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Molding Parabolic Primary from Acrylic or other plastic


PeterDavidowicz
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Hello stargazers!

What are your thoughts about making a primary with the following method?

Place a circular piece of mirrored acrylic on top of a (near)-parabolic mold of wood (of other material), and gently raising to the softening temperature of the acrylic until it 'melts' into the mold, then letting it cool.

You'd have to apply a mathematical calculation to adjust the mold shape to account for the stretching (and therefore changing width) of the expected width of acrylic, and possibly also for the stresses/stretches as the acrylic cools.

And you might need to make your own mirror out of a circle of mylar atop another plastic, if a suitable acrylic mirror can't be found.

But that's it, your mirror should be good to go, and an extremely cheap DIY telescope with large aperture could be made with this simple primary mirror.

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If that all can be done to an accuracy within a fraction of the wavelength of light and a plastic mirror can be made strong enough to not deform more than a small fraction of the wavelength of light when mounted in a telescope then I don't see why not.

I suspect that all may be a bit tricky though.

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I would have serious doubts with respect to the thermal stability and rigidity of such a mirror. There are optical quality plastics, but I would prefer to cast an anneal them properly, and grind and polish the mirror to avoid thermal stresses. Stretching of the mirrored surface would get you into all sorts of trouble regarding polarization effects. Even if you could avoid these problems, the figure of your surface would match that of the wooden mould, i.e. it would be pretty dismal. Figuring would be needed afterwards. After that you would need to apply a new coating to the mirror.

In short, I do not think it would work

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There are several reasons: Aluminium is not as stiff as glass, so will bend more easily. Thermal coefficient of expansion are another issue (which is why Pyrex, fused silica, and zerodur are preferred). It is also soft, so scratches readily (even my first mirror had a scratch-resistant coating over the aluminium to prevent scratches).

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Have you seen the price of thick aluminium lately? :eek:

Not nearly as bad as in the late 1800s. Emperor Napoleon III had special set of cutlery made from aluminium, it was more expensive than gold, I gather. In those days the idea of a thick slab of aluminium would be similar to a solid platinum mirror :eek:

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When i first joined this forum amongst one of my many stupid ideas was the idea to make mirrors from carbon fibre. the advantages for say a space payload are obvious I still think it may be possible one day perhaps with a graphene or sillithene overcoat to take the fine figuring and coatings all I need is a few million to research the idea from a beach in the canaries. welcome to sgl by the way

Edited by rowan46
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Hello stargazers!

What are your thoughts about making a primary with the following method?...... But that's it, your mirror should be good to go, and an extremely cheap DIY telescope with large aperture could be made with this simple primary mirror.

Its a crazy non starter... For the above and lots of other reasons...

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I suggest that you try it and then report back as to how bad the image is ( assuming you can find an image ).

I seem to remember that Acrylic was tried sometime in the 70's using the 'conventional' mirror making techniques. The best material for the lap was Plasticine. I have never heard another word about Acrylic mirrors for Astronomy. I wonder why??

Nigel

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Perhaps good enough for one of those shaving mirrors :-)

From my 3D printing experience I'd say the warping alone when the material cools will make it impossible to stay anywhere within the wavelength-accuracy required.

I did read a article about 3d printing magnifying lenses / casting them from a negative. Perhaps it would work better now with one of the SL/resin printers...

IF plastic mirrors or lenses where so easily made I am sure a company would produce them for amateur astronomy to reduce the cost :-)

Take some toy binoculars and you know it's possible but even at low magnification it's hardly usable. And those are manufactured with injection molding that is usualy more precice then what you can achieve at home.

I think the best use for plastic at home is with a 3d printer; Printing adapters, focusers, parts... and perhaps some mirror grinding tool/machine that spews out small, long focal length mirrors to get kids started into the hobby for little money. :-)

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And don't even get started on vacuum mylar mirrors!!!

Good energy collectors but NBG as an optical quality telescope.

The other alternative is a vacuum slumped thin glass disk - limited f ratios, but seemed to work OK....

