From: "Bob May" 
To: "atmlist" 
Reply-To: "Bob May" 


As to vignetting of the camera, you have to look at the optical system used
and the characteristics of the light source. First look at the light
source.  If it is having trouble filling the aperture of the mirror with an
even amount of light, you are not going to be getting a decent result with
any method of testing so we can throw that one out as a source of error in
the testing as it will be true of any method of doing the test.
The other is the use of the camera and how it is setup.  Note that the
light at the KE is essentially a point source (the theory is to have a
point source of light which gets imaged back to the KE as a point) and the
closer you get the lens to that point the less of the lens is needed to get
an image through the optical system of the camera. This is even noticed
when you don't have your eye all the way to the KE as you don't have the
mirror surface fully illuminated.  Thus, unless the camera lens has some
problems, you can pretty much eliminate the lens as a source of problems,
especially as you are able to get a simple camera lens (if you are doing a
LED source, you can use simple single lenses instead of complex
multielement lenses as you only need to focus at one color!) right at the
KE without fear of cutting your face up. As a result of the above, I'd
figure that the RoboFoucault version of the tester will be able to get
better readings from the mirror surface because of the better illumination
that can be derived from the lens being closer to the KE.  In addition, the
camera (and the accompanying electronics) will be able to sense the
difference in brightness better than the eyeball can becuse it is staring
at the whole image rather than the eye's problem seeing the two zones at
one time and successfully detecting the brightness difference between them.
Bob May
http://nav.to/bobmay
bobmay{at}nethere.com
NEW! http://bobmay.astronomy.net

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