Re: [ATM] On Axis Foucault

[aplanatic@aol.com]

 Hi Richard,
 
I've made several f/2 and faster spherical mirrors for Schmidt cameras, a collimator and a concentric Schmidt Cassegrain.  Once you get used to judging Foucault shadows across a wide field of view, I don't think you'll have any problem testing the reference mirror.  To reduce the eye and brain strain, try a two-lens 0.3-0.5X optical system behind the KE.  Using a pelicule beam splitter for on-axis testing is nice, but not strickly necessary as long as the light source and KE are kept very close together.  Nonetheless, if I had to do it over again, I'd probably build a little on-axis tester using a pelicule, and fasten an inverted telescope right behind the KE, attached to the housing.
 
If you need an f/2 reference mirror be sure to oversize it.  It's always the last 3-5 mm at the edge that's so hard to get right.
 
Don't stick anything through your pelicule.  The results of such an experiment are quite disappointing.
 
-- Dave
 
-----Original Message-----
From: Richard <cnc@cncservo.co.uk>
To: Alfredo <atm@atmlist.net>
Sent: Tue, 14 Jun 2005 16:53:50 +0100
Subject: Re: [ATM] On Axis Foucault


Hi aneves,

Tuesday, June 14, 2005, 3:29:50 PM, you wrote:

asb> I am trying to make an on axis focault tester for
asb> testing F 2 and faster optics, and would like some 
asb> ideas on how to achieve on axis alignment of the slit
asb> and knife edge. I was tempted to use a beamsplitter, 
asb> but I am unsure because of the spherical aberation it might induce. Any 
Ideas?

Someone will step in and disagree, but I thought Focault was
unreliable below F/4. As for the beamsplitter, we've been
discussing this over in the yahoo interferometry group just
recently. Conclusion is that if a cube beamsplitter is used in
an interferometer such that both test and reference beams pass
through it, then the SA is nulled out. In your case I don't
think it would be nulled, so I would use a Pellicle instead.
See http://www.nationalphotocolor.com/

Meanwhile, I have the same problem.

1) I am making a spherical wave interferometer and the reference
element is an F/2 spherical mirror. I need to test the mirror
but I don't have an interferometer that will do it. Go to (1).


-- 

Best regards,
  Richard in the UK



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