[ATM] Intro and slitless tester question

[scarab2 at cox.net (David Harbour)]

Gary,

I found your question interesting. I think you will find that the consensus
is that a pinhole offers no advantages, but rather, only disadvantages over
a slit. The knife edge can only analyze information lying at right angles to
itself; so there is no reduction in sensitivity of the test by extending a
single pinhole into a long vertical line of pinholes. More Light!!!

Call the verticle row of pinholes a slit. Your description of your seeming
ambiguity of finding focus with the knife edge (getting a completely
unequivocal, unambiguous null, at an exactly identifiable point on the
optical axis) is a normal experience, particularly for a longer focal ratio
mirror. That is the nature of the beast; it is why we are allowed tolerances
in knife edge positioning- one cannot physically find the single indivisible
point of center of curvature for any zone of any mirror. The faster the
mirror, the tighter the tolerances, and this translates into a less
ambiguous detection of the C.O.C. for the entire mirror (if it is spherical)
or any particular zone of the mirror, if it is not spherical. I wrote a
rather different kind of explanation of the meaning  of Knife Edge setting
tolerances, with newer kinds of illustrations, that helps visualize what
this means. You can see this article at:

http://www.atmsite.org/contrib/Harbour/Foucault.html

By the way, one may make an arrangement of the knife edge and the jaws of
the slit in such a way as to have the advantages of both a slitless tester,
and a slit equipped tester. My old number five, which you can see at:

http://www.atmsite.org/contrib/Harbour/Tester.html

had such an arrangement, though I do not think I touched on that in the page
about the tester. I have an email here somewhere that explains the
arrangement (the way to mount the knife edge and the slit so that you have
them both, functionally) if I can find it. Let me know if you would like to
have a description of how I did this. My tester showed the slightest surface
defects on a mirror, and also showed a mirror as fast as f/2 fully
illuminated (12.5" Schmidt Camera Primary I figured), without even the
slightest hint of the appearance of "false astigmatism". Let me know if you
would like for me to find that email, any of you folks who are interested,
and I can forward it to any one of you. It is rather longish, so unless all
of you guys demand my description, I would rather not post such a long note
here. Sorry I haven't commented on how I did this on my web pages.

>With the grating, it's easy to find inside and outside focus, but not very
clear exactly where focus is. With the knife edge, it also seems unclear.
The mirror darkens gradually seemingly >regardless of position. Do I need to
use a pinhole source to get to where the mirror illumination will suddenly
blank out? If so, does it need to be fixed? I feel like I'm missing
something >very obvious here.

>Any help appreciated!

>Thanks,

>Gary Fuchs