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Re: [ATM] Ball bearing for triangles,spherical aberration causedbythermal gradient in themirror :theresults...(long)
Hi Adrie,
The barlow I use is a powermate, which is normally aberation free.
I have to check again but if I remember correctly, I have the same amount of
spherical aberation (and of TDE) with the 6 mm radian or with the 6 mm
radian +powermate 2" 2X
About the paracorr, when I read what you've just write, I think I have maybe
misunderstood what you were saying.
I thought you were telling me to unscrew the screw that hold the ep and lift
the ep out of the paracorr.
And I'm just thinking now that maybe you were telling me to rotate the
"turnable top"? Is it that?
If yes, I'm so foolish ! I was puting the ep out without touching the
turnable top...
Thanks to help me!
----- Original Message -----
From: "A. Suijkerbuijk" <a.suykerbuyk@wxs.nl>
To: "Atm-List" <atm@atmlist.net>
Sent: Friday, January 07, 2005 9:59 PM
Subject: 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
> >> > >
> >> > > _______________________________________________
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> >> > >
> >> >
> >> >
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