Re: [ATM] fringe testing a convex and concave surface together.

[Mark Holm <mdholm@telerama.com>]

Mike Lockwood wrote:

> Additionally, fringes should be viewed from a distance of at least 10
> times the diameter of the pieces away from them in order to get an
> accuracy of 1/10 wave.  Typically I try to photograph the fringes with
> my digital camera from about twice this distance away.

Not personal experience here, but reading things in S&T and the late ATM
Journal.  Isn't the preferred method to use collimated light?  A fellow
writing in ATMJ (the very last issue, if I remember correctly) mounted a
Newtonian upside down and used a beam splitter at the eyepiece end so
the scope acted as both a collimator and decollimator.  He was also
using the water flat method.  As I recall the S&T article from even
longer ago, the author suggested using a Fresnel lens with a simple beam
splitter to collimate.  I don't know if he viewed at a distance or used
the Fresnel lens as a decollimator as well.  Evidently, the optical
perfection of the collimator/decollimator can be pretty awful without
messing up the test.  The only hitch I remember is that he recommended a
fairly high f-ratio Fresnel lens.  Most of the ones on the surplus
market are out of overhead projectors, and have fairly low f-ratio.


-- 
Mark Holm
mdholm@telerama.com

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