Re: [ATM] Two mirrors on one tool

[Ken Hunter <atm_ken_hunter@yahoo.com>]

Paul...
 
Alternating the mirrors on the tool will keep the 2 mirrors nearly identical in focal length.
Bear in mind that the focal length (of both mirrors) will always be decreasing if you stick to MOT. 
 
By alternating them MOT and TOT, you will be able to control the focal length changes and reach or remain at your desired actual focal length. 
 
It's common to find that you will have to spend more time at TOT to keep the focal length steady as there is more AREA to change in the outer parts of the glass when working TOT... About 4:1 area wise therefore 3-4 times longer at TOT than MOT to keep the focal length steady. 
 
All this is at the 1/3 diameter total stroke length.
 
Ken Hunter


Paul Bradley <paul@paul-bradley.com> wrote:

Hi all,

I'm making 2 mirrors which should be as near identical in focal length as I can make them
as they are to be used in a pair of newtonian binoculars.

I've read about people who have done this by using the same tool and doing alternate wets on
each mirror. That's what I'm doing, but some people said to alternate MOT and TOT as well.

Wouldn't this tend to keep the focal length around where it is? I've been working exclusively MOT. 

I'm only about half way through hogging, so it's not to late to start alternating MOT and TOT if I
need to, should I do this or will I still be keeping the focal lengths sufficiently similar by working
MOT? If it is OK to work MOT, could I do this throughout fine grinding too if I wanted to without
encountering problems?

Thanks

Paul
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