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Astatic cell

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#1 hakann

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Posted 01 August 2020 - 08:49 PM

Has anyone experiance whit the astatic cell design ?

The few amature telescopes I’ll seen whit this has been Frederic Gea/Luc Arnold on internet.
My guess this is a rather old reptage, still its good information.
My guess one can adopt a Plop cell or the average one from Mel Bartels web.
Most design here will give many times better than any plano mirrors ever will be.
And if I got it right, its so much friction and parts and balance arms etc it might be 5-10 maybe up to 20 times better than diffraction limit. ( in my way see it, it do sounds odd but if one shuut for 1 nm RMS in Plop it will be like a 1/80 PV wf and that is 20 tines better that diffraction limit.
Here the astatic might come to play.

On the load, say its a 18 pt ( 6 triangles but it can be a 9 pt aswell but still 6 ) and say the weight is 10 kg, so that means 1.66 kg at each triangle and say a lever at ration 1/5 that mean a added weight at 332 grams on the 3 astatic triangles.

I guess the other 3 triangles ( or pins ) hold the mirror in place and this can be used for collimation.

But how to tune this in ?
One idea is add a weight on the 3 astatic triangles at 1.66 kg and get them in level ( but in reality it might be hard vs where ’is’ level ? )
But maybe thats all needed or a fine tuning vs a star test ?

Ideas ?

#2 BGRE

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Posted 01 August 2020 - 09:17 PM

3 hardpoints points are required to set the location of the rear surface of the mirror substrate.

Additional kinematic constraints are needed for the edge.

The hardpoints can adjustable to facilitate alignment.

The astatic supports are adjusted so that there is some small axial load on each hardpoint.

#3 hakann

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Posted 01 August 2020 - 10:02 PM

BGRE,
Gea used flexure pins for edge support.

The ’hard points can be pins or triangles ( right ? )
You say adjustment is so each hard points has some axial load.
-How will that be done pratically ?

Let’s say mirror is on its back ond one adjust the weight on the lever, but how to know where the load is correct ?

Have you see this be done in any telescopes ?
And if so, did it work ?

On picture from Gea it seems a simple design so its not complicated to do it.

#4 hakann

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Posted 01 August 2020 - 10:09 PM


Gea ;


Adjustments

Adjustments of the mirror cell were made using the whole scope while doing the star-test in order to tune it. It is the mirror cell made for the 560 mm described on this website.

http://www.astrosurf...taz/astatic.htm

It's a 12-points cell with 3 fixed points without kneecaps, and 3 triangles on a kneecap and lever. The transmission of the effort is carried out by metal blades. Their first image was horrible because of a big astigmatism. After having removed the mirror of its support, it was easy to see that the mirror was not in contact with the 3 fixed points. After adjustments of these points to bring them into contact with the back of the primary mirror, the 3 counterweights of levers were brought to a point close to their minimal effect, where they generate less force. As the force of the levers is then insufficient, the mirror is support only by the 3 fixed points. At the eyepiece, there is then a triangular star. To see whether it is necessary to add force to levers, we remove all the push of the counterweight by raising them manually and we note that this triangular shape of the star image was worse. That means that the level didn't push enough, so we added some distance to the counterweight (same amount at the same time) and we saw the triangular shape slowly disappear. Needless to say that these operations are much easier to do with two people! This experiment is very interesting because it is incredibly easy to see how much a mirror, even a thick on, can be easily stressed by a small amount of uneven pressure. For a mirror cell with two circles of triangles, the procedure is the same, one finishes simply by the adjustment of the central circle, which is used to adjust the spherical aberration.

#5 hakann

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Posted 01 August 2020 - 10:56 PM

This guys has also make ’Chrysocal Levers’ instead of the piston and bearing ( that will create some friction )

Here is what they say, so as I read it this is better than the piston version.

but as far as I can see its still a piston (?)

 

its also a system at no piston but the besrings.

