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Re: [ATM] Ball bearing for triangles,spherical aberration caused bythermal gradient in themirror :theresults...(long)
I wouldn't expect Paracorr do exhibit spherical with
relatively small changes of the cone width at the
lens. It is supposed to work with cone width from
f.3.5 to f/8 w/o introducing spherical aberration.
If you are looking for a simple solution to fix
correction error, a thin concentric meniscus
at some distance in front of the focal plane
could do satisfactory job. For instance, if your
mirror is 1/4 wave undercorrected, such meniscus
of BK7 glass with radii R1,2=300mm (concave toward
mirror), 8mm thick, placed 80mm in front of primary's
image would correct spherical aberration if it results
from a conic error, without introducing coma or
astigmatism. It would induce some chromatism:
about 4" f/25 achromat level (would probably get
somewhat lower if optimized).
If the figure is not a conic (ellipsoid or hyperboloid),
but has different form of surface deviation (for
instance, max error at the 70% zone diminishing
towards center and edge), the meniscus would
not correct s.a. entirely, but still should have a
positive effect.
Vlad
----- Original Message -----
From: Raphaël GUINAMARD<mailto:rguinamard@infonie.fr>
To: Atm-List<mailto:atm@atmlist.net>
Sent: Thursday, January 06, 2005 5:14 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<mailto:rguinamard@infonie.fr>>
To: "Atm-List" <atm@atmlist.net<mailto: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<mailto:a.suykerbuyk@wxs.nl>>
> To: "Atm-List" <atm@atmlist.net<mailto: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<mailto:rguinamard@infonie.fr>>
> > To: <atm@atmlist.net<mailto: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<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/<http://www.atmlist.net/>
> > >
> >
> >
> > _______________________________________________
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>
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