Re: [ATM] TDE (That Darned Edge)

["jcmulherin" <jcmulherin@opticalmechanics.com>]

Hi Jerry:

We have two temp settings in the shop. In the winter we keep the thermostat
at 70 and use one teaspoon of linseed oil per kilo of Gugolz 64. We use this
formulation for polishing and figuring.

In the summer we try to keep the temp at 75 and use straight Gugolz for
polishing and figuring.

Pitch thickness is also a factor. A lap with a thick layer of pitch will act
softer than a lap with a thin layer. We polish a number of mirrors with a
lap but monitor thickness between mirrors. As you know, the pitch gets
thinner as the channels flow shut and you re-trim them. For larger mirrors
(12.5" diameter and up) we pour a full size lap .75" thick and work it until
it's about 3/8" thick. Then we re-pour it. We get about 10 mirrors out of
one lap. For smaller laps, and in particular figuring laps (4" to 10"
diameter) we start with 5/8" thickness and run till the pitch is 3/8" thin.
We figure about 5 mirrors with a set of figuring laps before re-pouring.

We do most of our polishing with full size laps. We set the machine to make
a normal 1/3 wide stroke with 1/6 overhang at each end of the stoke. The
table speed is about 5 rpm and the stroke speed is about three times that.
Assuming your lap is making good contact this technique will reliably
produce a decent sphere with a clean edge. We'll typically polish for about
10 hours of machine time to complete the polish. 

I haven't looked in a whit to see if John's a8" figuring project is still on
the ATM_free list. If it isn't I'll compile something and post it to our web
site. Let me know if you can't find it.

James

James Mulherin
Optical Mechanics, Inc.
jcmulherin@opticalmechanics.com
www.opticalmechanics.com
Tel: (319) 351-3960
Fax: (319) 351-3943

> -----Original Message-----
> From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf
> Of Jerry
> Sent: Thursday, April 27, 2006 1:45 PM
> To: 'ATM list'; Mike Lockwood
> Subject: Re: [ATM] TDE (That Darned Edge)
> 
> 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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