From: "CSC" 
To: "Atm" 
Reply-To: "CSC" 


Didn't I hear of a method by someone who does this professionally?   Like a
week ago?

The technique mentioned polishing in a certain amount of toroidal shape
then final touching up.

This makes good sense to me, because the general shape of this off-axis
disk to a rough degree is dominated by two different radii at right angles
to each other.

Jose sasian uses a toroid for the secondary in his off-axis newtonian, and
discusses his methods for creating toroids.

I'll be that creating the toroid will get you much closer to the shape you
want than the method you propose.  Your method is for touchup and final
tweaking.

The problem mentioned by another ATMer of not being able to locate the true
axis is a beaut!  Avoid that by marking the disk.

Colin

-----Original Message-----
From: owner-atm{at}shore.net [mailto:owner-atm{at}shore.net]On Behalf Of
Jerald F. Wright
Sent: Tuesday, December 10, 2002 1:54 PM To: Nils Olof Carlin; ATM List
Subject: Re: ATM re: Herchellians #2




    I am getting ready to start figuring on my 6".  This last weekend I
pried the mirror and lap apart after a few years of "thinking"
about it.  I must not have thought much.  I thought that mirror was f 8.
Turns out it is more like f 10.  That is better for what I'm doing so I
won't go back to grinding.
    The ROC is 118.5 inches as best I can measure right now.  I'll nail
that down better when I get the test rig set up.  The radius is measured
close enough to lay down the basic plan. The axis of the parabola will be
2.25 inches from the edge of the mirror so the mirror will be a segment of
a 16.5 inch f 3.6 parabola. Across my 6" part of that parabola I will
have almost .5 inch knife travel from one side to the other on the mirror.
    As for figuring I will start by just wearing down one edge to fit
the outer part of the parabola.  I have good idea in my mind of what shape
I want in the glass and what it should look like in various tests. I have a
lot to wear away on one  side so I will have time to learn to adjust the
stroke to wear the shape I want into the glass.  I will probably go at the
whole thing as lengthening the radius on one side and doing that
progressively less across to the other side. Its easier for me to visualize
how to lengthen the radius. I have a hard time thinking of a way to shorten
the radius near the inside edge of the glass closest to the parabola axis. 
I will try some different strokes to shorten the radius on that side but If
I can't find one that works I think I can just lengthen radii
progressively. The more I think about it the easier I think it will be.
    It will probably take about a week to get the test rig set up and
clear space to work. Then I will pour a lap and start stroking.  I'm
thinking when I actually get figuring that in one or two weeks I will know
how hard it is to make a good off axis parabola.

Jerry

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