Circular grinding - telescope mirror

[Chriske]

Hi guys,

It's about 42+ years I'm building telescopes and grinding mirrors.
Grinding mirrors, especially deepening the mirror with course carbo, I always push the mirror using very long W-stroke most of the time concentrating on the edge of the tool.

Thinking about 'concentrating on the edge of the tool', (while I was grinding..😁)I thought why not work a fully 100% on the edge of the tool avoiding the centre of my tool completely. There's only one way to do that and that is using circular strokes with the mirror on top of the tool, not touching the centre of the tool at all.!
And so I did...

Did it work and most of all is there an advantage using these circular strokes movements...??
The answer is yes/yes/yes
Indeed it works, and what's more this way of roughing out is the fasted way of (manually)grinding I ever did...😳
Even the classical very short straight strokes concentrated at fully 100% edge of the tool does not work this fast.
The third 'yes' is there because rotating mirror and/or tool is not even necessary. I still do from time to time because I'm used to it all these years, and just to be on the safe side.

Why didn't I think of it before...🙄
Maybe someone tested this way of working before, don't know. Never read about it.
Did a search on 'circular grinding telescope mirror' but didn't found a thing about it.
Except someone on the net(YT) suggested using circular movements parabolizing a telescope mirror.

Anyway, the first mirror I did this way, while roughing out, I overshot my target completely.
So if you try this, measure very often...!

[Chriske]

 

 

[davidc135]

Yes, it's a very fast technique although as the circular strokes get further out relative to the lower disc I've found that the two discs lose contact at their centres. And so some extra time is needed using just a slight 'throw' to true them up. Or a session with some CoC strokes.

I've been working two 6'' discs destined to be Schmidt corrector plates and have used just off centre circular strokes during 400 and 9micron stages to flatten the B270 float surfaces. I found that the edge zone was excessively abraded during fine grinding with this technique and added another half dozen wets with straight strokes to clear it up.

David

[Chriske]

Well doing some more tests I found that the centre is indeed much deeper then the rest of he mirror. As a matter of fact it is exactly the same phenomenon as a regular long straight stroke during roughing. It also needed some 'repairing' at the end of the deepening session just the same as in the past.
To correct this I did not use the classical third stroke as we always do but continued using circular strokes. But this time I used far smaller circular movement. And guess what, the correction using a shorter radius movement did took far less time to end up with a correct sphere.

It's all new to me too, but it looks promising...
 

[Chriske]

Apparently I'm not the first to do this circular movement thing.
A guy in the Netherlands used it and still do. And even posted animations about the technique he uses on YT.

[Chriske]

 

[skybadger]

Very interesting. Tx