Re: Alternative mirror technologies, WAS ATM Liquid-mirror telescopes

[Richard Schwartz <richas@idt.net>]

Kurt Hillig wrote:
> 
> On Tue, 24 Jun 1997, Chuck Knight wrote:
> 
> > > I could probably build a mylar
> > > mirror for even less!  (Say, have you actually tried that vacuum
> > > technique, or is it just theory?  I have a 14" embroidery hoop
> > > that's just "screaming" for an experiment!  Why would it form a
> > > parabola, specifically, instead of a spherical mirror?)
> >
> > I've got a question.  I've seen occasional references to mylar
> > film mirrors...pulled into a parabola with a vacuum.  What
> > makes it take on a parabolic shape?
> 
> That's the problem: it doesn't.  But I don't have ready access to a finite
> element program (or expertise with the math) to calculate just what shape
> it does take.  Maybe there's an ME out there who needs a little project?
> 
> (And I have yet to find an embroidery hoop that will hold a vacuum!)

Here is an even better idea:  use GRAVITY instead of a vacuum.  This
means that your mirror focal point will always lie on a plane that is
above the observing floor.  As the mirror is tilted to lower altitude
angles, the focal length gets longer per csc(a), where a is the altitude
angle.