A different kind of telescope mirror !

[Gasman]

Hi guys

In order to make a telescope mirror the usual way is to take two pieces of glass and grind one on top of another using finer grades of powder where one becomes concave and the other convex. The surface of the concave piece is gradually ground, polished and shaped ideally to a parabola then aluminized. I have actually made two mirrors like this a long time ago a 6" and a 8.75", the 8.75" served me well and still looks ok to this day.

I`m a retired industrial gas engineer and know all about flanges,gaskets, diaphragms etc and about 30+ years ago I had this idea of creating a telescope mirror using nothing more than a sheet of mirror like material (foil?) and creating a negative pressure behind it to form a concave surface and maybe it could be used optically?. I had no idea what shape it would make, at best a spherical/parabolic or at worse hyperbolic or nothing useable at all and so recently I printed off a prototype flange/diaphragm setup just to see if it was feasible. The flanges are at the limit of my little 3d printer and so I went for a 6" mirror. I can`t claim to be first with the concept as I gather a solar furnace was created some year back but I`ve never seen it used optically!.

 

 

The printed parts, note hole has not been made in the bottom for the suction tube.

 

 

Metal foil cut out

 

Back suction tube fitted

 

first try, gently sucked on the tube and then closed the tap  mmmm! Lots of stress lines from the foil and I put it away thinking no chance and nearly abandoned the project but got it out again and thought the middle 50% looks promising so if I could block out the outside 25mm maybe it was useable so made a 100mm stop to go over the front.

 

100mm stop being printed

 

 

fitted the 100mm stop in place and suction made.

 

looks much better. Thats where we are at present, using a torch side to side moving in and out of the mirror to get a rough centre of curvature it looks about 4ft giving a focal length of about 2ft.

Now I need to make a test rig with foucault setup to try and see what shape it creates.

I have two problems with it at present-

1 - not holding its shape as I made the gasket out of some crappy foam rubber I had and so could be leaking from there.

2 - or indeed the foil itself could be slightly porous

both could contribute to it not keeping its shape so further checks needed.

Would be great if it works as I remember all too well sat on a floor for hours on end rubbing two discs together, yes I know the experts will say I should have done it stood up but I preferred it that way ;-).

An interesting and very enjoyable brain exercise anyway!

cheers

Steve

 

 

 

Edited June 6, 2023 by Gasman

[vlaiv]

4 hours ago, Gasman said:

both could contribute to it not keeping its shape so further checks needed.

I'm not very confident that FDM printed plastic will be air tight as well.

Maybe look into putting some sealing paint / resin over it?

[Gasman]

2 hours ago, vlaiv said:

I'm not very confident that FDM printed plastic will be air tight as well.

Maybe look into putting some sealing paint / resin over it?

Good idea that vlaiv didn't think of the pla leaking 👍

[Astrobits]

Not a new idea at all. Just Google "Mylar mirror for astronomy" lots of hits and plenty of ideas there.

Nigel 

[pipnina]

This sounds similar to a video by an optics youtuber (very impressive videos) who tried to use the same principle to make a variable shape mirror

I like the ingenuity of the design principle!

[Gasman]

2 hours ago, Astrobits said:

Not a new idea at all. Just Google "Mylar mirror for astronomy" lots of hits and plenty of ideas there.

Nigel 

Thanks for that, I didn't claim it was a new idea just I'd never heard of it used optically before!

[John]

Interesting idea.

I can recall a company called Dalserf Optics in Scotland experimenting with a similar approach back in the 1980's. I may still have some info on what they were trying if anyone is interested.

 

[Astrobits]

Using this technique thin plates of glass are converted into corrector plates for Schmidt-Cassegrains. The suction pulls the material into a parabolic shape and the concave surface is then ground and polished to spherical. When the vacuum is released the figure is what is required for the corrector plate.

Nigel

[Gasman]

Ah that's interesting Nigel, I thought the SchmitCass mirrors were mass produced spherical with a front lens to enable focus?

Steve

[Mr Spock]

Interesting idea. Can't say I fancy it though - give solid glass any day.

[Peter Drew]

2 hours ago, Astrobits said:

Using this technique thin plates of glass are converted into corrector plates for Schmidt-Cassegrains. The suction pulls the material into a parabolic shape and the concave surface is then ground and polished to spherical. When the vacuum is released the figure is what is required for the corrector plate.

Nigel

Is this a current technique?, my understanding was that the SCT corrector plates were vacuum deformed against a master former and the exposed surface ground and polished flat.  When released from the vacuum the plate assumed the correct profile for the corrector.      🙂

[Astrobits]

16 hours ago, Peter Drew said:

Is this a current technique?, my understanding was that the SCT corrector plates were vacuum deformed against a master former and the exposed surface ground and polished flat.  When released from the vacuum the plate assumed the correct profile for the corrector.      🙂

It is probably easier to go for a flat surface as the radius of curvature would make the normal testing for a sphere with pinhole and knife edge very difficult. For a 9.25" corrector, with full correction on one side, the ROC would be about 163ft. If half of the correction  is to be on each side then the ROC would be double that. I don't think that I would like to try doing a Foucault test at those distances. 

This is just one of the ways to produce that complex corrector plate. In ATM book 2 they used several polishers of various sizes to figure the plate without the deformation technique. Testing was much more difficult though as they had to assemble the telescope for each test run.

BTW the definition of a flat surface is that it is "the surface of a sphere of infinite radius".

Nigel

Edited June 8, 2023 by Astrobits

[Peter Drew]

I made a small number of 8" F20 Schmidt-Cassegrains wih optics made by Jim Muirden.  Jim hand figured the correctors by the ATM book 2 method that you mentioned.  The secondary obstruction was no more than 25%.  A user who contributed observations of Jupiter to the BAA planetary section was challenged as to how he could make such detailed observations with a 8" SCT assuming that it was a Celestron of that era.   🙂

[rotatux]

On 06/06/2023 at 12:50, Gasman said:

creating a negative pressure behind it to form a concave surface and maybe it could be used optically?

Certainly. As said it has already been done. AFAIK this kind of deformation gives a "chainette" curve, which is a hyperbolic-cosine, which in turn is near hyperbolic and parabolic. 

Other deformation based telescope mirrors about which I read/views in the past 40 years :

• using vacuum behind a spheric mirror to turn it to nearly parabolic
• using mechanical traction with a screw behind a spheric mirror to turn it to nearly parabolic (I think it was in old S&T)