Polishing pads

[dark star]

I am polishing a 20 inch mirror. I had a wide turned down edge. I am spin polishing on a fixed post machine using a 15 inch pitch lap. After around 15 hours of polishing the TDE has reduced in width and is right at the edge. But after more hours of polishing does not seem to be getting any better. I am using a over hang of the pitch lap varying from 2 inches to 3 inches, most of the time I have been using a 15% of the tool over hang, as seems to be recommended with fixed post machines.

I have now picked up some shallow scratches. I think maybe from a wire brush I was using to scratch the pitch lap, some bristles may have come off and got stuck to the lap-although I don't know how as I always scrub the pitch lap with water and a brush before using it on the mirror.

Does anyone know where to buy polishing pads for optical work in the UK? I have found some on line, but it is not specified if they are for optical work on glass. Has anyone used them? I would try to polish out the scratches with them. If this does not work I will go back to fine grinding using 320 grit. I will need to make a new coin tool.

I could then use the polishing pads to get a quicker polish again before going back to using a pitch lap. Some people have said that polishing pads can lead to TDE, if this is the case I obviously don't want to use them!

I am waiting for some grit to be delivered in case I have to go back to fine grinding so am going to try some more polishing with a 3 inch over hang and see what this does to the TDE. It is of course possible that the TDE is so bad that it will never go away.

But it is strange as a few months ago I took it to a telescope making class I go to. There is someone there who has been making and testing mirrors for many years. I appeared to have a large over corrected outer zone and under corrected inner zone but no TDE. It seems that I got the TDE when trying to figure. I think I did W strokes that were too short. This was at home-not at the class. I only take the mirror to the class occasionally as the class is after work and I have to take the mirror and a large test stand with me in the car-I am usually too tired to do this. 

David