Telescope Aberrations

Aberrations are imperfections in the image produced by an optical system.

Chromatic aberration is a lens defect. Starlight consists of all the colors of the spectrum. When starlight passes through a lens, the lens focuses different colors (wavelengths) at slightly different distances. This variation blurs the star image with spurious colors (Figure 2.10). An achromatic lens, a combination of two or more lenses made of different kinds of glass, counters this defect.

A properly curved mirror reflects all the colors of starlight to a focus at the same point. The image formed by a reflector has no blurred colors.

Starlight

Lens

Blue Red _ liaht light

Blue Red _ liaht light

Starlight

Starlight

Figure 2.14. (a) Chromatic aberration. A lens bends blue (shorter) light waves the most and brings them to a focus closer to the lens than red (longer) light waves. (b) Spherical aberration. An improperly curved mirror does not reflect light waves to a single focus.

Spherical aberration is a mirror defect that blurs a star image. It is a defect of spherical surfaces, hence the name. Parts of the mirror at different distances from the optical axis reflect starlight to slightly different focal points (Figure 2.14).

A parabolic mirror avoids this defect. Its paraboloid shape is curved less at the edges than at the center, and so it properly reflects starlight to a single focus. A catadioptric telescope, a refractor-reflector combination, has a correcting lens or plate at the upper end of the tube to correct the aberrations of a primary mirror with a spherical shape.

Why should you have the best-quality optical parts in your telescope?_

Answer: To avoid image aberrations.

You probably wonder which type of telescope is better—a refractor or a reflector. The answer depends on the application involved since each type has advantages and disadvantages over the other.

Small telescopes for hobbyists can be of either design. Refractors, with their sealed tubes, are rugged and require less maintenance. But reflectors offer greater aperture for the price and are easier to make at home. The Dobsonian telescope, a Newtonian reflector on a simple mount, is popular because it is easy to use and cheapest for a given size. Although more expensive per unit of aperture, catadioptric telescopes such as the Schmidt-Cassegrains and Maksutov-Cassegrains are the most compact and portable.

Whatever design you choose, the stability of your small telescope mount is

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