RE: ATM Autocollimation Testing via Flats

["CSC" <optic@gwi.net>]

Thank you Roger for the great info.

I understand that what I will see in the test will be the 8" f/15 disk with
the irregularities of the 16" flat's zones superimposed (and a slim
elliptical hole where the 1.5" perforation is).  I realize that the zones
will be foreshortened and seem dramatic, and that I won't be able to view
the top/bottom 4" of the flat. Testing on different diameters would be
necessary, I believe.

Won't the testing be done in a band across the middle of the setup? How do
you think it's best to locate the zone position?  Would you put a scale on
the 8" or the flat or maybe have a friend mark them with a sharpie during
the test?   I understand that testing for convex/concave is done by
interpreting the astigmatism induced by the flat.  I believe the flat is
actually fairly good, just rough and zoney.


Using a sharper angle than 45 (this setup will be 30 degrees) would tend to
compress/foreshorten the zones a lot. As you say, makes the zones more
dramatic, but much narrower and harder to locate.

I'll bet just a touch of silver will make a big difference, and won't use
much chemical.  Macintosh's book mentions some formulas that can be stored
for a long time and give a thin coat quickly for testing.

I plan to use a spindle and slow hand correcting using various laps.
Hopefully the ATM articles on flats and my past experiences will help.  I
believe the full thickness 16" flat is close to 1/4 wave over the 16"
diameter, but quick interference testing against a good 10" flat shows a
~1/4 wave zone and a TUE at the perforation and the periphery.

A 4x riflescope to view the 8" mirror 20' away is another item I plan to
use.  When I made the f/15 sphere, it seemed darned far.

I've never successfully used the Dall or Ross null, but bought a 1/8 wave
lens from Newport that I had planned to use for this.  Perhaps when the
paraboloid is corrected, I can reverse engineer and check a Null lens setup.

Checking and fixing my tools.  I hope to wind up with a smooth 1/10 wave
flat, and know where the 1/10th wave error is.  I think it's not worthwhile
(or likely for me) to go much better than that with a Pyrex disk.  As it is,
this is a major project for myself and the other glass pushers in the club.

Regards,

Colin

-----Original Message-----
From: owner-atm@shore.net [mailto:owner-atm@shore.net]On Behalf Of Roger
Ceragioli
Sent: Tuesday, August 20, 2002 5:56 PM
To: atm@shore.net
Subject: ATM Autocollimation Testing via Flats



Hi, Colin.

I've used the Ritchey-Common test a number of times over the past few
years and here's my little bit of knowledge/suspicion.

As others have said, it's a challenge to align the mirrors for the test,
but it can be done o.k. starting with a flashlight.  The equations given
in Tex and Malacara for deducing the residual curvature of your flat
assume a geometry in which the flat is tilted 45 deg. to the spherical
test mirror.  This produces a dim foreshortened (i.e. elliptical) image
on the return.  Silvering your flat, as well as your spherical mirror
(which is absolutely necessary), can help a lot.  In order to do the
full test, you need a knife edge which cuts into you pinhole source at
two right angles, as Tex describes.  Not too hard to set up, but a small
challenge.  If you're just testing for zones so ROC of the flat, even a
Ronchi grating can work.

The zones, if there are any (as there usually are) appear elliptical
too.  This can make isolating them and measuring their position/width a
challenge.  The test is extremely sensitive to slope errors, and so
light even trivial zones can look quite bad.  Moreover, it can be
extremely hard to make a flat without a moving spindle.  Even with a
spindle, you often get zones when you wouldn't during normal spherical
work.  Tom Waineo did an article in one of the later ATMJ volumes which
is very useful to read and covers some issue not touched in the ATM
volumes.  You might wish to read it.

The 45 degree geometry also means to you can test a larger flat using a
smaller sphere, but the limit will be a sphere which is 0.707 times the
size of your flat.  So your 8" sphere is too small for a 16" flat,
unless you increase the tilt between flat and sphere/light source, which
is a bad idea.  Also with a subdiameter test sphere, you vignette the
top and bottom of your flat, further confusing the return image.  I.e. a
12" diameter reference sphere will work much better.

What about a null lens for your mirror?  I large Ross null lens, say 8"
dia. wouldn't be too expensive or difficult to make compared with a 16"
flat, and could be used for may other mirrors too.  Takes some
calculation and ray tracing to work out the test geometry but it's not
too bad.  Personally, I strongly dislike autocollimation testing, though
Mike Spooner and others on the list are quite good at it and can give
you lots of pointers.  IMHO the knife edge with Couder mask with star
testing for confirmation can do a good job on your mirror.  But that's
just me.

Two problems with the autocollimation test: 1) unless the parabola and
your flat are quite square with one another AND you can put your light
source/eye on the same axis, then rays striking the parabola on the way
out will not retrace the same path, which causes a slight averaging of
errors on the surface; 2) if you use the Ronchi test to evaluate the
asphericity, then remember that if you put your source/eye inside or
outside of focus in order to see several Ronchi bands, you will not be
testing at the focus and infinite conjugate.  In other words the Ronchi
bands SHOULD NOT look quite straight for the correct parabola.
 Likewise, beware of using the knife edge test or Ronchi if your
source/eye are NOT at equal distances for the mirrors.  At focus, though
the mirror should still null out.

Well, enough blabbering for now.  Good luck!
Roger Ceragioli