From: Guy Brandenburg 
To:  atm{at}shore.net,  novac{at}his.com
Reply-To: Guy Brandenburg 


I have a projector lens that I purchased really cheaply a few years ago,
intending to make a finder scope. It looks like the lens is plastic for the
most part, and is highly curved, and its diameter at what looks like the
front end is about 85 mm (~3 1/2 inches). Today I finally got around to
trying to see what its focal length was so that I could make a finder scope
out of it. The clear diameter of the lens is about 65 mm (~2 1/2 inches),
and the widest part of the plastic lens holder assembly is about 135 mm (5
1/2 inches).

To my surprise, the distance from the back of the back lens component to
where the sun forms an image on a wall is - get this - about 30 mm (about 1
1/4 inches)!! Thus, this thing has an f ratio in the neighborhood of 0.5,
or perhaps, less.

Whew.

It occurs to me that I could perhaps lengthen the focal length with the
equivalent of a barlow lens, which, I understand, is simply a negative
lens, preferably an achromat. I imagine that I will have to play around
with this to get it to work at all.

Any suggestions?

Guy Brandenburg

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