Re: spherical mirrors matching the curve of parabolic


<dkelvey@xxxxxxxxxxx> wrote in message
news:1114642204.391360.86150@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>
> jtaylor wrote:
> > The stuff I read says they match well enough for small mirrors that
> it
> > doesn't matter which you use - but this is for a comparison of two
> mirrors
> > each centred on the optical axis.
> >
> > What I'm wondering is if you could use a small pair of spherical
> mirrors
> > tipped a bit towards a single flat one and two eyepieces to make
> light cheap
> > binoculars...lighter at the far end anyway...
>
> Hi
> 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.

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").


.

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