RE: [ATM] RE: [atm_free] The zonal Foucault test is free ofinherentcorrection bias - some supporting graphs

["john sherman" <atm@mail.johnspics.com>]

Hi all,

>I think that the actual kind of test should be 
>better defined in the discussion. "Foucault" has 
>been sometimes used to mean just about any test 
>with KE on axis (not by you!). "Couder" can mean 
>masked testing 

As far as I know, Foucault never used a zonal mask. So doing a test requiring a mask probably should not be called Foucault, even tho nearly everyone does (even me, sometimes). Foucault (as far as I know) only did null testing with his knife edge.  I like the suggestion to call modern masked zonal testing "Couder" (even tho he was not the first to use a mask), and keep "Foucault" for nulling a sphere or ellipse. Did Foucault ever do a "measurement of longitudinal aberration"?  In any case, wasn't the first mask already used before Foucault's time?

This is similar to the problem of Enke's Division...

>I think our simulations show that under ideal 
>conditions Foucault performs very well.

Thanks for doing all that work! A bit of evidence: my friend has been making 28" f/3.7 mirrors using the "Couder" test (Foucault!). He reports that he has no problem getting the mirrors very close to perfect before taking them out to star test (and being a career professional, he is very critical). I've observed through several of them, and the seeing has always been the limitation to resolution, not the mirror. 

About half way down the page at:
>http://www.atmsite.org/contrib/Carlin/couder/ 
is:
>The third order x³ term will make the zone on 
>one side appear a little brighter near both its 
>edges at one side, and a little fainter near its 
>edges at the other side,

Evidently the two halves are NOT mirror images. Perhaps this can explain the discrepancy that some observe?

>He [John Strong]points out that Foucault tests 
>are sensitive to parallax, 

Not being a mathemagician, still I calculate that I've jumped off of the deep end before, and lived, so why not do it again??

Here's a reason why the parallax might be:

Every good program has to compensate for the error of having a flat mask in front of a curved mirror. In the center zone, you are looking flat on to the mask, so there is no real problem. The outside zones are physically close to the mask, so there is only a little problem. But at the 60% zone and thereabouts, the light rays go in and out of the mask at an angle, and hit the mirror at a different radius than the actual mask openings. 

Now enter a lateral spacing between the source and knife (creating the parallax error/demonstrable astigmatism). Are the zones you are actually measuring on the mirror still at the same radius on each side?? Apparently not! 

It seems the rays would go in and out, and show a slightly different zone on the mirror in the left mask opening than in the right. This is easy to see in a diagram I just made (hopefully, I made it accurately enuf to be worth something). Far be it from me to do real calculations (as I said in a recent post). But I can imagine that this is indeed a problem, and it might explain some of the posts that were made.

The little diagram that I made also indicates that lateral separation of source and knife means you are actually now seeing a smaller zone of the mirror than before. Almost like your zone's aperture is now actually a smaller effective aperture. Perhaps this allows diffraction effects to have an even greater effect than normal? Should the zones at ~60% use wider openings to compensate? 

Do any programs compensate for any of these effects? (I can't read the math on Nils Olof's web page) Can a program offset the mask holes to compensate, by asking for the lateral separation in the current setup?

Okay, I'm mostly just babbling. I'll
shut up now.

John























_______________________________________________
ATM mailing list http://www.atmlist.net/