From: "Mel Bartels" 
To: 
Reply-To: "Mel Bartels" 


For those who would put foward the hypothesis that diffraction is caused by
the spider's edges, please explain these two observations

1. take a refractor of 3 - 6 inches aperture and place a single thin bar
say 1/16 inch thick across the front of the lens.  Note the diffraction on
a medium brightness star.  Now double the thickness to 1/8 inch.  The
diffraction noticably increases, yet the length of the edge of the bar
(read spider) does not.

2. make a central obstruction equal to 50% of the aperture from cardboard
and place it over the front of the scope.  Note the diffraction.  Now make
the thinnest possible thread or strand and stretch it across the front of
the scope.  Why is the latter's diffraction incredibly less in extent
despite both situations (central obstruction, stretched strand) having
equal edge values?

Mel Bartels

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