Re: [ATM] Back to original "ball bearing for triangles" thread?

[Raphaël GUINAMARD <rguinamard@infonie.fr>]

Hi Jay,

> Raphael, I'm sorry, I think I misunderstood your original question. It may
> not be that your triangles don't balance, it may be  that your large
> triangles exert enough pressure on the collimation bolts that when you
turn
> the collimation bolts, that twists the triangles out of their position in
> their plane. Is that what's happening?
Not exactly: my triangles are not exactly balanced (because I cut them 1/2"
larger than what plop said because my pads are 1" in diameter), but that's
not what bother much because I know that when I'll have a good pivot point,
it'll be very easy to adjust the center of G by putting some weight behind
the triangle.
What's bother me is that my pivot point in my big triangles doesn't rotate
well, certainly because
1 the conical mill i've done to the bottom part of the triangle is not well
done
2 the accorn nut i've in my pivot point has a small "shoulder"  that could
bother the rotation, see the picture
http://www.atmlist.net/contrib/rguinamard-at-infonie-dot-fr/600f3.3/triangles%20inf%2brotule1.JPG
So my big triangle is not horizontal and to put it horizontal, you have to
provide a force .
See for example the weight I have put as a test :
http://www.atmlist.net/contrib/rguinamard-at-infonie-dot-fr/600f3.3/triangles%20inf+contrepoid%201.JPG
I think this force necessary to put the triangle in the same plane as others
could deform my mirror.
At first I thought it could cause the spherical aberation I see in my star
test star, but TomKrajci and a french professional mirror maker pointed out
it should put astigmatism instead of sherical aberation.
The french has emitted another hypothesis for my spherical aberation: he
thinks the 2 faces of the mirror could be at different temp (because one
sees the cold space) and on a thin mirror like this could deform it.
Anyway I have to get rid of the problems 1 by 1 and that's why I was looking
for ball bearing instead of my bad pivot point.
To answer your other post, yes I think a 300 g force (3N) could cause deform
ation in my mirror: for example in the 1.5 m mirror i was referring, the 30
triangles have to hold each about 3.3 kg of the mirror. It is approx the
same amount of weight for the 9 triangles touching my mirror (22 kg/9=2.4
kg). For their astatic triangle, they have do have better accuracy than 0.9N
on the actuator tobe sure not to deform the mirror by more than lambda/4...

> If so, the very best way to solve this is not to use the pivots of the
> three triangles as the collimation points. There was a recent(?) thread on
> this group where several people were of the opinion that doing so was not
a
> good idea for the very reason that this could stress the mirror.
> The legendary TM guru Robert E Cox believed that it was better to both
> move the three collimation points further toward the edges of the mirror
> and out of the equilateral triangle arrangement

You're certainly right, but my mirror cell is already done and I would like
only small modifications to do.
If I have to redo it completely, then I'll choose an astatic cell with 2
levels: one for the cell and one for collimation bolts...
Thanks for your reply

----- Original Message ----- 
From: "Jay Kirkland" <jaykirk2@compusmart.ab.ca>
To: <atm@atmlist.net>
Sent: Sunday, September 05, 2004 1:46 AM
Subject: [ATM] Back to original "ball bearing for triangles" thread?


> Raphael, I'm sorry, I think I misunderstood your original question. It may
> not be that your triangles don't balance, it may be  that your large
> triangles exert enough pressure on the collimation bolts that when you
turn
> the collimation bolts, that twists the triangles out of their position in
> their plane. Is that what's happening? - any why you're looking for a
> frictionless connection between the triangle and the collimation bolt?
> If so, the very best way to solve this is not to use the pivots of the
> three triangles as the collimation points. There was a recent(?) thread on
> this group where several people were of the opinion that doing so was not
a
> good idea for the very reason that this could stress the mirror.
> The legendary TM guru Robert E Cox believed that it was better to both
> move the three collimation points further toward the edges of the mirror
> and out of the equilateral triangle arrangement into what he called an
> "orthogonal" system which he said was preferred by professional opticians.
> So if you had a square mirror box the collimation points would move toward
> one upper corner and the two bottom corners. Because of the weight of my
> mirror and cell, I put a fourth non-collimating  weight-bearing pivot
point
> at the 4th corner. The orthogonal arrangement means that when you move a
> collimation point, the mirror moves up up/down and left/right at right
> angles, not diagonally, and the wide stance makes possible a more
sensitive
> collimation. And the whole mirror cell can become a mechanically stronger,
> more stable unit in the process.
> Mr. Cox did a lengthy article on "better" mirror cells in Telescope Making
> magazine. This was one of their later issues, I believe, late 1980s?
> If this was your original concern, I apologize for my confusion. D'oh! And
> hope this helps.
>
> Jay
>
>
>
>
> _______________________________________________
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