Re: ATM An $18.00 Null Test

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

Rick Wagner wrote:

> That might be an interesting project for the future.  I have a surplus
Ross
> lens I bought off Peter Ceravolo.  He says it is quite good but I have no
> way to test it.  I have a 10" perforated sphere that I was going to try
as a

Which is the ever present dilemma of course.

I believe thought should be given to how we can bootstrap ourselves from
nothing and make a verifiable null lens.

This is possible by making it in the form of a meniscus with concave back. 
There is no reason it has to be plano.  This has been traditional because
it made calculations easier but the availability of raytracing software
voids this restriction.

1.  Make and test concave back - eg R = 600 mm

2.  Test and figure front surface through the back.  This will impose a
slight conic on the surface. 

3.  Calculate this conic.

4.  Record the lens specs. R1, R2 ,  conic deformation, and t.
An example is R1 = 600mm, R2 = 300 mm(conic -0.2389) , t = 12 mm. (BK7)

4.  When using it, raytrace for spacings using these parameters.

5.  Now test and figure the mirror with it.


This has the advantage that every stage is verifiable.

Unfortunately, the ray cone is reasonably wide when testing through the
back.  But it is possible.

It may be possible to pick up a surplus lens which is close.  Then test and
refigure if necessary.

Of course, if you have access to other trustworthy testing methods, this is
all unnecessary.

All the best,

Peter Smith.