Re: [ATM] Ross null test - would this surplus shed lens work?

[Ric Rokosz <radicell2@yahoo.com>]

--- Mark Cowan <toolontop@yahoo.com> wrote:

> problem with fused silica - just use the proper
> refractive index in the position calculation.  And
> post your results, it sounds like that could be a
> really nice lens for a Ross Null setup...

Just an added point,you need to know the index of
refraction for the wavelenght you plan on using.

Also you need a buffer zone around the edge of the
lens that wont be used,so you need to size the lens
accordinly to take that into account.

The lens can be fairly "lousy" on surface
tolerance.What you'll see in the test is a milky type
of haze - degree of this unclarity will vary with the
degree the convex surface departs from a sphere.Not to
worry about it as Ronchi bands will stand out in any
event and will show the mirror's defects.

If you have a convex pyrex tool from a previous mirror
use that - polish it out of course - as you know the R
value to a high degree and the focal lenght will be
long.Trust me,I've done it and compared the results to
a custom BK7 lens I made.

Ric

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