Re: DIY: Parabolic mirror

18 Aug 2008

      Yes,, BUT,,
it may be good enough for a low sreq radio signal,  but in this casting 
they still had to make the reflecting surface the same old way,  
grinding with grits, to even out the surface,,  then polish it,  then 
paraboloze it.
all the spin casting does it take the "Rough Grinding" portion out of 
the process, since 90%+ of the concave shape is raedy.
But as is, it's still no where near as accurate as it needs to be for 
optical work.
Radio, maybe,, optical  nope.
mvm wrote:
...
Joe,
??
As you surely can see this is an example of rotation casting. In this 
case of a large mirror.
Joe schreef:
...
And your point is?
mvm wrote:
...
http://medusa.as.arizona.edu/lbto/tech/mirror.htm
Greg D. schreef:
...
Best "form" is probably not to have one.  Wasn't there a post 
recently talking about making a mirror by spinning casting resin in 
a shallow round dish?  Perhaps a good use for that old record (*) 
player gathering dust in the garage...
Greg  KO6TH
(*) an ancient mechanical instrument for storing music in an analog 
form

...
Date: Thu, 14 Aug 2008 15:59:26 -0400
From: ka2pbt@arrl.net
To: amsat-bb@amsat.org
Subject: [amsat-bb] Re: DIY: Parabolic mirror
agree ..... what would be a good form to start with rather than 
the bottom
of some random water tank??
Assuming you had a proper form ... I think the construction 
process has
merit.
The next step would be to coat the fiberglass shell with something 
metallic
... foil ... screen ????
On Thu, Aug 14, 2008 at 2:04 PM, Franklin Antonio wrote:
...
At 07:34 AM 8/14/2008, Joe wrote:
> Too bad it's not parabolic,  and probably far from even spherical.
>
>
Precisely.  Apparently the author thinks "parabolic" just means 
some kinda
symmetrical concave surface.
This guy obviously isn't an antenna guy.
I've found Make magazine to be a crock in general.  Too bad.

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Joe

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