Re: What are parabolic mirrors?

Hi, Rianna - great question! Actually, any mirror above 6"
in diameter is generally paraboloid rather than spherical.
(A 4.5" or 6" is about the smallest reflector anyone makes!)

Now if you're asking WHAT a paraboloid is, as opposed to a
simple section of a sphere, imagine this: Slice the top off
a hollow sphere (a globe). Then stretch the sides of your
top-piece, until it is JUST wide enough that it will never
fit snugly back into any hollow sphere, no matter HOW BIG.
You've now created a paraboloidal section! :")

The shape of the concave mirror MUST be paraboloid (or a
pretty close approximation to it), in order to bring light
from all across the surface of the mirror to focus at a
single point... A spheroidal mirror would focus light from
its edges at a different spot than from its center!

BTW the reason smaller mirrors, six inches/15 cm and under,
are generally spherical is that their surfaces are just so
small: the difference in focal points between a spherical
6" mirror and paraboloidal 6" mirror just isn't noticeable.

Clear skies Rianna!
Lew Gramer
Medford, MA, USA

[Moderator's note: most astronomy textbooks will have a section on
telescopes and mirrors, if you want to see pictures of how the
different mirror shapes work.]