Re: ATM Astigmatism

["David Harbour" <scarab2@cox.net>]

Dear John,

You are right that Foucault should be used over Ronchi, and I believe that
the reason so many do not embrace the wonderful Foucault test is that it has
never been really well written up with all of the necessary concepts
presented logically in sequence, and also properly illustrated. That is why
I spent about five hundred hours writing and illustrating "Understanding
Foucault". I have a folder in this computer bulging with comments from many
newcomers who read it and say that it made the concepts clear when other
authors did not. And that is what I set out to do when I wrote it. I invite
all who have shied away from Foucault to read it, and I can pretty much
guarantee that a thoughtful reading of it will make it clear to anyone of
average intelligence.

And my most deepfelt hearthanks to all of you, so many dozens of you, who
have read and appreciated it. It is now up on five different web sites and I
guess has turned out to be about the only thing I have accomplished of much
value in this life. I invite those of you who have not looked at it to do
so- I have tried to make it interesting from the very start, to kind of draw
one in, so to speak. It looks very well on the other sites it is on, but I
seem to be able to retrieve the atm site URL with the least effort, so if
any of you want to look at it, click on this link:

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

My thanks to the many who have allowed me the privilege of passing on my
knowledge and experience in optical work to them-

Sincerely,

Davey


----- Original Message -----
From: "John Sherman" <shermj@netzero.net>
To: <atm@shore.net>
Sent: Tuesday, June 25, 2002 8:26 PM
Subject: ATM Astigmatism


>
>
>
> Dear List,
>
>
> Recently Jerry had problems with astigmatism, and as a result there has
been
> some misleading information going around, which I will address. Here are
two
> facts, which I have determined from much experimentation:
>
> 1)  Astigmatism cannot be seen with the Foucault test or the Ronchi test,
>          unless it is horribly severe.
>
> 2)  Astigmatism can easily be seen with the Spot test.
>
> Jerry described using a Ronchi grating and having straight lines in a few
> orientations. He was told that means he has a good sphere. However, this
is
> not a good enough test to see astigmatism. The problem is that the Ronchi
> grating is a one dimensional test and astigmatism is a two dimensional
> defect. At each orientation he was performing a 1D test. Some might say
that
> since the Ronchi covers the entire mirror that it is 2D, but that is not
the
> case. If you have very severe astigmatism and you rotate the Ronchi
grating
> you might see a faint ying-yang pattern. Maybe. If you're lucky. I have
set
> up very severe astigmatism and I have never seen this pattern.  Yes, it is
> possible to detect (as opposed to see)  astigmatism with the Ronchi
grating
> by rotating it to different orientations. You will need many orientations
> (not just a few) say 0, 20, 40, 60, 80, 100, 120, 140 and 160 degrees. And
> here is how you do it:
>
> 1)  When you rotate the grating you must make sure it is EXACTLY the same
> distance from the mirror at all times,
>
> ---------AND---------
>
> 2)  At each orientation you must count the number of lines and spaces and
> fractions thereof. Astigmatism will manifest itself as a variation in the
> number.
>
> If you cannot satisfy condition 1) then don't even bother with the test
2).
>
> And here is how you can prove to yourself that I am correct. Make your own
> astigmatism!! Last night I performed yet another experiment. This time I
> used an 8" f/8 128" roc paraboloid mirror. This is an excellent Richard
> Fagin mirror known to not have astigmatism by using the eyepiece
> Suiter-style star test. Using a pinhole light source and a Spot tester,  I
> set up with the spot at the 70% zone. When there is less than one inch
> lateral distance between source and spot the ring shaped shadow of the
spot
> was round, showing no detectable astigmatism in the mirror or the setup.
At
> two inches lateral separation the ring is very elliptical shaped. This is
> the visual manifestation of simple astigmatism, you can see it first hand.
> At three inches the astigmatism is so severe that an image was not
> obtainable with the 2D Spot tester. Yet with the Ronchi tester I still saw
> normal lines at each orientation. Since this is a paraboloid mirror, the
> lines are slightly curved to match the curvature of the mirror (with a
> spherical mirror I would see perfectly straight lines). Even at six inches
> lateral separation between the light source and the Ronchi grating the
> Ronchi still showed normal lines at each orientation. No matter how I
turned
> the gratings (I have two, 100 lpi & 150 lpi), astigmatism was not visible.
> Using a slit light source instead of the pinhole might make some small
> difference, but the slit has to be aligned with the grating. If they are
not
> aligned or if something is not square to the optical axis then you might
see
> a ying-yang effect because of that.
>
> Even though the Ronchi tester makes measurements at every point on the
> mirror, it is not a 2D test. Why do so many people insist on using a
Ronchi?
> It is pre-Hitler technology, found in dusty old books. A month ago I made
a
> post with this same subject line. You could refer to it for more
> information.
>
>
> Thank you,
>
> John
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