Gea ;
-Using a "chrysocal" blade (called flexural pivots), it's even less cause for friction, it's an "elastic" lever made with a metal that always comes back to the same position. For the transmission of the push of the counterweight, it is the same thing. Some astatic cells use a transmission part (a "piston") that can give some friction but it's possible to use a chrysocal blade or other metal to transmit the effort with very minimal problems, so the push that was given to levers sometime to release tension is very reduced, a simple movement at the begining of the night is enough. Astatic levers only transmit a pressure, a strength, not a movement. Parts don't move. It's not a question of moving parts but a question of effort applied with little friction in order not to hinder motion. The triangles always apply the same force to the glass. When the 'scope is moved down, the counterweight regularly gives less and less push and lets the lateral support carry the weight. When the scope is point up, the counterweight has more and more "power" to carry the load, always the same way because the position is still the same. This difference, compared to a classical cell is that it is totally reproducible and always gives the same thing

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Edited by hakann, 01 August 2020 - 11:53 PM.


#6 BGRE

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Posted 01 August 2020 - 11:36 PM

BGRE,
Gea used flexure pins for edge support.

The ’hard points can be pins or triangles ( right ? )
You say adjustment is so each hard points has some axial load.
-How will that be done pratically ?

Let’s say mirror is on its back ond one adjust the weight on the lever, but how to know where the load is correct ?

Have you see this be done in any telescopes ?
And if so, did it work ?

On picture from Gea it seems a simple design so its not complicated to do it.


Integral load cells within the hardpoints can be used to measure the load carried by each hardpoint.
ESO do this in various telescope rear support systems.
Probably more important with a large number of astatic supports to help avoid lifting the mirror off one or more the hardpoints when the astatic supports are misadjusted.

#7 hakann

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Posted Yesterday, 12:00 AM

BGRE,

my guess as text say the only way here is do startest, but first a pre set just small load.

He say even a thick mirror tend to easy bend under very little pressuare.

 

i checked many telescopes where a triangle was lose under tilt and if lose, it can be to much load to.

this idea should avoid that, but in reality.



#8 don clement

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Posted Yesterday, 12:31 AM

This is interesting from https://www.bbastrod...XlZogGDR88UEIg 

"The mirror that really turned my head and got my attention was David Davis' 16 inch x 1/4 inch thick slumped F3. The mirror was supported on a bed of compliant 1/4 inch diameter heads that looked like marshmallows arranged in a grid. The regular figure with complete absence of astigmatism was astonishing.

My new 3-axis alt-az-alt mount"


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#9 hakann

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Posted Yesterday, 08:27 AM

Don Clement,
Out of topics maybe.
Mel Bartels meniscus shape and polishing is my guess over rest of us heads.;-)
How he can get it at a good parabola and smooth so thin and backed it up as the curved back and use a simple cell in plywood, is probably more of a riddle !

We can only lift the hat, but Mel is not new into ATM.

It's way over my understanding, that' for shore.
I understand his idea of meniscus as he mean the shape get it stiffer and he like avoid balance arms to go 27 or 54 pts as to much friction, so he used a 9 pt cell bit with big pads he mean each pad create another 3 pts, so a total of 36 pts.

If this reach 1.000X in a good sky I can't say, but if : then it is totally amazing.

I seen it via my Nikon HW 17/14 mm 102 degree EP anyway + 100X. ( very nice ! )



#10 hakann

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Posted Yesterday, 08:34 AM

This I can't get.

Se picture above the 2 L block ( schims between ) and a rod is going in a hole like a 'piston', but there is NO bearings as this is the chrysocal blade deesign.

But Gea say it is no frictions bearings or a piston, but picture show it's a 'piston'.

 

 

Gea ;

Using a "chrysocal" blade (called flexural pivots) Some astatic cells use a transmission part (a "piston")  that can give some friction but it's possible to use a chrysocal blade or other metal to transmit the effort with very minimal problems, so the push that was given to levers sometime to release tension is very reduced. Astatic levers only transmit a pressure, a strength, not a movement. Parts don't move. Their is no "piston" movement that raises the mirror from it's position on the 3 fixed points.



