Grinding mirrors with holes in.

[skybadger]

Hi all, 

I'm committing myself to figuring a mirror I've got hold of , a f/3.5 16". The mirror is already partially figured but needs completing.

To get there I'm going to do a 12" first and I've already got a 6" on the polisher. 

My question for now is, the mirror is already pierced with a central hole. Does that condemn me to a half diameter lap at best or is there an established process to use a larger one. I recall texereau cut his plugs out and then reinserted using plaster but that was still at the rough grinding stage. 

The problem will appear when creating a new lap against the existing mirror, it needs a whole mirror to form the lap curve using plaster or cement and has the same needs to press the pitch to find the curve for polishing. 

It occurs to me I could use the 12" lap as long as the f/ratio is the same and I take the larger mirror back to rough grinding to take on any differences . Any other suggestions ?

 

Edited October 26, 2022 by skybadger
Spulling

[davidc135]

Yes, it's awkward. For forming the lap base a rough plug could be fitted and the gap covered with tape. Similarly when first pouring the pitch lap. The  centre of the lap could be further pressed/flattened out of contact when polishing/figuring.

A window with a central hole that I figured suffered a 1/4 wave or so surrounding zone that stayed put. Not too big a deal but I'll redo another window and this time cut out the centre after I've finished.

David

[Astrobits]

A 16" f3.5 parabola is very steep and a polishing tool greater than 1/2 dia will struggle to get the figure. You should try using the 1/2 dia lap and see how that progresses. You will almost certainly need even smaller laps for final stages of figuring.

Nigel

[skybadger]

Thats actually a plus! I'm glad you mentioned it. 

When I finish the mirror box I'll be able to safely take  pictures of the mirror on edge to work out its current state of figuring.  Do you have a recommendation on how to form the  lap - I could prop the mirror level for the radius of interest and then warm press on that (remembering I have a central hole to manage ) but there will be an underlying asymmetry in curve from that. I guess no greater from any moulded lap used off centre though. 

cheers