Re: [ATM] Updated pictures of 8"f6 mirror............

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

And the debate continues....

Jerry wrote:
> Which came first, temperature or pitch hardness? Which has a lever 
> attached for easy adjustment?
> Once the lap is made, in most cases it will be easier to control 
> hardness by either controlling the temperature or controlling when
> or where you work based on temperature.

In my experience it takes a fairly large rise in temperature (10F or 
more) to make hard pitch significantly softer.  So, I choose to use 
linseed oil to soften my pitch rather than crank up the thermometer. 
(Warm water works too, but getting the mirror and lap into equilibrium 
is tricky.)

Keeping my work/testing area closer to the temperature it naturally 
wants to be makes for less air currents when testing, too, and that is 
a big consideration for useful testing results.  It also costs less in 
terms of electric and gas bills.  A few teaspoons of linseed oil is 
WAY cheaper.

> Channel closing time by itself is not a good indicator. There are 
> many factors involved in that time. Depth and width and angle of
> the channels, stroke length and speed, weight, slurry quantity and
> dilution are some. All those variables mean that until you have
> settled into a personal standard of methods, channel closure time
> won't mean much.

Well of course other factors can affect the channel closing time, but 
this is the best we can do over email when we are on opposite sides of 
the world.  Let's face it - most people aren't going to build a pitch 
tester, so this is the next best thing.

> John is in the experimenting phase. He wants to play a bit to see 
> what happens. That is a good thing to do what with all those
> previously mentioned variables and more. He tried 1/4 stroke. It
> happened that the work space was warmer than with previous work. He
> considered that the lap may be too soft. It might be. I was letting
> him know I think that is not the problem.

The problem is we still have no measure of how the pitch is behaving, 
we can only guess.  And that's what we've been doing.

If we are ever going to learn to help people with mirrors over this 
list we need some standard questions and indicators that will help us 
determine what is going on.  So I ask again, how long are the channels 
taking to close up?

Mike Lockwood
http://bi-staff.beckman.uiuc.edu/~melockwo/index.html


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