Re: [ATM] Ball bearing for triangles,spherical aberration caused bythermal gradient in themirror :theresults...(long)

["matt" <mariusrf@bellsouth.net>]

There's a device called SAFIX that corrects for spherical aberration.
Otherwise, design a corrector for the scope .
You will not be able to warp the mirrors into correct shape using a large
number of screws for actuators . You need actuators with variable force not
variable displacement. You also need a wavefront sensor to monitor the
wavefront accuracy continuously and adjust it periodically . The mirror
cell, scope trusses, all mechanical parts are changing rapidly at the
wavelength size scale . Pro scopes which are more rigid than ultralight
portable dobs correct their active optics actuators in the Hertz range,
nothing fixed would work for more than a few seconds and would not be
repeatable once the scope is touched, changes position, temperature .

best regards,
matt tudor

----- Original Message -----
From: "Raphaël GUINAMARD" <rguinamard@infonie.fr>
To: "Atm-List" <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>
> 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
> > > >
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> > >
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