parabolic or spherical

[Galileo3269]

Which is type of mirror is better, "parabolic" or "spherical"? And what is the difference?

Steve

[KeithBC]

Define "better".

A parabolic mirror reflects all light from a point source to a single focus. A spherical mirror cannot do that without additional corrective optics. With the necessary corrective optics, it can bring light to a focus. Since most modern multicoated lenses lose very little light, the difference is not particularly significant.

Where it makes a difference is if the spherical mirror is uncorrected, or if the corrective optics are of poor quality.

[Galileo3269]

Thanks KeithBC, what are "corrective optics"?

Steve

[KeithBC]

Thanks KeithBC, what are "corrective optics"?
Steve

Eyeglasses for your scope.

In this case, a lens installed in the light path specifically designed to correct spherical aberration. In Bird-Jones telescopes, the corrective lens has magnifying properties and is often mistaken for a built-in barlow.

[Galileo3269]

If i buy a corrective optic. With that improve my views?

Steve

[KeithBC]

If i buy a corrective optic. With that improve my views?

Steve

I have never heard of spherical aberration lenses being available as after-market items. Either it is already built into the scope (Bird-Jones design) or the scope is junk.

[Galileo3269]

Ok

Steve

[jenniferchristine]

I'm not sure it is a matter of buying a lens and plopping it into the light path.
If you're building a telescope with a spherical mirror, the light path and length thereof should be calculated for the 'correct' corrector to bring the light to focus in the correct place.

[pikaia]

The advantage of a parabolic mirror is that light coming along the axis of symmetry can be brought to a focus at one point. However, off-axis the light does not come to a sharp focus.
Spherical mirrors do not bring light to a sharp focus so they require a correcting plate, but as a sphere has no particular axis of symmetry light can be brought to a focus regardless of its direction, which means that you can get a wide field of view and a very small focal ratio. They are also easy to make.

[Galileo3269]

I didnt know the diff. when i bought my scope, or i would,ve payed the extra $30 and got the next one up from mine which had the "parabolic" mirror. Live and learn right?.

Steve