Re: [ATM] Which tool for a 14 inch blank ?

[Mike Lockwood <melockwo@uiuc.edu>]

Eric,

Eric Royer wrote:
> I'm starting a new project : it will be a 14 inch f/5 binocular 
> telescope. The first step is to make the mirrors. So far I've ground 
> only small mirrors (6 and 8 inch). I wonder what is the best tool size 
> for a 14 inch mirror : full size or smaller ?

Well done on choosing a challenging project.

There are a number of techniques for hogging out mirrors.  If you can 
afford it, pregenerated curves on both blanks will save time.

After hogging, using a single full size grinding tool for both mirrors 
gives you the best match of ROC for the two mirrors.

The size of the polishing lap depends on you - a 14" lap is not easy 
to push (as Richard Ozer pointed out), and subdiameter laps work 
slower (because you're polishing less of the surface with each stroke).

If you're in good shape and like a workout, 1 hour with a 14" lap in 
good contact will be doable.  I did it with my 16".  (The grinding 
stand had 100 lbs of weight on the base, but I was still rocking it.)

Figuring of a 14" F/5 can be completed with a full-size lap. 
Personally, I'd probably use a full-size lap to get about 80% 
corrected, and then switch to an 8" lap to sneak up on the final 
figure.  However, working with a subdiameter figuring lap requires 
patience and practice.

> I think I will use a plate glass tool, because I don't have a garage and 
> I don't want to mess with mixing plaster in my living room. The intended 
> mirror blanks are 350mm (14 inch) in diameter and 38mm (1.5 inch) thick.

I also recommend using a plaster tool.  If the curves are 
pregenerated, a single plaster and tile tool (costing ~$20) can be 
used to grind both.  Also, you can dig the plaster out of the tools 
(thereby channelling it) and this greatly reduces the chance of 
"locking" the mirror and tool together, and makes separating them 
after a wet of fine grinding much easier.  (A 14" tool is heavy, and 
separation can be delicate at 5 micron.)  Also, I think I remember a 
claim by someone that channelled tools work faster due to better grit 
distribution.

	Mike Lockwood

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