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Re: [ATM] Ball bearing for triangles,spherical aberration caused bythermal gradient in the mirror : the results...(long)
I don't know if you use a Paracorr, Raphaël. By moving it a little bit
relative to the eyepiece, a small correction for spherical aberration is
possible.
Regards,
Adrie Suijkerbuijk.
----- Original Message -----
From: "Raphaël GUINAMARD" <rguinamard@infonie.fr>
To: <atm@atmlist.net>
Sent: Wednesday, January 05, 2005 11:21 PM
Subject: [ATM] Ball bearing for triangles,spherical aberration caused by
thermal gradient in the mirror : the results...(long)
>I don't know if you remember the thread I posted in september about the
> spherical aberration I have on my 24" f/3.3 scope.
> Here is the story, the results, and one question:
> My mirror is very thin: 40 mm at the edge => 28 mm at the center. I have a
> 27 point classical Kriege style (calculated with plop)
> I have approx (according the star test...) 1/4 wave of spherical
> aberration.
> I suspected 3 things could produce spherical aberration:
> 1) it is in the glass itself => nothing to do except redo the mirror or
> use
> an astatic mirror cell that could correct some spherical ab.
> 2) it is caused by a temperature difference between the top of the mirror
> (cold because it sees the 3K temp of the sky) and the bottom of the mirror
> (warm because it is at the outside temperature if you wait enough so that
> it
> equilibrates the ambiant air). The gradient temp between the 2 faces of
> the
> mirror would then deform it. A french professional mirror 28mm thick
> (meniscus), 1.5 m diam, that had this problem (which lead to 10 wave of
> spherical ab...) solved the problem by heating the aluminium layer using
> electricity joule effect.
> 3) it is caused by my mirror cell because my 3 big (bottom) triangles
> didn't
> seems equilibrate because of the friction in the acorn nut I use as the
> pivot center for the triangle (Kriege design)
>
> I have now some answers of the points 2 and 3:
> Point 2: I put the scope outside at the beginning of the night with its
> primary dust cover on it so that the top faces dosesn't sees the 3K sky.
> I waited 3 hours or so so that I'm quiet sure the mirror is at a uniform
> temp - the same as the outside tem-. Them I put off the dust cover and
> make
> the star test immediately.
> Nothing had changed with the spherical aberation: it was the same as
> usual.
> So I think that if the gradient temp could have a big effect on huge
> professional very thin mirror, it has certainly a non detectable effect on
> my 24" thin mirror.
>
> Point 3 : To be sure the friction in the pivot center on the big triangle
> of
> my cell was not the problem, I put ball bearings like in Bruce Sayre' 22"
> bino
> (http://www.foothill.net/~sayre/images/22-in.%20spherical%20bearing.jpg )
> It changed nothing neither, but it showed that the 9 little triangles are
> not really coplanar.
> They are not coplanar certainly because I put 3 little shanks between the
> big triangles to avoid rotation and these shanks doesn't seems to have
> exactly the good lenght => they bend a bit and so does the now-flexible
> triangle. It could also be that I drill the triangle at the COG given by
> plop, forgetting that the outside part (not shown in plop) of the real
> triangle moves the COG a bit.
> Anyway I have to change little things on my cell to put them coplanar and
> say definitively that the mirror cell is not the cause, but i'm nearly
> sure
> now that it's not the cause because I'm near a perfect cell.
>
> According the deformation drawing of spherical aberation I found in
> D.Suiter's star testing astronomical telescope, I also tried to deform my
> primary by putting washers under the 3 inner (upper) triangles and add
> counterweigh under the external part of the 6 external (upper) triangles.
> Nothing changed neither about the star test pattern showing spherical
> aberation.
>
> The only thing I have to test now is to deform the secondary so that it
> gives an "inverse" spherical aberration => I hope the resulting image
> would
> be free of spherical ab.
> My 130 mm secondary (20 mm thickness) mirror is glued with silicone on my
> aluminium secondary holder (wich is smaller than the secondary, 80mm * 140
> mm in lengh).
> I think I could easily deform my mirror by puting some (the less the
> better!) small screws through my secondary holder acting like fixed
> "actuators" for my secondary mirror. I could then adjust the pressure of
> the
> screw on the sec mirror to have the finest star test -aberation free
> image-.
> Not too much because I don't wan't the mirror to unglue from the silicone
> of
> course...
> My question is :
> Knowing that my primary spherical aberation is undercorrected (so the
> contrary shape of the fig 10-3 p175 of Suiter's book), do you know if I
> have
> to put screws near the center of the sec mirror, on the 2/3 part of the
> sec
> mirror or on the outer part?. If it's on the outer part, I can't because
> my
> secondary holder doesn't cover all the secondary....
> I think to give my secondary an inverse spherical ab of my undercorrected
> primary, I certainly have to give my secondary an overcorrected form (so
> "actuators" screws on the 2/3 part of the ellipse mirror), but I'm not
> sure...
> What do you think, guys?
> Thanks in advance for your help. I hope my english is understandable
> Raphaël Guinamard
>
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