Re: ATM Real Time Automated Foucault Tester

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

If you've got a full illumination of the mirror from a pinhole source, you
are good as to distance.  The shorter FL of a F4.5 vs. a F7 or so mirror
makes that distance more critical.  Note that a pinhole or short slit is
where the test needs to be done from for this particular spec.  An elongated
slit will show a vertical fill but won't show a horizontal fill.  The
aperture of the lens is what also determines the distance you can put the
camera behind - the ordinary geometric ratio of angles is what is in effect.
That point spot on the KE needs to not fill the aperture for valid results
and the smaller the diameter of the mirror's image on the lens makes the
lens work better for imaging and focus results.  I might note that a
mechanical iris will shrink the effective diameter of the lens.  Thus, you
can use a particular setup if the light is dim but lose that fill of the
mirror's surface when a brighter light is used and the iris closes down.
In a very dark testing enviroment, you actually don't need a lens at all but
you do need to put the imager close enough to meet the criteria of the
diameter of the image of the mirror doesn't flow over the size of the
imager, usually from 1/4" to 1/2" on the diagonal measurement these days.
Bob May
http://nav.to/bobmay
bobmay@nethere.com
NEW! http://bobmay.astronomy.net