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From: Minus XVII on 20 Jun 2005 19:10
wht is the diameter of a parabola?

NG, Sing wrote:

> It depends upon the f/d ratio. If fast (4 to 6) then it goes out of focus
> quickly off axis. If the mirror is slow (10 to 14) then it does not. there
> is a crossover at the larger f/d ratios (over 10) where the mirror can be
> spherical and not parabolic. Therefore the rays as you ask above would be in
> focus, the same focus.
> (f,d =>focal length, diameter)

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From: Steve Willner on 21 Jun 2005 16:45
In article <42b7409e$1(a)cfanews.cfa.harvard.edu>,
willner(a)cfa.harvard.edu (Steve Willner) writes:
> Just for fun, I tried ray-tracing a 100 mm diameter f/10 paraboloid.
> As expected, rays parallel to the axis focus (ignoring diffraction)
> to a perfect point at a distance of 500 mm from the mirror vertex.

Sorry, I should have written "f/5 paraboloid." It had radius of
curvature 1000 mm, focal length 500 mm, focal length/diameter = 5.

> Rays coming in 1 degree off-axis produce an image with an rms radius
> of 28 microns.
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