Re: spherical mirrors matching the curve of parabolic

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.

Provided that the radius of curvature R is long compared to the diameter of the mirror D then the spherical mirror is a close enough approximation to a parabola. Faster mirrors need to be parabolised. 

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...

You can do it, but you won't get what you expect. Try it out instead with an aperture mask placed in front of a larger telescope.

Regards,
Martin Brown
.

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