Slumped mirrors?

[GTom]

I am thinking on grinding my own lightweight mirror (first f4 16", later f4 24"). The lightest and cheapest option is to get a thin blank and slump it in a decent kiln.

Anyone has longer term experiene with slumped mirrors? Overall doesn't seem to be more work than a normal (not pregenerated) blank.

Do I have to grind the backside as in case of normal flat back mirrors to avoid astigmatism? How do people support a convex back while grinding? Does it make sense to grind a hole in the middle for additional support? I am remotely considering a convertible Newtonian/Cassegrain system anyway...

Edited January 14, 2020 by GTom

[Glasspusher]

Hello, I have slumped my own blanks in a kiln but gave up due to the expense of running the kiln. To properly anneal a large blank takes many hours of controlled cooling which is expensive. You then have the problem of supporting the meniscus blank during grinding which is not straight forward.  It is more economical to go the monolithic blank route which is well tried and tested. Hope this helps.

John

[GTom]

Thank you for your thoughts John. The ultimate goal is a portable big Dob like Mel Bartels. Portability is an absolute must for me, a conventional 1.5" thick ("thin") 24" is already too heavy.

[Merlin66]

I agree with John. Trying slumping a couple of 12" disks, one was about 6mm thick the other 10mm (?) Used the services of the local ceramic group and their Kiln.

Making a "proper" mould to slump into proved difficult, temperature control/ annnnealling became a PITA!

Supporting the disks for grinding - sand bags/ air bags/ various disks of carpet didn't work for me.

An alternative was to vacuum distort a thin blank (which already had a "reasonable" finish) - supported on an edge O ring within a closed cell and hand vacuum pump connected to the rear. Yes, I think it would have eventually worked, getting around f10.

Don't even think of stretching a Mylar mirror and vacuum!!!!!