Monday, October 26, 2009

Astronomy is for Everyone, Part Three

In this installment we consider what many typically think about first when considering amateur astronomy--choosing a telescope.


At some point in their observing, every amateur astronomer considers whether a telescope can aide them in their observing. The light-gathering and magnifying power of telescopes brings out details of the Moon's surface. It reveals Jupiter's larger satellites and its banded clouds, as well as markings on Mars and the rings of Saturn.

There are three popular types of telescope configurations:

1. Refractors, which use lenses to collect and focus light.

2. Reflectors, which collect light with a large mirror.

3. Catadioptrics, which are a special class of telescope that use lenses as well as mirrors. They are considered by some as modified reflectors.

Refractor Telescopes

The familiar long tube telescope, with the lens in front and the eyepiece in back, is the standard design of the refractor telescope. This design is commonly seen in department stores. While all look generally the same in advertisements, quality varies tremendously.

Beware of advertising claims of extremely high magnification. These are usually achieved by pushing the telescope to its limit, and then the images are not satisfactory.

Look for sturdy mechanical construction. A spindly mount will wobble at the slightest touch and ruin the view. Favor models with low to medium power eyepieces of good quality, rather than those with high-power, low quality eyepieces. Fittings for the eyepieces, diagonal prism, and accessories should be of machined metal, not molded plastic.

Newtonian Reflector Telescopes

The Newtonian reflector (invented by Sir Isaac Newton) is a very popular and economical telescope. Its simple high performance design provides tremendous light grasp at the lowest cost per unit of aperture of any type of telescope. Many observatory telescopes are Newtonian designs. Small Newtonians are very portable because the tube can detach from the mount.

Because the light is gathered and bent by mirrors, the image is rotated and usually appears upside-down or sideways. Their large aperture makes them ideal for deep-space views of galaxies, star clusters, and nebulae. The optical design results in sharp, high-contrast planetary and lunar views.

Compound "Catadioptric" Telescopes

Compound telescopes combine the best features of refractors and reflectors into very compact, lightweight instruments. They use both mirrors and lenses, resulting in telescopes only about twice as long as they are wide. Unlike the basic refractor and reflector, these telescopes are distinctly modern 20th century designs, the products of high-technology manufacturing techniques.

The features are many -- the closed tube, lightweight, rugged designs are easily portable, and the superb optical performance is better in nearly every respect than any single telescope. Little if any maintenance or alignment is required. The lightweight optical assembly allows very strong mounts to be made very light in weight. Camera adapters and many varied accessories are widely available and easily attached. The one significant disadvantage is just what might be expected: compound telescopes cost more than other types of telescopes.

Maksutov-Cassegrain Telescopes

The Maksutov-Cassegrain telescope was introduced by D. D. Maksutov in 1944. It uses a deeply curved, thick front corrector lens, with a reflective spot on the corrector acting as a secondary mirror. Large diameter models are very difficult to manufacture and take a long time to reach thermal stability at night.

Schmidt-Cassegrain Telescopes

The Schmidt-Cassegrain design was made commercially economical due to the optical production innovations of Tom Johnson at Celestron International in the late 1960's. His techniques for producing the complex-curved Schmidt corrector plate were the foundation for every major manufacturer in the business.

Unlike the Maksutov, the Schmidt-Cassegrain has a separate, adjustable secondary mirror mechanically attached to the glass corrector plate. The most popular sizes are 8" to 11" diameter models on fork mounts. As with Maksutovs, large diameter models take a long time to reach thermal stability at night.

In our next installment we will examine telescope mounts...


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