Re: [ATM] [atm_free] RE: data and musings on thin mirror

["Jerry" <wa4guu@verizon.net>]

Dale, Steve, Vlad, Mel and others following this....

I have done this with several different mirrors and Ronchigrams (not mine)
and it works. There may be cases where it doesn't work well, but I think
that if you understand that 100 % of Focault knife correction could be in 10
% of the curve, you can reason the mirror curve needs to be made smoother.
In making the Ronchi lines more correctly shaped the test becomes more
accurate.

No one looking at the Ronchi of the ultra-thin mirror will think the curve
is smooth. All of the correction of that mirror is in the outer half of the
curve. Using my experience judging that one, I can see that it is actually
better not to be informed of the grating distance from COC. You can figure
it out better than some people can measure it.

I found John's ATM web page of a 19 inch f5.3 mirror.
http://homepage.ntlworld.com/glassandgrit/testing.htm

He says he used Mel Bartel's matching Ronchi test. Why would we think we
could determine what a Ronchi test would look like at a certain grating
distance inside or outside COC if we think that we cannot determine the
distance COC to grating by the observed Ronchi test?

If someone wants to try it out, using the line count method we are speaking
of, look at the Ronchi images of that mirror and see if you can verify his
stated grating-COC positions and the mirror correction. You don't have to
scroll down the page to look at his Foucault data until after you are
finished with the Ronchi analysis.

Are his stated grating positions correct?
If not which way and how far is it off?
Can you tell if he is using a fixed source or moving source tester?
Does his Foucault data reasonably fit the Ronchigrams?

You can do it. 

Jerry





-----Original Message-----
From: Mel Bartels
 
> I could be wrong, but I think my analysis is correct.  This is an
> interesting disagreement, because I think it is one that we can prove
> one way of the other by example, theory, or both.  It's possible that
> we are not yet talking about the same thing, so  we should nail this
> down, first.


I suppose I have as much Ronchi experience as anyone having used it
extensively for 25 years and taught classes with it.  Visually even from an
experienced practitioner's view, I cannot deduce the RC without moving the
Ronchi grating.  

It's not the spacing that matters as much as the shape or the spacing up and
down the bands.  I've run extensive simulations and concluded that the range
of RC values that would fit what can be observed in the band spacing and
their shapes is too great and thus invalidates this method.  This is from a
visual eye use.

Too me this is analogous to using Hartman without moving the screen and
doing the star test at the scope without being able to move the focuser up
and down.

Mel Bartels


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