if the spherical aberration is
there. Clever experience to do!
Unfortunately, my secondary mirror is away because I had a problem with it
=> I'll do the test at the end of september
He told me it's a long story to dispose of the problems one by one a so thin
mirror could lead, but if the miror is good (what I think) it's worth it
So I have to get rid of this stiction first, then see other potential
problems
>
> What kind of edge support are you using?  Any photos or diagrams you can
> provide us?

I use piano wire support. I've put photos in the file section (folder : 600
f/3.3) :
http://www.atmlist.net/uploads/index.cgi?action=panels;left=/600f3.3/;right=/;order-left=name;order-right=name
I've put also pictures of the triangles and of the "weight" I need to put on
one of the big triangle to maintain it horizontal. Otherwise, it's inner
apex is lower than the inner apex of the other big triangle. I though this
could lead to the miror surface like the one in page of Suiter's "Star
testing astronomical telescope"
On one other triangle, I need to put (less) weight on the other side of the
triangle because its inner apex is higher than the others. I agree with you
that it shouldn't lead to radially symetrical default like the spherical
aberation

>
> Bruce's use of ball joint bearings will probably be better than what you
are
> using now...but at this time I'm not sure if we've properly identified the
> cause, because the symptoms are not what I would expect.
>
> If you bump/shake/jostle the mirror in it's cell...does that improve the
> star test?  If yes, then you have stiction issues that need correcting.
> Ball joint bearings can do that.  So can compliant hinges.

I have to test this also!

>
> If you are curious about compliant hinges I can provide a few more ideas
and
> diagrams.

I would like very much to receive your diagrams/ideas, it's the same idea as
Nils mentioned in his post this morning?
Thanks again for your help!


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