From: "CSC" 
To: "Atm" 
Reply-To: "CSC" 


        I recommend the knife edge test, and carefully measure the longitudinal
motion (spherical abberation or zonal error) necessary to get the wavefront
error.  I used this test when I was pursuing "perfect" spheres
for flexing.

        I found the point at which I could detect general shadow motion inside
focus, then measured the distance to where I could detect shadow motion
outside focus. I also made sure that any slight zones (and I mean really
faint at the end of figuring) were also extinguished at this knife setting.
Since there were no other defects visible on the surface within this range,
I assume that all errors were within this wavefront difference.

        According to Diane Lucas, High Priestess of Telescope Making, (didn't
know
we had one?), the knife edge travel (longitudinal abberation) for 1/4 wave
WAVEFRONT error is:

assume stationary light source

longitudinal abberation=16* wavelength*f^2/y^2

wavelength is .000022"
f is the focal length of your mirror y is the radius of your mirror

My 8" f/7 would be:

LA=16*.000022*(56^2)/(4^2)

LA=.069"

a moving light source would be half that.



I found no error in the final figure within .007" travel, using a
moving source, so I calculate I was within:

.007/(.069/2)= .2  of the above formula, or 1/20 wavefront pv

I also used a laser pointer with the lens removed as a point source of
monochromatic light.  Using a 4mm orthoscopic eyepiece to view the
diffraction image, I watched the results of the figuring as it showed in
the star test.  The slight astigmatism was isolated by rotating the mirror,
and was found to be due to the lateral offset in the test rig.  Could have
been designed out.  With a diverging beam, an uncoated surface and high
magnification, the diffraction image was not too bright.

At 711x, I figured to the point where I could not tell any difference in
the star test. You would not believe how beautiful this test is. It is
supposedly accurate to 1/40-1/60 wavefront under lab conditions. Any
over/under correction shows obvious patterns, and coupled with the
foucault, it is easy to get a superb sphere.

If you setup the laser as above, you might also try Peter John Smiths
Shearing interferometer.   Simply place a vertical plane of glass to
intercept the returning rays from the laser, and by shearing the wave front
(folding it back on itself), you can see true interference bands which will
be perfectly straight when null (sphere).  See it at

http://www.users.bigpond.com/pjifl/page15.html

Let me know if you need any more info.  I think making a great sphere and
flexing it is THE way to a fine scope.

Colin

-----Original Message-----
From: owner-atm{at}shore.net [mailto:owner-atm{at}shore.net]On Behalf Of
Ray{at}J-Engineering.com
Sent: Sunday, February 09, 2003 7:29 PM To: atm{at}shore.net
Subject: ATM Tesing a spheriod?



Hi All,

When testing a spheroid,  how can you quantify the figure?

As far as I can tell, the edge is good.  I used an oversized lap which
seemed to work rather well.

If I set things up just right, I can get the Ronchi lines (grating /
grating) to bend just a bit.  Hardly noticeable.  It seems to only work at
the few positions where the diffraction images combine or merge just right.
How can I tell how far off the surface is?

In a slitless knife edge test,  at ROC, I can just eek out a few shadows
just before the whole image gets too dark.

I'm working on getting a feel for using these tests and trying to associate
some kind of number to what I see.

(8", f/6.3)

Thanks for any help or web links,

Ray

--- BBBS/NT v4.00 MP