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<DIV><FONT face=Arial size=2>Wow! That is really cool. Thanks for the
suggestion!</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Brian</FONT></DIV>
<BLOCKQUOTE
style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: black"><B>From:</B>
<A title=RickInNoCal@aol.com
href="mailto:RickInNoCal@aol.com">RickInNoCal@aol.com</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A title=simpits-tech@simpits.org
href="mailto:simpits-tech@simpits.org">simpits-tech@simpits.org</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Saturday, December 20, 2003 2:56
PM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: [simpits-tech] rear
projection screens</DIV>
<DIV><BR></DIV><FONT face=arial,helvetica><FONT lang=0 face=Arial size=2
FAMILY="SANSSERIF" PTSIZE="10">In a message dated 12/20/03 8:58:20 AM Pacific
Standard Time, <A href="mailto:hangr18@hotmail.com">hangr18@hotmail.com</A>
writes:<BR><BR>
<BLOCKQUOTE
style="PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #0000ff 2px solid; MARGIN-RIGHT: 0px"
TYPE="CITE">I'd LOVE to do a setup like this someday, but building a
giant<BR>curved mirror is gonna be a
bitch....<BR></BLOCKQUOTE><BR><BR> Well,
actually, no, not really. The mirror needs to be curved in two dimensions, not
just one, which actually makes it easier.
<BR><BR> 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.
<BR><BR> 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.<BR><BR> 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.<BR><BR>Richard</FONT>
<P>
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