Possible mirror blanks

[Bentley]

HI everyone, 

I salvaged 2 mirrors from a couple of optical comparators. one is a 17" diameter that's 1.250" thick and the other is an 8" diameter and 1/2" thick and to me I think they are plate glass. Both are dead flat. With the proper support, would they be usable? I'm thinking of using them in the future. For my 1st mirror making adventure I think a mirror grinding kit is the way to go. Any advice is welcome.

 

Dave

 

[Demonperformer]

You can make mirrors from plate glass, but that does not have the low-expansion qualities of other products. Also, I think you will possibly find them a little on the thin side. 400mm (about 16") mirror blanks are usually about 50mm (2") thick, I believe. That said, they could be good practice ...

[Astrobits]

The 17" is fine at 1.25". But the 8" is a bit thin if you have no experience. My 16" is 1" thick and performs fine. Go to Mel Bartells site and see how to make a 25" which is 0.5" thick.

Mel's web site:  www.bbastrodesigns.com

Nigel 

 

[andrew s]

If they are high quality optical flats then they might be worth a significant amount.

Do you know what they were in and or the surface quality?

If they are good you could sell them and get low expansion blanks or buy a mirror.

Regards Andrew 

[Demonperformer]

2 hours ago, Astrobits said:

The 17" is fine at 1.25". But the 8" is a bit thin if you have no experience. My 16" is 1" thick and performs fine. Go to Mel Bartells site and see how to make a 25" which is 0.5" thick.

Mel's web site:  www.bbastrodesigns.com

Nigel 

 

I stand corrected.

[Astrobits]

Andrew is quite right.  If they are optically flat they will be of more value than same size mirror blanks. You just need to get them tested. If they are coated then it's tricky to do the tests. If they are uncoated then carefuly put the smaller one onto the larger and allow them to thermally stabilise for a few hours. You should be able to see fairly straight interference bands if both are optically flat. This won't work if only one of them is flat. 

Nigel 

[Bentley]

Hi. Thanks for the replies. The 17" mirror came from a 30" Jones and Lamson optical comparator that had magnification up to 100X. I would imagine that its dead flat, the front surface coating is pretty bad. Maybe it is worth something.  The other one came from a no-name comparator. I did remove the coating with ferrric chloride. The coating on that one was all but gone. In any case I want my 1st mirror to be 6", Should I go with a kit or just by the supplies individually?

[Astrobits]

As you don't have any 6"  glass you might as well get a complete 6" kit.  The 8"  x 0.5" mirror you have would be a suitable tool for making an 8" f/8 mirror.

On the Cloudynights forum there is some discussion on this topic and one small mirror there  is claimed to be accurate to 0.2 wave.  However, I have my doubts that the 17" is anywhere near that level of accuracy, I do not see that the design of an optical comparator would warrant the high level of accuracy needed in an optical reference flat, and consequent cost.  Perhaps your local astronomical society can help you to get the mirrors tested.

Nigel

 

[Bentley]

Thanks, I'm always scavenging items for future use. Well I have it in any case. I'll put it on the shelf and get a kit.