Re: [ATM] Open Question Concerning Pitch Laps

[Mike Lockwood <melockwo@uiuc.edu>]

Jonathan,

Jonathan Bishop wrote:
> You are going to hear a lot of contradicting advice on figuring a 
> mirror. So I expect many other people will have opinions that sound much 
> different than mine...

When you boil it all down (not the pitch, the advice!), the list 
advice is fairly consistent - there are many ways to make a mirror, 
some easier than others.

Bryan, the most important advice that I saw multiple times was that 
you should practice and try to perfect your Focault testing.

Jonathan, I was impressed with how well you used a subdiameter lap to 
figure your mirror a while back.  It's not easy, especially for a 
mirror of that size.  (I think it was an 8"?)  It was also not the 
typical path to the parabola.

> I think your pitch is much too soft. I also used soft burgundy from WB 
> and had similar frustrations. I remember boiling it for a couple of 
> hours to harden it. After I did that I was able to quickly get back to a 
> sphere using short W strokes. In fact what actually happened was that I 
> raised a hill in the middle which gradually got bigger and bigger until 
> it covered the whole mirror, at which point it was actually more 
> spherical than I had ever seen before. So I really am convinced we 
> should stay away from this soft pitch for this reason.

This recommendation is based on experiences in making how many mirrors?

The goal of making a Newtonian primary mirror is not to make a sphere, 
it is to make a smooth parabola.  For hand work, hard pitch may more 
readily form spheres, but soft pitch more readily forms smooth 
parabolas.  The (approximate) sphere is just a stage that we pass 
through on the way to a parabola.  The necessities before parabolizing 
are a good edge and lack of serious defects (deep holes or rings, poor 
polish, prominent zones).  I'm not saying you CAN'T make a smooth 
parabola with hard pitch, but I am saying it is a lot easier with soft 
pitch.

At the risk of starting a "pitch war", to me Bryan's pitch sounded 
like it was in the range to make a nice smooth parabola.  He just 
needs to get close enough to a sphere so that he can start figuring.

	Mike Lockwood

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