RE: [ATM] Another Astigmatism question, but at the sphere stage

["Jerry" <wa4guu@verizon.net>]

I did not reply because I don't know if Jay's test method is valid or not. I
think it is flawed but since I know little about Sixtests, the order and
timing of his 3 sets of readings on 2 diameters and how he chooses those
diameters I think it is too difficult to offer help by email. So not knowing
I thought I should stay out of it.

But.....

I really suspect he is chasing a "boogie man". If he has taken the usual
well known and documented techniques to avoid astigmatism it is unlikely he
has any more than a small residual that most any such mirror will have. Then
on a first mirror, and I assume that is what this is, he may be doing
something wrong. 

My best advice is that if he is near a sphere astigmatism should show quite
well with a simple Foucault test. Care should be taken that the knife edge
is parallel with the slit and that the slit and knife is squared vertically
with the mirror axis. If this is not done you can have an appearance similar
to astigmatism when testing a mirror with none. That taken care of it either
nulls all over or it doesn't. Since it is oblate with a TUE it won't null
but he should be able to see asymmetry on at least one diameter if they are
not 90 degrees to each other. And even if they are at 90 degrees he would
have to be fairly unlucky to have chosen the diameters that would not show a
change. Being close to a sphere with the shadows not contrasting starkly he
should still be able to see asymmetry if he has significant astigmatism.

As far as the order of his test runs, it can matter. The mirror does change
shape with time unless the mirror is at the same state of stability
throughout the entire series. Unless the testing environment is controlled
more carefully than is possible for most people to attain, you will get
different readings at different times of day depending on whether the
temperature is rising or falling. That rise and fall may be slow and slight.
Many will think they have stability when it is not perfect.

My point is that to set up and take a number of readings on one diameter and
then do a series on another assures that there is a passage of time from the
average time of the readings on one diameter to the average time of the
readings taken on the other. For example, that has the likely outcome that
one set is taken in the morning when temperature is probably rising. And the
second readings are taken later, maybe at noon when the temperature is
rising faster or maybe in the evening when the temperature is falling. The
mirror being a different shape at the time of testing the different
diameters, the readings being different is not surprising. 

If one wishes to test for astigmatism by testing two or more diameters by
Foucault readings a better method would be to interlace single reading on
each diameter. Test both diameters. Then test both again, both again. And on
to whatever you have chosen as for the number of runs you want to average. 

Obviously this is more tedious. However, it is not as tedious as chasing a
boogie man.

I still don't think I like this method for testing astigmatism. But since
I'm not sure, I can't say Jay is wasting his time.

Jerry

   


-----Original Message-----
From: Michael Peck
Sent: Sunday, August 28, 2005 10:07 AM

At 19:47 8/27/05, J Killea wrote:
>
>HOWEVER, I just did three sets of Foucault measurements along each 
>of 2 diameters and Sixtests (set to "Target b=0" with variable ROC 
>calculation) came back with the ROCs of each diameter differing by 
>about 0.5mm.  
>

Where is everybody? I can't believe nobody has offered a helpful 
response after over 12 hours. 


Mike Peck



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Michael Peck
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