"Robert Morein" wrote in message
...

" wrote in message
ink.net...
A while back I was teasing Morein about getting some measurements wrong
with
respect to a telescope. Actually the Hubble telescope.

Mikey, I had the diameter of the telescope wrong.

And refused to admit it.
And refused to admit you had ever had the discussion.

However, Mr. Allison is incorrect about the use of the pinhole light
source.
He states, "A pinhole CANNOT, under ANY circumstances, create ANY
approximation of a parallel beam unless it is placed approximately an
infinite distance away."

However, the Focault knife-edge test has been used as a method of
examining
the figure of a mirror for hundreds of years. If the mirror is perfect, a
pinhole source placed at the focus of the mirror will cause a reflection
in
the form of a parallel beam.

http://www.answers.com/topic/amateur-telescope-making

When I was a member of the Franklin Institute telescope builders' group,
we
had a pinhole lightsource located approximately 60 feet from the mirror
mount of the optical bench. This enabled us to check the focal length of a
mirror with an approximation of a parallel beam. However, in order to
check
the figure of a mirror, the pinhole source must be placed at the focal
point.

Please let me know if you have any further questions.

Why are you such a fool?