Re: [ATM] TDE (That Darned Edge)

["Jerry" <wa4guu@verizon.net>]

Boy!  The pain medicine must have been working when I wrote that description
below on figuring. I should have just used 4 numbered zones such as used in
Texereau' standard 8" testing in "How to Make a Telescope".  What I said
seems to be correct but how I said it is awful. I doubt any one could follow
it. Probably just as well. Maybe Robert has tried it (what he thinks I said)
and he can report back...

I have collected a number of ideas on making a good edge. I will have a sack
full of things to try on my second mirror.  That is the one with the perfect
edge.

James Mulherin's direction of the figuring an 18 inch mirror had some
mention of making a good edge. I went to the webpage of Optical Mechanics,
Inc. and looked at those edges.  Real nice and of course the figures too. If
I could get an edge good enough for him to have to think a moment before
rejecting it, I would be happy.  

I am going to re-read the figuring of the 18 inch by John Abrahamian.  I
think that is posted on the atm_free Yahoo group. If I remember correctly, a
standard overhang relative to the diameter of the lap was mentioned. And
think there is something about a proper weight psi on the lap.

On the Obsession Telescopes web page, I saw a picture of James at work
polishing or figuring 
http://www.obsessiontelescopes.com/optics/index.html

Can any one read the thermometer there?  
I think that is going to be important for the edge.
James, what does that thing read?

Mike...  Our different take on Robert's mirror might be that I hate to have
to fix the zone just inside the edge. I have thought that everyone else does
too. If correction was to be uneven I prefer the center to be ahead of the
outer zones slightly. That way as I increase correction near the edge, I can
choose where farther inward I am taking correction from by shortening the
radius of the zones between the over and under corrected zone. If I have to
play near the edge much very slight misjudgments of pressure or location
etc. lead to edges worse than I otherwise end up with. The last thing I want
is overcorrection in a zone near the edge.

I will have to see if I have left out some good ideas by thinking that way. 

I'm going to have to go spend another night at a Holiday Inn Express.

Jerry



-----Original Message-----
From: Jerry
For example...Results of pressure as I view it...

Lets take the outer 3 inches of the mirror and say that the inner inch of
the 3 to the middle inch is overcorrected slightly. And the outer zone is
under corrected a little from the middle zone.  Working the pressure across
the middle zone will decrease the correction from the inner to the middle
zone. It will increase correction from the middle to the outer zone. Two
birds with one stone. Hence easier to fix an overcorrected inner zone if
some outer zone is under corrected. Inward from the inner 1" wide zone of
those 3 zones would not change much. The work goes right on the middle of
the 3 and changes its relation to the zones on either side of it and not
much else.  

If the stroke was applied across both the inner and the middle of the 3 one
inch zones, from the inner to the middle would not change much and the
middle to outer zone correction would increase. There would be a decrease
from inside those zones to the innermost of the 3 outer zones.

That is my reasoning on that.




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