Re: ATM : Re: Astigmatism

["John Sherman" <shermj@netzero.net>]

Hello,

Yes I like to eat bread, and I usually sit in a chair while doing so. I'm sorry, I do not mean
to belittle or invalidate any of those things which came before me. Without them I would be
nowhere. Whatever I am doing now is based on those things remaining valid. I think you should
keep doing what you're doing. And, if you want, you can also do it in 2D. I agree the Ronchi
grating has many uses. I am speaking only of the subject of astigmatism. Someday someone will
say "Aha, the spot test only does this or that, my test is better!". And I'm sure he'll be
right.

>...the amount of deformation comparable to Suiter's illustration of...

Suiter also says Ronchi abandoned his own test.
Still, I apologize for my lame sense of humor.


Thank you to those who described what I should look for to see astigmatism with a Ronchi
grating. Yes I now see the ying-yang. With an astigmatic-spheroid on-axis with one line showing
I can see that it is curved. And I see that it rotates around the center. Still, this is just an
effect. You can detect the presence of the astigmatism, but how do you measure anything? A spot
test shows me the axes automatically, just like the star test does.

I don't have rms numbers, no, but I can say this. I am bending an 8" f/5 80" roc spheroid in a
jig to give it astigmatism on-axis. When the roc of the two zones differ by 0.2" I can hold the
knife in there at 45 degrees to each axis and see a nice black and red ying-yang. This is what I
call severe astigmatism. What I think happens is that this is the effect on a 1D test in a 2D
environment. The shadow of the knife changes sides as you go through roc. Normally there is a
null when it changes sides. But it can't null the astigmatism, and it can't fully see it, and
it's forced to do something. So what happens is that the shadow rotates around to the other
side. The Ronchi does the same thing.

The less astigmatism you have, the smaller the spot you can use. This is the first measure of
severity. At the best focus there is a distorted square or cross shape. Inside focus it turns
into a line exactly oriented with that roc. Outside focus there is a line directly showing that
roc. Neither axis has to go through the center, and they don't have to be separated by 90
degrees. I can change the push part of the jig from a bar across the center to a pad at each
end, and I can see the difference it makes. A 1D test can't easily distinguish these things. The
least difference I've measured between a line inside and a line outside roc is 0.07" (moving
source/fixed spot). In this case the jig is barely touching the mirror. Unfortunately this
spheroid is more oid than sphere, I'll have to fix that when I get time.


Later,

John