why is it so difficult to create a good parabolic mirror vs. a spherical one?
i think it has something to do with the weight warping it but im not sure
Chief’s Domain - http://www.seas.ucla.edu/~ravi
Weight only becomes an issue when the mirror is large, and effects both types of mirror. For small mirrors it’s mainly a matter of complexity, there is a simple method of producing spherical mirrors using glass disks and abresives, there is no simple method for producing parabolic mirrors. I got this from an article about plans for a giant telescope:
Another cost saving is realized by the use of a mirror surface with a spherical figure rather than a parabolic optical figure. A spherical surface results in a less-than-perfect focus, but spherical surfaces are much easier and more cost effective to manufacture.
Yeah, what he said. Spheres are very easy to polish. The polishing piece is just a piece of a bigger sphere and always fits on the mirror and you just rub it around until everything is smooth. You can’t do that with a parabolic mirror.
Right, a sphere is a surface with constant curvature everywhere. Much easier than having to deal with different curvature at different places.
Spheres are also much easier to test optically. A parabola converts a point light source into a parallel beam or vice versa. So you need a device to make or evaluate a parallel beam as wide as the mirror - typically another parabola or lens with the same diamter, or a flat mirror that reflects the light from the mirror back. On the other hand, a spherical mirror reflects light from a point source at the focus onto itself, so you only need to do somethig clever at the focus. No other big pieces of optics required.