Could you spin mold amateur mirrors?

[NickK]

http://www.npr.org/2012/01/26/145837380/want-to-make-a-giant-telescope-mirror-heres-how

Now obviously health and safety.. but does anyone do this? Get a blank and heat it then spin and cool?

Just curious..

[Astrobits]

In principle, yes, but is it worth the time and effort to do? I guess it depends on what you call an 'amateur' mirror

You will only be eliminating the coarse grinding stage. I used to take 1 1/2 hours for an 8" and 2 1/2 hours for a 10" to coarse grind to curve. Heating, spinning and cooling blanks of these sizes will take days if not weeks.igel

N

[NickK]

Yup, I was wondering if the curve accuracy you get initially would lead to just the polishing stage.

[cassiewoofer]

I don't think you would get that level of accuracy.

However having recently read of mercury telescopes.... it does make you think,......

I've never seen an image taken with a mercury telescope, and how practical are they really? or are they just a demonstration of centripetal force and parabolic surfaces?

Will have to go and google!

[FraserClarke]

It wouldn't be worth it for small mirrors. It's only to save the curve generation stage (and, I presume, the amount of glass you get rid of, which is not an insignificant cost on an 8m mirror!).

More interesting for amateurs is slumping; where a flat piece of glass is heated up and 'slumped' over a former (e.g. a convex piece of plaster). The glass doesn't fully melt, just softens and 'droops' with gravity. This is how the mirrors for the VLT and Gemini were generated before polishing. It has a similar effect to spinning, and would allow you to use much thinner pieces of glass. A 20" x 1" thick mirror at F/4 would become quite attractive, as you wouldn't have to remove a huge amount of the glass from the centre.

It's something I've thought of trying to do, but priorities and all that... You'd still need a good kiln with proper temperature control, as you'd need to cool the glass down in a very controlled fashion to anneal it properly... I doubt the equipment is beyond the realms of someone already into pottery and/or ornamental glass work...

[Glasspusher]

Fraser, I have been experimenting with slumping and fusing for some time now with mixed results. As you suggest a 20 inch x 1 inch thick meniscus (slumped) blank has interesting possibilities, a meniscus is an inherently stiffer structure than a traditional flat blank. Going bigger than a 20 inch would involve fusing thinner disks to build the structure and then slumping. This is exactly what I am working towards. A pottery kiln is not ideal for this as the heating elements are located in the sides of the kiln producing localised heating at the edge. With glass a more uniform heating is desirable for obvious reasons. A glass fusing kiln has elements in the lid as well as the sides of the kiln resulting in a much more uniform heating of the glass. I have attached an image showing a 20 inch fused blank.

John

[NickK]

Only thing is looking at the equipment - to make a 20" slumped mirror would require a kiln such as this http://www.creativeglassguild.co.uk/prod/hobby-fuser-2-glass-fusing-kiln-kilncare but much larger (the example has 200mm internal space - good for an 7-8" mirror).

[Astrobits]

Look further down that page and there are a couple of much larger kilns, one which will take up to 1000mm x 800mm, so big enough for your proposed 20". At £4600 you will need to do a lot of mirrors to make it worthwhile. However, as they do ones that size there might be someone who owns one that will do the work for you.

Nigel