Re: [ATM] Initial large thin mirror grinding question

["Mel Bartels" <mbartels@bbastrodesigns.com>]

> I am lucky to have a library that has a rather complete book on how to
> grind telescope mirrors. It's called Standard Handbook for telescope
> Making by N.E.Howard. He is very thorough on the procedures involved.
> Since this is my first attempt at grinding a mirror, I'm just trying
> to get the big picture down as to how all this works.

Howard's book is excellent and I highly recommend it to this day over other
sources.  However, if you follow his recipe you will likely fail in large
thin mirror making.  His recipe is tuned to smaller 6:1 ratio motors.

> Are thin mirror ground MOT first? Or are they ground on the bottom to
> have maximum support? Would it be be beneficial to grind a large thin
> mirror on an 18 point support system similar to a mirror cell?

Thin mirrors are ground TOT with subdia tool.  Support the mirror on a
flexible backing.  Do not use a 18 pt support.  The extra friction caused by
pressing down when grinding will likely give a bad result.  Just use a
backing like Kestner and I recommend - keep it simple. 

> Can anyone recommend a thorough complete text on gringing large thin
> mirrors other then Mel Bartell's site?

In the back of Berry's The Modern Dobsonian is a treatise by Kestner - also
see his articles in the old Telescope Making magazine.

Mel Bartels


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