Mirror materials

[jambouk]

Someone asked me why glass was used to make telescope mirrors, especially massive telescopes. “It would be much lighter if they could just put a reflective aluminium layer on a lighter material!”

“So why not make massive mirrors out of carbon fibre, shaped and buffed to a high degree and then coat that with a reflective surface? Or other materials?”

The only thing I can think about is the smoothness of the grind, and the thermal stability of a massive chunk of glass, but these may well not be the reasons.

Thanks for any help.

James

 

[vlaiv]

Reasons that come to mind:

1. holding the shape - rigidity (carbon fiber might bend under its own weight for example).

2. ability to be polished to needed degree (ease of figuring it)

3. Thermal stability (but also thermal expansion, some glass types are better at this then others - have expansion factor close to zero)

 

[johninderby]

Good article here.

https://www.cmog.org/article/reflections-glass-telescope-mirrors

BTW glass isn’t actually a solid but has properties of a liquid even though it isn’t really a liquid.

Edited October 7, 2019 by johninderby

[Davey-T]

16 minutes ago, johninderby said:

BTW glass isn’t actually a solid but has properties of a liquid even though it isn’t really a liquid.

Read an article somewhere a while ago saying that light hit the mirror and sent out ripples like a pond that bounced back off the mirror edge ?

Dave

[Captain Scarlet]

2 hours ago, jambouk said:

“It would be much lighter if they could just put a reflective aluminium layer on a lighter material!”

They've done just this for the James Webb I believe: a reflective aluminium gold layer on Beryllium...

[johninderby]

Beryllium is a metal with unusual properties that make it great for telescope mirrors. So why isn’t used instead of glass in most telescope mirrors. Cost is roughly $748.00 per 100 grams and is also a deady poison. 🙀

 

How and Where the Beryllium Mirror is Made

The beryllium being used to make the Webb Telescope's mirrors was mined in Utah and then purified. The particular type of beryllium used in the Webb mirrors is called "O-30" and is a fine powder of high purity. The powder is then placed into a stainless steel canister and pressed into a flat shape. The steel canister is then removed and the resulting chunk of beryllium is cut in half to make two mirror blanks about 1.3 meters (4 feet) across. Each mirror blank will be used to make one mirror segment; the full Webb mirror will be made from 18 hexagonal (six-sided) segments.

Once the mirror blanks pass inspection, they are molded into their final shape, polished and temperature tested to ensure they can withstand the frigid temperatures of space.

Beryllium is much more capable than glass to handle the frigid cold of space. The James Webb Space Telescope will face a temperature of -240 degrees Celsius (33 Kelvin). Beryllium contracts and deforms less than glass -- and remains more uniform -- in such temperatures. For the same reason, the optics of the Spitzer Space Telescope were entirely built of beryllium metal. It is thanks to beryllium that the James Webb Space Telescope will be able to see further back into the universe and back in time than any other space telescope operating today.

[DaveS]

Which is one of the nastiest materials in common use,

[johninderby]

16 minutes ago, DaveS said:

Which is one of the nastiest materials in common use,

I had some model boat propellers once made with Beryllium which came with dire warnings not to grind or reshape them.

Edited October 7, 2019 by johninderby

[DaveS]

Page from an old copy of Industrial Toxicology

Not something I'd want to get involved with, even as a chemist.

[Carbon Brush]

Ref John. The propellors were probably a Be/Cu alloy. No hazard until ground up and inhaled!

Be/Cu alloy is often used in electronics to make springs with excellent electrical conductivity.

For James Webb. I think the $$$$ per Kg of payload launch cost would justify thinking of materials other than glass.
The effort of lifting a big dob into the garden is a bit different.😁

David.

 

[Robert72]

Yes, carbon mirrors. I read today that future Earth based VLTs in the 30 - 100m range will use Carbon mirrors, as glass at this scale is a non starter.

I don't think stiffness is an issue, but as vlaiv states, thermals and grinding may be problematic.  Also mirroring may be an issue.  I believe they use Rhodium for such mirrors.

I don't know why the big makers have not started yet, it is probably just quicker to go with known technology than with a relative unknown like this.

Before I go, let me paint you a picture......

Large, deep section, honeycomb cell backed, carbon fibre mirrors with a Rhodium deposition.  A fraction of the weight and cost of current 12" mirrors,

with large 1m+ mirrors possible for a modest sum and mirrors weighing a few kilos.  Available for ATMs to use with large, lightweight portable scopes.  This in my mind is the essence of ATMing and it is the ATMs that will

push this technology forward along with the VLTs.

This will possibly be the next evolution of the Dobsonian philosophy, increasing large telescopes availability and usability even further.

Am I off the mark? Possibly, but we can all dream.

Would be a fantastic project, something I am seriously considering tackling next after the carbon tube. 

Edited October 7, 2019 by Robert72