Re: [ATM] 12.5 inches BVC advice for parabolizing

["Bill Kelley" <flex@commspeed.net>]

Hi,

RML wants to know if carefully measuring offsets and using something like 
WinRonchi can produce a good mirror. Dale Eason says no dice,  use Foucault.

Zonal testing and Ronchi comparisons both have subjective elements.
I've done a couple (not very fast) with WinRonchi comparisons and carefully 
measured offsets. One later later tested very well by autocollimation. Did I 
just get lucky?

Bill Kelley



> I'm very surprised.
>
> I tought that using ronchigrams, (with one of the many free software, with 
> offsets varying lets say from 0.3" inside ROC to 0.7 outside...) was 
> sufficient, and that with successive testings and photo-shoots and 
> comparisons between the ronchigrams and actual mirror images, I could make 
> the figure look allmost exactly like the ronchigrams, then I would have a 
> good mirror...
>
> I also have different gratings, and I'l try to find the most precise by 
> trying them at the different offsets.
>
> Thanks all for your continuing input...
>
> RML
>
>
> ---------- Original Message ----------------------------------
> From: Dale Eason <atmpob@yahoo.com>
> Date:  Sun, 10 Jul 2005 21:14:22 -0700 (PDT)
>
>>In a recent post you said you do not understand how to
>>do a knife test.  You will need to understand how to
>>do a Foucault test or else you will not know when to
>>stop or when to change stroke.
>>
>>How are you judging when you will be done?  Ronchii
>>images will not help you at that point.  Suiter has
>>shown in his book that for fast mirrors like yours it
>>is impossible to tell a good figure from a 1/2 wave
>>one using only ronchi. In general they can only tell
>>you about the smoothness of the surface and edge.
>>
>>Dale Eason 


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