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Re: [ATM] Ball bearing for triangles,spherical aberrationcausedbythermal gradient in the mirror:theresults...(long)
Hi Raphaël,
I have a old Paracorr, without tunable top. I simple slide the 2"/1 1/4"
adapter a little bit further from the corrector. I do the same with the
eyepiece, which is now about 2 cm's farther from the corrector. Of course it
is a cheap solution, I don't know if it really improves the image quality,
but it improves the star test.
I don't know anything about the Paracorr design, but once I raytraced a
corrector for photography. The amount of overcorrection. can be controlled
by the distance from the corrector to the focal plane. Also it is known that
spherical correction can be controlled by changing the conic constant of
the mirror. So there is interaction between mirror and corrector, and the
placement of the corrector.
As the conic constant of the mirror can not be changed, it is a matter of
playing with the distance of the eyepiece to the corrector to see what
happens. Personally I would use the Foucaulttest
to really know the quality of the mirror..
Regards,
Adrie Suijkerbuijk
----- Original Message -----
From: "Raphaël GUINAMARD" <rguinamard@infonie.fr>
To: <atm@atmlist.net>
Sent: Saturday, January 08, 2005 2:42 AM
Subject: Re: [ATM] Ball bearing for triangles,spherical aberration
causedbythermal gradient in the mirror: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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