Re: ATM An $18.00 Null Test

["PETER JOHN SMITH" <pjifl@bigpond.com>]

That this is really an extension of the Dall Null Test has been mentioned.

Just a few more thoughts.

In S&T Sept-76 there is an article about extending the Dall test to other
conics. Mention of varying the distances from a centre of curve position is
made, but the computations were too hard to summarise simply so could not
be included in the article.

I am sure if the author, or Dall, had raytracing software available that
would find the position of, and evaluate the quality of the setup they
might have used it. Computing power is magic in optics !!!!

I imagine a tube which mounts the lens and knife edge or ronchi or whatever
at a measured spacing may be used.

The area of the lens traversed by rays is of the order of 5/8 and 3/8 inch
diameter depending on orientation (useful nulls occur for each orientation)
 This small area makes it easier to get a small lens of sufficient
accuracy.

BUT 

My calculations seem to suggest that, while the test might well indicate
zones which hide in the Foucault test, it will be very difficult to use to
judge the amount of parabolic correction required because 0.001 inch
variation on the KE - lens spacing, or 0.1 % error in R of the lens produce
a null with gives a mirror with about 1/8 wavefront error when used.

This would make it overly sensitive to unavoidable errors.  Have I made
some stupid error ??? 

All the best,

Peter Smith.