Re: [ATM] fishing for a lil polishing advice!

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Jerry wrote:
> Let's see if I can start a firestorm...   Forget all you have ever
> heard or read about turned down edge. It isn't caused by what you
> think. That would be what you have read in books and on the web. 
> Having forgotten about turned down edge, you will be free to learn
> to figure. You do that by correcting zones. That is the most
> important thing to learn about figuring.

I like that advice very much.

> I think I just recently told someone... Contrary to common belief
> the edge is a small area of the mirror. It is better to have a
> turned down edge surrounding a smooth accurate parabola than to
> have a perfect edge that is only a band around a lousy parabola.

Yes, it is a small area.

The main problem with TDE is that it is a large slope error, so light
is scattered quite far from where it should go.  Bad for planetary
viewing, bad for viewing faint objects near bright ones, and very bad 
for CCD imaging (stars bloat).

> Truth be told, I would have to recommend learning to figure by hand
> first.

Agreed.

Turned edges receive excessive attention.  A mask will cure them, and 
so will a diamond hone or grinding stone.  When the rest of the mirror 
is good, that's the best way to be rid of them - just abrade them away 
gently.

That said, I am certainly not going to tolerate one on a mirror I am 
working on, though.

	Mike Lockwood

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