Re: ATM Robo-foucalt...is it calibrated?

["Bob May" <bobmay@nethere.com>]

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@nethere.com
NEW! http://bobmay.astronomy.net