RE: [ATM] Ross null test?

["lou krajci" <loukrajci@comcast.net>]

>From: "Rob" <polaris@gsinet.net>

>I am thinking of using a null test for my next project, a 10" f/6.
>Is the Ross Null generally the best, easiest for an ATMr like me?

>Where can I get a reasonably priced reference lens?
>Ceravolo offers one but it's > $500.

That high cost is one reason I recommend ATM's pursue the Maksutov/Waineo
null test.

See:  http://www.melbartels.com/dnld/waineo96.html (in English)
and: http://www.astronomer.ru/data/library/books/sikoruk/glava2/2_32.htm (in
Russian)  See Fig. 59 in middle of web page to see start of discussion of
this null test.

Why do I recommend this test?  It requires a concave, spherical mirror,
about 1/2 the size of the  mirror you want to test.  In your case that's a
five inch mirror.  I bet you can make a five inch sphere for far less than
$500.  (And you can null test it yourself...to be certain of its accuracy.)

No lenses are used, so you don't have to worry about filtering light to
avoid chromatic aberration problems with a nulling lens.

Also, this test can be used to null test any concave asphere...hyperboloids,
ellipsoids, etc.  You just have to change the spacing of the spherical
mirror and mirror under test.

I hope this helps.

Tom Krajci
Albuquerque, New Mexico
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