[simpits-tech] rear projection screens

[Rob Hommel]

Hey Rick:

This sounds worthy of a complete write up for a Simpits article. Teach us
all how to do this it is something many of us have thought about doing. At
least I know that I have.

Keep 'em Flying
Rob Hommel
----- Original Message ----- 
From: <RickInNoCal at aol.com>
To: <simpits-tech at simpits.org>
Sent: Saturday, December 20, 2003 11:56 AM
Subject: Re: [simpits-tech] rear projection screens


> 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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