[Author Prev][Author Next][Thread Prev][Thread Next][Author Index][Thread Index]
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/
> >
>
>
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
_______________________________________________
ATM mailing list http://www.atmlist.net/