Astrophysics (Index)About

spherical aberration

(mis-focusing by spherically-curved mirrors and lenses)

Spherical aberration is a type of aberration (distortion) in the image of an optical instrument (such as a telescope) due to the shape of a mirror or lens. The image made by a parabolic mirror is truer than one with a spherical curvature, and the additional distortion of the latter is termed spherical aberration. Lenses of various shapes show the same issues.

A paraboloid mirror focuses light that's arriving perfectly parallel to its optical axis to a point (ignoring diffraction, i.e., the Airy disk), and has a characteristic aberration for point sources off the line of the axis. Near its vertex, the curvature of a paraboloid is quite close to that of a sphere, and a spherical mirror approximates what a paraboloid mirror accomplishes, but the further from the optical axis, the more the sphere-shape diverges from a paraboloid and focuses light to the point in space further from the mirror, which on the focal plane, turns a point source in the image into a finite-sized spot. This distortion is spherical aberration. The larger the percentage of a sphere that the mirror represents, the more it diverges from a paraboloid and the greater this aberration, and one way to limit it is to shape the mirror as only a small percentage of a large sphere, which is another way of saying "use a longer focal length". Spherical mirrors are easier to construct, so there is a quality trade-off regarding the resources you put into a telescope's construction. Schmidt cameras use a spherical mirror, compensating for it somewhat with a purpose-built lens (Schmidt corrector plate) but it creates its own problem: a non-flat focal "plane".


(telescopes,aberration)
Further reading:
https://en.wikipedia.org/wiki/Spherical_aberration
http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/aber.html
https://www.telescope-optics.net/spherical1.htm
https://www.handprint.com/ASTRO/ae4.html#spherical

Referenced by pages:
aberration
anastigmat
field curvature
Schmidt camera
Schmidt-Newton telescope (SNT)
Southern African Large Telescope (SALT)

Index