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For the record I know It's a pretty very crazy idea XD, But I haven't read much about it. I'd love to read any material anyone has on similar attempts. I'd like to give it the best shot I can, even just to say it absolutely won't work ;)

Perhaps good enough for one of those shaving mirrors :-)

From my 3D printing experience I'd say the warping alone when the material cools will make it impossible to stay anywhere within the wavelength-accuracy required.

I did read a article about 3d printing magnifying lenses / casting them from a negative. Perhaps it would work better now with one of the SL/resin printers...

IF plastic mirrors or lenses where so easily made I am sure a company would produce them for amateur astronomy to reduce the cost :-)

Yes, I was hoping that with a really well machined (or 3d printed) mold, and some careful sanding, you could get within macroscopic tolerance, and the thickness of the deformed acrylic could correct for microscopic imperfections.

Assuming you could get such a good enough mold, you could (pre-emptively) mathematically correct the mold's shape for the expected elastic thermal expansion strain due to cooldown of the material to operating temperature.

For thermal expansion strain during operation, could it be minimized by having a small thickness, or two layers sandwiched?

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I enjoy the speculation. I claim independent (thought) invention of the "Mercury Scope". <G> If you've got a large shed, decent foundations, breathing apparatus (optional) etc. But I have vague memories of spinning liquids, "araldite" etc. in Scientific American(?), back in the 70s. Various "novel" techniques (Wikipedia) have been around for quite a while, it seems. :)

ALthough the Meccano trusses looked cool, I have to admit my 6"

"shaving mirror Newt" was not a rip-roaring success either, but... :p

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I enjoy the speculation. I claim independent (thought) invention of the "Mercury Scope". <G> If you've got a large shed, decent foundations, breathing apparatus (optional) etc. But I have vague memories of spinning liquids, "araldite" etc. in Scientific American(?), back in the 70s. Various "novel" techniques (Wikipedia) have been around for quite a while, it seems. :)

ALthough the Meccano trusses looked cool, I have to admit my 6"

"shaving mirror Newt" was not a rip-roaring success either, but... :p

You are doing very well for your age, Macavity :grin: The first description of a liquid mercury telescope was in 1850! http://home.europa.com/~telscope/LMT.txt

Although spinning liquids to produce a parabolic shape was noted by Newton.

Nigel

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I'm most worried about the thermal expansion during operation. If anyone has any ideas to compensate or correct for it, I'd love to hear them :D

I'd also love any resources anyone has on the physics behind the thermal expansion stresses of lenses.

Some possible solutions to correct for it:

#1 Multiple removable/replaceable primaries,designed to have the correct shape at each temperature. You'd need multiple molds and multiple mirrors.

#2 Use a heat strip to control the temperature, with the 'barrel' of the scope covered with an insulating material. Big problem here would be the power source required, and the additional cost of the heat strip, I'd probably design it for a car battery)

#3 Make a 'brace' around the outer edge (or inside a groove beveled inside the outer edge) with a ring of some material to constrain the expansion (or contraction)

#4 Really thin mirror, but then you still need a structure to hold it with (machined to parabolic shape, and still with the same expansion problems)

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I'm guessing the lack of plastic primary mirrors is a pretty good indication that it won't work :(

surely if it was possible to make cheap, quality plastic mirrors, skywatcher, celestron, meade etc would be pumping them out by the truckload ?

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I'm guessing the lack of plastic primary mirrors is a pretty good indication that it won't work :(

surely if it was possible to make cheap, quality plastic mirrors, skywatcher, celestron, meade etc would be pumping them out by the truckload ?

I totally agree with what you're saying. But I'd also like to argue that the market for telescopes is rather small (sadly), so much so that the amount of profit to be made by selling more expensive lenses outweighs the consumer benefit of cheap products. There's also the perceived inferiority of 'cheap', but well-designed products that can really hold a product back. I have a 10$ toaster that I swear is the best one I've ever used, but the market for 10$ toasters is pretty small.

SO! I think it can be done, hopefully it just requires some engineering research. The big thing I'm buying in on is cheap high quality plastics that have saturated the market only in the past 10-20 years or so, as well as cheaper methods for applying the reflector.

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