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Re: [ATM] Ball bearing for triangles,spherical aberration causedbythermal gradient in the mirror :theresults...(long)
It surprises me a little, Raphaël. I can improve the star test from a
slightly overcorrected mirror by moving the eyepiece farther away from the
corrector. If your mirror is undercorrected the eyepiece should be placed
closer.
Also I wonder about your barlow. Once I tested five barlows, only two gave a
good star test. Did you test your barlow separately?
Perhaps it is best to test your mirror with a Foucaulttest.
Regards,
Adrie Suijkerbuijk
----- Original Message -----
From: "Raphaël GUINAMARD" <rguinamard@infonie.fr>
To: "Atm-List" <atm@atmlist.net>
Sent: Thursday, January 06, 2005 11:14 PM
Subject: Re: [ATM] Ball bearing for triangles,spherical aberration caused
bythermal gradient in the mirror :theresults...(long)
> Well after 1h40 of cool down time the images were still dancing too much
> to
> make a real star test using different position of my 6 mm radian in the
> paracorr (itself in the 2X powermate).
> Anyway, it seems to me that the effect is not obvious on the sherical
> aberation... It is still there ...
> ----- Original Message -----
> From: "Raphaël GUINAMARD" <rguinamard@infonie.fr>
> To: "Atm-List" <atm@atmlist.net>
> Sent: Thursday, January 06, 2005 9:10 PM
> Subject: Re: [ATM] Ball bearing for triangles,spherical aberration caused
> bythermal gradient in the mirror : theresults...(long)
>
>
>> Adrie,
>> Yes I have a paracorr. I'll test what you say right now.
>> Thanks !!!!
>> ----- Original Message -----
>> From: "A. Suijkerbuijk" <a.suykerbuyk@wxs.nl>
>> To: "Atm-List" <atm@atmlist.net>
>> Sent: Thursday, January 06, 2005 8:38 PM
>> Subject: 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
>> > >
>> > > _______________________________________________
>> > > ATM mailing list http://www.atmlist.net/
>> > >
>> >
>> >
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