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

["matt" <electro_optic@bellsouth.net>]

the French software from Project Roddier :

http://www.astrosurf.com/tests/roddier/projet.html#download

seems to be the tool . All one needs is a lowly webcam, a cheap laptop and
the free software from above . I have tried the software with images from a
Logitech Quickcam . I have used a small subnotebook the size of a VHS tape,
a Toshiba Libretto for the image acquisition and analysis . A complete setup
can be had for a couple hundred $ , sub Gigahertz laptops are real cheap
(and they have SERIAL and PARALLEL ports ) .

What the above setup lacks is a way to do a sanity check , or better said,
how to make sure there are no systematic errors in the setup or processing .
While the problem seems trivial, it is not.
The Roddier software measures only a complete telescope .
All bench methods which do not involve large autocollimation flats measure
only the primary mirror .
So bottom line, I am left staring at a primary mirror bench measurement
stating this and that wavefront error, and also staring at a totally
unrelated complete telescope measurement, with a completely different error
map.
I don't mind staring but I found no way to reconcile the 2 , or find a
correlation , short of measuring the complete telescope on the bench with a
large autocollimation flat . How do I measure a 16"+ aperture scope, when I
don't have or want to make such a flat , and compare the measurements with
the Roddier method to find out if there is a systematic error ?
It's great to have several tests available and avoid a Hubble style fiasco,
but if they're impossible to correlate, it is impossible to draw any
conclusions other than something is fishy and not know what .


best regards,
matt tudor

-----Original Message-----
From: Mel Bartels <mbartels@bbastrodesigns.com>
To: ATM List <atm@atmlist.net>
Date: Wednesday, July 06, 2005 12:10 AM
Subject: Re: [ATM] Question about "Null-testing"


>>    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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>

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