Newby question re mirror making

[SheldonE (Australia)]

Here's a simple question, can a primary mirror be made out of something other than glass? Considering the problems with making a tool, then grinding the glass and the popularity of cheap 3d printers, would a printed blank work. I realise it would need to be silvered.

Thanks,

Sheldon.

[Wavytone]

3D printing is nowhere near good enough, you have a lot to learn about:

- the surface smoothness required to make a useable mirror,
- the dimensional accuracy required to make a useable telescope mirror,
- the mechanical stiffness of the material (ie does it deform under its own weight outside the above tolerances),
- the thermal characteristics of the material chosen, ie does it expand/contract, soften, deform or sag,
- can the material be polished ? (many plastics cannot be polished)
- how to apply a reflective coating,
- whether the material can be coated in a vacuum (metallic vapour deposition) or a liquid bath (silvering process) or not,
- whether the coating will stick or fall off quickly,
- ability to survive cleaning.

Basically you can forget plastics for anything bigger than 5 cm. The main problem is they are too mechanically weak (they sag under their own weight), achieving the desired surface accuracy is next to impossible, and the uncontrollable deformation (warping) that occurs when the material is warmed.

The materials I'm aware of for successful mirrors are:

Speculum metal (copper/tin alloy),
Obsidian (natural volcanic glass),
Mercury (as a liquid, spinning),
Glass, common borosilicate or flint;
Pyrex and variants,
Fused quartz,
Natural diamond,
Cervit, Sitall and Zerodur, which are all glass-ceramics;
"Siliconised" silicon carbide,
Beryllium, aluminium, gold
Optical plastics http://www.plasticoptics.com/optical...materials.html
for which processes to cast or grind & polish exist. Some can be reflectively coated, some can't.

Composites, foams and microsphere arrays have also been tried, finding a way to achieve an adequate polish, accurate figure and apply a reflective surface is the main problem as they out-gass in a vacuum, so a vacuum-deposition process can't be used. Mostly these can be used to create a mirror with a surface created by casting against a polished surface, though the surface accuracy is rarely as good as 1 wavelength - which is nowhere near good enough for visual astronomy.

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA532018

Small mirrors are easy - use glass. Past 15 cm, with glass the mechanical stiffness (it sags under its own weight) and thermal expansion becomes an issue and while thin glass mirrors are used for dob mirrors up to 40cm, Pyrex is a lot better material. Beyond 50cm Zerodur, Cervit etc.

There is one other material whose name I have forgotten - its a low density porous sintered black glassy ceramic, on which a top layer of glass has been applied that is thick enough to grind and polish to make a mirror. It has been around for 10 years or so and has been used for amateur mirrors.

[Satchmo]

Make it at least F7 and it should work fine

[astronobob (Bob)]

Hi Sheldon, 1st Question, Why would you not want to use Glass, it is the proven Medium, though, Mid to late 1900's, astro Mirrors & Tools were also made from Steel, Ceramic Tiles, and prob a few other appropiate mediums in them days ?
BTW, Making the tool, well, that is usually another blank piece of glass anyway, & it comforms to shape as you rough, med & fine grind the mirror blank, Easy its all in the process !
Next, is the need of the Pollishing Lap, Which youll need to hunt down the materials for, normally from the same supplier as the astro mirror Optical quality abrrasives were resourced, with detructions : )
Laps, as far as I know can be made from two mediums, A) Bitumin, which was the more common material for amatures in the late 1900's, and it works fine. B) The newer 'Tropical Lap' made from Resin & something else, memory fails, as its been a while !
Mind you, that these methods are probably most useful for mirrors upto 12-14 inches at a pinch, 18, 20 or even 30" may involve other techniques Im unaware of as I have only made upto a size 12 , ,

I assume you prob know all this, and prob looking at a new method, which, I doubt I could help on that.
Just covering Bases, & hope this has been useful ?

[SheldonE (Australia)]

Thanks for the responses, if I am reading the it right, the main issue is printing accuracy.

Originally Posted by Wavytone

The main problem is they are too mechanically weak (they sag under their own weight)

I'm not sure what you mean here, but there are a few plastic we can print now including nylon and poly-carbonate. What mechanical properties are we specifically looking for?

Originally Posted by astronobob

Why would you not want to use Glass

and prob looking at a new method

Partly, but also cost. Plastic is significantly cheaper than glass, just seeing what materials have been used and if plastics have even been considered.

[DavidU (Dave)]

The accuracy needed for a paraboloid mirror is within a wave length of light.Also a very smooth surface is needed.
http://stargazerslounge.com/topic/95...face-accuracy/

[wavelandscott (Scott)]

I do not know the quality control capability at the moment for 3D printers regardless in time I have no doubt that it will be great (if it is not already). The idea of printing a perfectly shaped "mirror" is very interesting to me.

I was remarking just the other day on how revolutionary in terms of manufacturing this could be. Once the optimal "part" was designed it could be copied endlessly without the need for scaled manufacturing...radical stuff.

When the time comes I wonder if "human hand crafted" will be able to compete with mathmatical "perfection"?

I hope you try it!

[ZeroID (Brent)]

Plastic deformation with temperature change would be a problem. And being a poor conductor getting a stable mirror would be problematic. Mind you, glass is a pretty conductor as well.

[Wavytone]

Originally Posted by SheldonE

Thanks for the responses, if I am reading the it right, the main issue is printing accuracy.

I'm not sure what you mean here, but there are a few plastic we can print now including nylon and poly-carbonate. What mechanical properties are we specifically looking for?

Hehe what you need to find out is how much a disk of the material will sag under its own weight, and how it deforms with temperature when used as a telescope mirror. It is a function of the mechanical properties of the material (young's modulus, density, the thickness of the disk, and the spacing of the supports).

The tolerance is very small - less than 0.1 wavelength, or 50 nanometers. Even a glass mirrors made from say 25mm plate glass, say 30 cm in diameter, will sag under its own weight more than this if supported on just 3 points. Localised thermal expansion with differential temperatures across the disk of just a few degrees C can also cause hills and valleys to form in a mirror that also exceed this tolerance.

No plastic I have ever heard of is dimensionally stable to that kind of tolerance - not even close.

The surface smoothness required is of the order of 0.01 - 0.001 wavelength, 5 to 0.5 nanometres. Optical plastics used in camera lenses and spectacles can meet this by being cast in polished moulds, however its not something you can do as a home hobbyist.

Plastic is significantly cheaper than glass, just seeing what materials have been used and if plastics have even been considered.

They've been tried many times before - and rejected very quickly. You're far from the first to think of it.

If you are trying to make a "quickie mirror", the easiest is know of is to cut a disk of aluminised mylar (as used by florists to wrap flowers) and support it as a membrane by it's edge so that it forms the top across a cup. Using a modest hand pump, suck the air out of the cup and atmospheric pressure will deform the glass into a curved surface. The result is good enough to work as a solar barbecue, but not for forming an image.

You could also use thin disk of a rigid plastic (acrylic, perspex, whatever) or even glass say 1-3 mm thick, like the stuff used for picture frames, it will give a better result than mylar, but again it will but won't be good enough to form a decent image useful for astronomy. Thin disks will also sag under their own weight, and perhaps this will be a good way for you to find out why this doesn't work for making telescope mirrors.

[SheldonE (Australia)]

Originally Posted by Wavytone

(young's modulus, density, the thickness of the disk, and the spacing of the supports)

Thanks, that's what I was looking for, young's modulus really tells the story.

[Wavytone]

Originally Posted by SheldonE

Thanks, that's what I was looking for, young's modulus really tells the story.

Yup, by 2-3 orders of magnitude.