Re: [ATM] Turntable and weight

[Mike Lockwood <melockwo@uiuc.edu>]

Jerry,

Jerry wrote:
> And now a question for you and others on the list....
> 
> I notice that you are pressing with a weight. I know I am in the minority
> here but what does the weight do that the weight of the mirror or tool
> wouldn't do? It seems to me that the weight serves two purposes.
> One is to flex the mirror and tool out of shape and press the lap into the
> flexed shape. 

This flex will be fairly minimal if the weight is a reasonable weight, 
it is distributed fairly evenly, and if the piece on the bottom is 
properly (uniformly) supported.  I find mirror/tool flexing to be a 
problem only for cold pressing with LOTS of weight applied to a ROCK 
HARD lap, like Gugolz 73, which is great for flats.

> The other is to thin the lap and close the channels more quickly causing the
> tool life to be shortened. 

Tool life shouldn't be an issue unless you want to use the lap for 
more than one mirror.  (I have used the same lap for three mirrors and 
it could have done more.)  A lap should easily last long enough to 
complete a mirror, even if it is trimmed 10 times.  If it gets too 
thin after many trimmings, it was either poured too thin to begin 
with, or it was way too soft.

> I leave the mirror and lap together and do not press with weight. When not
> working, cover it with a dish pan or bowl of some sort to keep dust from
> settling on it and a damp towel around it to keep it from completely drying.
> The towel does not have to touch the disks. It just keeps the humidity high
> under the dishpan slowing evaporation from between the disks. If it does dry
> out you can squirt room temperature water into the channels and let it soak
> 15 minutes or so. Pitch does not need more weight than the disk alone to
> press into good contact. You cannot get a better press.

The problem with leaving the mirror and lap in contact, either before 
or after work, is that cerium reacts chemically with the glass, and 
the lap pattern will imprint itself in the glass.  Admittedly, this is 
only a real problem during figuring, and likely only with Cerium.  (I 
don't like Rouge.)

Want to see this?  Here's a photo of a not so great Cave mirror before 
refiguring (yes, really, they are not all perfect) that I think 
illustrates this.  Click on the Foucault image to get a larger one:
http://bi-staff.beckman.uiuc.edu/~melockwo/telescopes/12inF6tube/12inF6tube.html

Granted, it may not have been caused by etching, but it sure looks 
like it to me, given the regularity of the pattern.  That mirror had a 
turned edge, too.  (This is not the worst Cave mirror I have seen. 
The worst was a 12.5" F/4.2 that was HALF corrected.  Seriously.)

> I do not warm press before figuring. Warming the lap or mirror prior to a
> figuring session seems to me to guarantee that the mirror and lap are
> changing shape in an uncontrolled manner while you figure.

Agreed.

	Mike Lockwood


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