Re: [ATM] Question about "Null-testing"

["Mel Bartels" <mbartels@bbastrodesigns.com>]

>    Effectively, that's what he's doing.  If you use a Ronchi in the 
> focuser
> AT (or very near) focus, you can easily use the edge of ONE band as a 
> knife
> edge.  It's very simple to position since you can use any band out of 
> dozens
> near the center of the field.  As one drifts by, wait for the next one.
>    I was able to see a bend in the shadows that was hardly detectable with
> the usual star test.

Right.  I totally agree.  However, the original thread had to do with a true 
knife-edge being easy to use, and why aren't more amateurs using it. 
Texereau used timed exposures on the 82 inch.  I downloaded the French 
analysis tool and it shows great promise in analyzing total optical system 
defects.

I'd like to see a digital imaging tool that can be easily moved from scope 
to scope to analyze the optics.  We could learn a lot from a quick survey of 
many scopes at a large star party.

Mel Bartels 

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