[ATM] re Ross Null Test-would this lens work?

[Mark Cowan]

The Ross Null test is a great sanity check.  Even with
an off-the-shelf optic of dubious quality (I have a
Surplus Shed coated PCX lens just that purpose) it
will show you things like surface smoothness and
(particularly) edge defects that are hard to spot with
COC tests on fast paraboloids.  To use it for
finishing optics I think you really need to have a
well characterized nulling lens, otherwise you're just
guessing.

Best,
Mark Cowan
Salem, OR

--- DrewClan at aol.com wrote:

> Hey, Gil! I've done a couple of mirrors with similar
> Surplus Shed and  
> Edmunds pcx lenses and think that for personal use
> on amateur telescopes  they work 
> fine. There is nothing like a null test! If you look
> back in  the old TM 
> magazine (issue number 49 I think but don't quote
> me), you'll see a  well known 
> professional saying the same thing about cheaper
> lenses in the  letters to the 
> editor. I don't think a pro would be able to sell
> mirrors  done this way, but us 
> amateurs don't necessarily have to spend an arm and
> a leg  on a pcx lens. <snip>

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