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progress on grinding for 12.5 mirror

Started by BABOafrica, 10/26/2012 11:26PM
Posted 10/26/2012 11:26PM Opening Post
Here's a picture of my 12.5" mirror. I finished #400 grit. I'll be moving on to the #800 soon.

I had gotten to #800 once already but got a nasty little scratch. I'll have to be more careful to clean out all the little crevices from the tool. I learned the hard way that the "sucking" action with such close contact between mirror and tool at that stage sucks some hidden grains of old grit into action.

But it was just as well. I decided to go back to #100 grit and grind down the sagitta a bit more. Now I'll have a f/5.9 mirror instead of a f/6.6 mirror.

Clears,
Joe

Attached Image:

BABOafrica's attachment for post 56596

In lumine tuo videbimus lumen.

8O Home-made 10” Dob / Home-made 4” refractor

EPs: Konig 32mm (1.25") / Zhumell WF 30mm (2") / Nagler 13mm T1 / Orion Sirius Plossls 25 & 10mm / Zhumell Plossl 9 mm / Meade MA 9mm
Posted 11/03/2012 10:30PM #1
It wouldn't hurt to do a sharpie type test on the mirror now you are closing in on polishing time.

I use a thin slurry and move the tool over the mirror a few times in all directions after drawing a grid pattern with a permanent marker on the mirror blank surface.

If you see for example the mirror is in contact at the edge but not the middle then you need more wets tool on top to bring them together and vice versa when the middle is in contact and not the edge.

Starting polishing especially on a big piece of glass like you have and seeing grey not go away through a non spherical surface can be disconcerting but it is easily avoidable.

I do sharpie type tests from 25 micron down and polishing always goes well.

re the scratch we all get them I now cover the tile tool with a pizzabox type setup if I'm leaving it for any length of time. You can't be too careful.