Re: [ATM] Ultra Thin 16" f3.5 kinda long

[Raphaël GUINAMARD <rguinamard@infonie.fr>]

Hi David,
Your story about heat reminds exactly what I could have with my miror.
Did you read the mails about ball joint bearing and then about an hypothesis
to explain the spherical aberation I have on my 24", f/3.3, thin (40 mm)
mirror?
Did you already observe with your miror? If yes, have you got spherical
aberation when you observe (but not when you test your mirror inside your
home)?
Here is on part of the mail, see the archive for more:

----- Original Message ----- 
From: "Raphaël GUINAMARD" <rguinamard@infonie.fr>
To: <atm@atmlist.net>
Sent: Thursday, September 02, 2004 11:24 AM
Subject: 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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