Secondary making

[woodbob123]

Has anyone made there own secondary mirror?

Whilst getting my head around figuring my 600mm mirror and making a tester decided that I would go for a complete DIY build. From memory surface plates would be made in 3s alternating on top and bottom to cancel out any curve. Would this method work ok? If not where can I find info on grinding, polishing and of course testing. Also do I cut blanks round or square does it matter? Starting from no knowledge so any info, ideas would be great.

Bob

[Glasspusher]

Hi Bob,

Making a secondary flat mirror is not easy. For a one-off flat I would suggest that you get hold of three plate glass disks all the same diameter. The disks are ground on each other in the sequence 1 on 2, 2 on 3 and 3 on 1. I generally grind for about 10 minutes before going onto the next pair in the sequence, keep repeating until the grind is complete then move to the next abrasive. The advantage of plate glass is that it is fairly flat to begin with so the first abrasive can be 400 silicon carbide.

After grinding, each of the flats is polished and they are tested on each other to find the curvature and flatness. There is some software on the net to make this testing easier, sorry can’t remember the url. I think the software is called threeplanes, or something similar.

When one of the flats is good enough you can either trepan out an elliptical or cut it into an octagon, which is a little easier. This is just a quick overview of the procedure, if you decide to have a go I can provide more details.

Hope this helps.

John

[woodbob123]

Thanks John will be back for more info when I get started, have to build a small grinding machine first and sort out a trepanning cutter to fit in my pillar drill , what material is best for the cutting/grinding part of tool.

Bob