Re: spherical mirrors matching the curve of parabolic
- From: Martin Brown <|||newspam|||@nezumi.demon.co.uk>
- Date: Thu, 28 Apr 2005 09:28:19 +0100
jtaylor wrote:
<dkelvey@xxxxxxxxxxx> wrote in message news:1114642204.391360.86150@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
The only issue with tilting mirror is that you increase the error from a parabola as you move off the optical axis of a parabola. A tilted mirror is most correct when it fits into the location on the surface of a parabola that has the same relative focus point and source. Imagine going up the side of the parabolic surface and drawing a circle to cut out that piece. Now use that piece at the same offset and angle from the original parabolas axis. Dwight
I knew that.
What I don't know is how far from the centre you could put a spherical mirror of some specified dimension before the error would be above acceptable limits.
David Knisely has already posted the quantitative limits. Your two spherical mirrors will deviate from the exact parabolic surface by an amount which increases as you move away from the optical axis.
And as a practical matter, how closely matched, in terms of focal length, am I likely to get two, say, 3" mirrors (buying, not making, me making them would make the answer "not at all close").
Try it out with an ordinary telescope masked to give the same working aperture shape before you waste your time making this device.
Regards, Martin Brown .
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