Re: [ATM] webcam and foucault test?

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Jeff Rowe wrote:
> I have often wondered but not tested, the improvement that might occur 
> if a high quality, flat field lens was used in these tests, such as a 
> Nikkor or Rodenstock enlarger lens, or an APO process lens.  I have read 
> the great debates on this list and others concerning the use of webcams 
> and the like for foucault testing. To me it seems like the really weak 
> link in this approach is the imaging lens. Could the cheesy lenses that 
> come with webcams or the non flat field mini scopes that are added to 
> testers be the source of  less than desirable results?  Is this the 
> Achilles heel, of all these tests? We use great eyepieces and mirrrors 
> in our scopes, are less than prime imaging lens the source of errors in 
> our test equipment?

I suspect that non-uniform performance of CCD chips contributes more 
error than poor lenses, but I can't prove it.  I've never seen any 
calibration data.

 > Mike Lockwood any further thoughts?

I just wanted to warn people to be careful when using something other 
than their eye, because once the neat-looking image comes up on the 
screen, it is tempting to forego the calibration/verification of the 
system and just start taking readings.

It sounds like some imaging systems are working, so that's encouraging.

We (Mike Conron and I) tried to test our 30" F/3.8 with a digital 
camera and it didn't yield the same figure as visual testing.  A null 
test late on a steady night, using a line of a ronchi grating in the 
telescope as a knife edge and Polaris as the source, revealed a high 
zone exactly where my visual Foucault testing idicated.  The same null 
test on my 16" F/4 yielded a beautiful null, again in agreement with 
my visual Foucault testing results.  So, I feel I'm fairly well 
calibrated using my eye.

James Lerch wrote:
 > BTW, the Human Eyeball is a terrible lens / detector system,

For the case of a beginner, I will agree with that.  But for for the 
case of a practiced operator who has learned to be objective, I 
disagree.  The human eyeball has something the camera doesn't have - a 
brain connected to it.  With practice it can LEARN to see (or see 
through) turbulence, surface roughness, or grunge on the mirror 
surface, and the operator can learn to take highly accurate, 
repeatable measurements.

 > and an eye at the knife edge is industry standard for several well
 > known companies.

That's fine with me, so long as some final astigmatism check is conducted.

	Mike Lockwood

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