> On Mon, 24 Jul 1995 MbWanderer@aol.com wrote:
> >
> > I think what the person who sold you that mirror meant was that the mirror
> > was not a paraboloid. A paraboloid is the shape needed to construct and
> > newtonian reflector, the only kind of telescope that most amateurs can build.
The primary problem (sorry, couldn't resist the pun) with a Newtonian using a spherical mirror is spherical aberration (blurry everywhere) not coma (blurry off axis).
Fast spherical and fast paraboloidal mirrors in Newtonian telescopes will both have lots of coma (the same amount in fact), however the spherical abberation of the fast spherical mirror will be so bad you will never notice the coma.
Slow spheres are pretty much the same shape as slow paraboloids. The often quoted example is a 6 inch f/10 sphere has just less than 1/4 wave (peak to peak wavefront error) of spherical abberation. That is the surface of the sphere differs from a paraboloid by just less than +/- 1/16 wave at the worst spots. This is usually considered "diffraction limited." Now f/10 is not a magic number, it only works for a 6 incher. A 12 inch f/10 sphere has 1/2 wave of spherical aberration. This is usually considered "not good enough" by us hotshot glass pushers. :-) So you really can only use a spherical mirror in a Newtonian if it is in fact very very close to the desired paraboliod shape. You might as well just call it a paraboloidal mirror at that point.
Chuck