[ATM] RE: Ball bearing for triangles

["lou krajci" <loukrajci@comcast.net>]

>From: Rapha?l GUINAMARD <rguinamard@infonie.fr>

>I have a 24" f/3.3 dob.

>My mirror is very fine (40mm) an I have built a 27 point mirror cell (9
>small triangles on 3 big triangles).

>For the "rotation" of the big triangles I have used a carriage bolt that I
>have drilled so that the collimation bolt could pass though, and I have
>beveled the bottom part of my triangle so that the spherical part of the
>bolt could fit in it.

>I have put only a very very small amount of grease.

>My big triangles are quite heavy (6 mm stainless steel) and my problem is
>that there is friction and my triangles needs each approx 300 g of  force
to
>be really in the same plane.

>I think this could deform a little bit my miror because I see some
spherical
>aberration in the star test.

>My question : does anyone has example of using bearing to support the big
>triangle (close picture, reference where I could find the bearings) ?

>I've seen Bruce Sayre page
>(http://www.foothill.net/~sayre/22-in.%20binocular.htm) but I can't get in
>touch with him to have more details .

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.

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?

What kind of edge support are you using?  Any photos or diagrams you can
provide us?

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.

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

I hope this helps.

Tom Krajci

Albuquerque, New Mexico

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