Re: [ATM] Best webcam/ccd for Foucaultgrams?

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

Most 1/4" imagers are the same similar size so the 4mm is probably a good
figure for them.  You can overfill the vertical (cut off the top and bottom
of the image of the mirror) if so desired to fill out more to the edge of
the sides of the monitor.  If you feel the need occasionally to see the top
or bottom, a simple rocking of the camera will show those areas which are
not significant in the Foucault test although they are useful for the Ronchi
test.  When you boil the formula down to it's minimum, it becomes ~6-8 *
focal ratio for a 75% to 100% fill for the typical webcam with a 1/4" sensor
in it.  There are also 1/3" and 1/2" sensors but they are a lot less common
than the little ones.
Lens size can be an approximate (shorter FL lenses than desired will make a
smaller image while longer FL lenses will make a larger image) and, if you
are using a colored LED, can also be a single lens rather than a achromatic
doublet as there is no color correction needed to focus the light from the
mirror.  If you get the lens right up against the KE, you can even get away
with a 1/4" diameter or so lens!
Bob May
bobmay@nethere.com
http://nav.to/bobmay
http://bobmay.astronomy.net

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