Re: Re[2]: [ATM] Question about "Null-testing"

["David Harbour" <scarab2@cox.net>]

Gentlemen:

Null tests not requiring auxiliary optics will not lie to you. At the risk
of repeating myself, Texereau was one of the last men to demonstrate this
when he used a knife edge suspended on cross wires to refigure all of the
mirrors of the McDonald observatory's 82" optics; if memory serves me, one
mirror was out by one and a quarter waves at the focal plane and the other
two by one half and or three quarters. Using his knife edge, and a bright
sstar (no auxiliary optics) he nulled tested the mirrors back to
beautifullly near perfect curves (somewhere as near as I recollect, about a
tenth wave on the wave front). It seems to me that the only way this test
could go awry is if someone couldn't figure out where the focal plane is, or
couldn't interpret the shadows. And that is not a case of the test lying to
you; that is the case of you being a dummy.

Why do atms resist this wonderful test, which is so easy to do (tape a razor
blade across the opening of the eyepiece draw tube?). Does anyone have a
theory they would like to elaborate for us why atms seem to avoid this test,
so much more sensitive than a Ronchi grating over the eyepiece opening
instead?

Dave

P.S. The Ross test nulling lens can lie through its teeth, sometimes, if you
don't get certain things right. The other only requires a star; all of them
produce parallel light, which works so well for null tests made at focus.

----- Original Message ----- 
From: "Richard" <cnc@cncservo.co.uk>
To: "Bob May" <atm@atmlist.net>
Sent: Monday, July 04, 2005 12:58 PM
Subject: Re[2]: [ATM] Question about "Null-testing"


> Hi Bob,
>
> Monday, July 4, 2005, 6:47:31 PM, you wrote:
>
> BM> Like any other test, the Ross and other null tests can lie to you if
you
> BM> allow them to do so.  The test is still a variation of the Foucualt
test
>
> How so?
>
> BM> Using interfrometery (there's a Yahoo group for that which is fairly
active)
> BM> that will show errors a lot better than what the geometric tests like
> BM> Foucault and so forth.
>
> Exactly, but that's the way a Ross null test is done. I assume
> you know of a method of using a Ross null lens with a Focault
> tester that I don't. If that is the case, the immediate question
> it would pose to me is what effect would the off axis effect of
> Focault have on the results?
>
> -- 
>
> Best regards,
>   Richard in the UK
>
>
>
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
>


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