[simpits-tech] rear projection screens

[RickInNoCal at aol.com]

In a message dated 12/20/03 8:58:20 AM Pacific Standard Time, 
hangr18 at hotmail.com writes:

> I'd LOVE to do a setup like this someday, but building a giant
> curved mirror is gonna be a bitch....
> 

       Well, actually, no, not really. The mirror needs to be curved in two 
dimensions, not just one, which actually makes it easier. 

       To make, for example, an 8' diameter curved mirror, make a 8' diameter 
wooden disk out of a couple of sheets of plywood, with a lip about 6" high 
all round the edge. Paint the inside thoroughly with epoxy paint to make it 
airtight. 

       Drill a hole in the lip at some point and epoxy in a piece of plastic 
tubing. Cover the top of this dish you've made with a thin sheet of shiny 
mylar - the stuff they make helium balloons out of - pulled as tight and wrinkle 
free as you can get it, and carefully glued all the way round. Then just use a 
hand held vacumn pump (The sort of thing you use to bleed your brakes) to pull 
a slight vacumn inside the dish. This will suck the mylar down into a 
perfectly parabolic concave shape. You adjust the focal length by adjusting the 
vacumn.

       I used this technique at high school....um, "some" years ago.. with 
clear film and silver spray paint (No silver mylar back then!) to make a 
projection telescope to show a large image of a lunar eclipse for an astronomy club 
open night. Our mirror was 16' in diameter, and 2' deep, and it worked no 
problems. We weren't perfectly airtight, so we had to keep tweaking the vacumn, but 
that was probably a workmanship issue.

Richard
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