Re: [ATM] Mirror clips

[Mike Lockwood <melockwo@uiuc.edu>]

Jason,

Jason Hissong wrote:
> Well, I started figuring, but I am having trouble keeping things secure 
> on my turntable when I am at a large overhang, so I got some zones 
> starting to happen.  The surface is getting a little rough from things 
> not being very secure.

Turntable?  Secure?  Maybe you should post a photo of how you are 
working.  My article assumes walking around the barrel, or working on 
a slow, strong turntable that doesn't stall when you do a stroke.

Roughness comes from bad contact, hard pitch (my article assumes 
fairly soft pitch for figuring), and too much force during figuring. 
Strokes should be smooth and fairly slow.  Contact must be good.

> However, after the first session of figuring the 
> slight oblate sphere became more spherical.. i.e., the center finally 
> started to deepen so that it was meeting up with the raised zone.  It is 
> neat to actually see things change like they are.  I think I solved my 
> turntable problem... however, there is still a turn down edge that I may 
> work on.  Going back to some polishing strokes to resolved that.. I 
> could also just mask it as my mirror is really an 8.125" mirror and it 
> looks like just the outer 1/8 is turned down?

You can practice your figuring for a while if you like, but then go 
back to a sphere and FIX THE EDGE!  As Ken said, you can easily buy a 
mediocre mirror.

> I was going to use felt or cork for the surface that would touch the 
> mirror, but I like your idea Mike.
> but Mike, your explanation on figuring 
> made the light to go on and I think I have a very good idea on how the 
> various strokes work.

Glad these were helpful.

I still encourage you to learn Foucault testing.  Using both test 
methods would provide confirmation that they were each being 
interpreted correctly, especially with a faster mirror.

I think Robert H. hit the nail on the head with the vignetting theory 
for outside ROC ronchi images.  My camera does the same thing when it 
is too far away (during Foucault testing).

	Mike Lockwood

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