#11 MitchAlsup

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Posted Yesterday, 10:57 AM

How he can get it at a good parabola and smooth so thin and backed it up as the curved back and use a simple cell in plywood, is probably more of a riddle !

It turns out that if the front and back sides of the mirror have the same shape (equal thickness all the way across) that the mirror acts like a plano mirror with respect to zenith gravitational forces. This simplifies the mount needed to hold said mirror.



#12 hakann

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Posted Yesterday, 11:49 AM

Mitch,
Mel mirror is not equal thick as he got it slumped way faster than he thought so if my memory is correct the parabola is at f/2.6 and the curved back is like f/+3 something, so the centre is thicker than the edge, and it also went way thinner what he had in mind at first.
( I think edge is like 14 mm and centre around 18 mm )
I shore think it is stiffer than a plano at the same edge height but several has said and vs a French tess that is not stiffer, and actually less stiff than a plano.
But a good thing is heat might not get stuck in the end of glass.
I do think Mel's optic's get ready to use very fast and he use no fans a closed back, plus edge is rest on 2 pts.
How this works and the curved edge is not at any COG so one can think about triangle load under tilt.
But he star tested this and seems very pleased, so it is just hat off big time here.

 

 

-I has not a meniscus in my mind as it way to complex and very few do like make one anyhow.

 

 

But more feed-back on the astatic’s should be nice.
Gea says in web about a 'classic' astatic, but I has not seen any ATM been build this way, more than those guys in France ( on internet )


Edited by hakann, Yesterday, 11:54 AM.


#13 hakann

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Posted Yesterday, 03:41 PM

I would like to reach Frederic Gea.

If anyone had Mr. Gea's email address, please PM it to me.


Edited by hakann, Today, 11:39 AM.

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#14 hakann

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Posted Today, 10:26 AM

I feel's like it is lack of information on the subject, or few are into this ideas.

If I learn more I come back. I had this topic out here before at CN but it went to more questions, and I has not been able to get in contact whit Gea, and the contact info on his web site has not work.

 

When I read on Gea tuned in the the weights and how a glass is sensitive for no load or little to much load it feel like somethings need look into.

 

( Then if a blades or bearings will do the 'move' I can't say.


Edited by hakann, Today, 11:41 AM.


#15 PrestonE

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Posted Today, 12:29 PM

Mitch,
Mel mirror is not equal thick as he got it slumped way faster than he thought so if my memory is correct the parabola is at f/2.6 and the curved back is like f/+3 something, so the centre is thicker than the edge, and it also went way thinner what he had in mind at first.
( I think edge is like 14 mm and centre around 18 mm )
I shore think it is stiffer than a plano at the same edge height but several has said and vs a French tess that is not stiffer, and actually less stiff than a plano.
But a good thing is heat might not get stuck in the end of glass.
I do think Mel's optic's get ready to use very fast and he use no fans a closed back, plus edge is rest on 2 pts.
How this works and the curved edge is not at any COG so one can think about triangle load under tilt.
But he star tested this and seems very pleased, so it is just hat off big time here.

 

 

-I has not a meniscus in my mind as it way to complex and very few do like make one anyhow.

 

 

But more feed-back on the astatic’s should be nice.
Gea says in web about a 'classic' astatic, but I has not seen any ATM been build this way, more than those guys in France ( on internet )

Mel's slumped mirror started out as an F2 and he ground it back to F2.6...just reread the article yesterday smile.gif

 

Best Regards,

 

Preston



#16 hakann

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Posted Today, 01:07 PM

Yes it's at f/2.6 but centre is thicker as back side is not curved the same, so maybe back has the slumped curve as that make sense on get centre thicker.

Amazing mirror and project, and now the new 30" binoscope is coming !



#17 Pierre Lemay

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Posted Today, 07:42 PM

I would like to reach Frederic Gea.
If anyone had Mr. Gea's email address, please PM it to me.


Hank,
You can reach Frédéric through his company at Stellarzac@free.fr


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