Re: [ATM] 8" F/4.2 progress

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Jim Burrows wrote:
> At 2005-10-25 23:31 -0400, Jason Hissong wrote:
>>I wanted to get the scope together so that I can star test during the
>>figuring.
> 
> In general, using star testing during figuring will make your figuring go 5 
> (10?) times slower.  Use good ol' Foucault, (or maybe the LWT (Lateral Wire 
> Test) - Foucault's hard at f/4.2) which doesn't depend on waiting for a 
> cloudless night, good seeing, etc., etc. - also it's hard to tell from the 
> star test what the next figuring spell should try to accomplish.  The star 
> test is a good second opinion before sending the mirror off for coating 
> when the bench test says, "You're done!"

Just to be clear, because Jason is enjoying the learning process of 
making his mirror, I suggested (a few messages back) star testing to 
him during figuring so he could learn how star images changed as he 
got closer to a parabola and easily check for astigmatism.

However, I recommend that he makes figuring decisions on the mirror 
based on Foucault testing results, as I do in my figuring tutorial on 
my web page (to which I referred him).

With some practice, Foucault testing testing gives you information on 
the shape, smoothness, and the edge condition.  Other tests may give 
more/more precise information on ONE of those qualities (Caustic test 
- shape, phase contrast - smoothness, edge condition - Ronchi), but 
Foucault probably gives the best balance of info of all three 
qualities for a beginning mirror maker - at least that's what I think.

Yes, I'm not forgetting the star test, but it requires good seeing and 
LOTS of practice to interpret.  But, an equilibrated mirror on a 
steady night can definitely confirm if you are doing the Foucault test 
properly!

Jason, I haven't used the lateral wire test, so I can't comment on it. 
   Many on the list can and I hope they will.

Keep us informed of your figuring progress.  Remember, you can always 
go back to a sphere - you know how to make one.

	 Mike Lockwood

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