Since Galileo introduced the telescope to astronomy in 1609, the device has been refined and adapted several times. Amateur and professional astronomers can find a telescope for any purpose, and which kind to use depends on budget, experience level, and what you’ll be viewing Three of the most popular versions are refractors, reflectors and Cassegrains.

Refractors

Refractors have a long, thin tube, and use a glass lens to gather and focus light. Refractors are sturdy, require little maintenance, and have sealed tubes to prevent dust or air from interfering with viewing. Larger refractors produce clear, high-contrast images, and smaller models are popular as starter telescopes because they’re also inexpensive. Refractors can reveal details of the planets and the moon, even when viewing them under city lights. However, because refractors generally have small apertures, they may not work well for viewing faint, distant objects like galaxies and nebulae. Refractors can also be used for daytime land viewing, particularly if used with a “correct image” diagonal, instead of the “star diagonal” (or right angle mirror) usually used.

Reflectors

Reflectors were designed specifically for astronomy, and use a concave mirror located at the back end of the telescope’s tub to gathers and focus incoming light. Reflectors are large and have a fat tube; they are also heavier and bulkier than many other kinds of telescope, and the mirror alignment must occasionally be adjusted. However, they are popular with amateur astronomers, because it is less expensive to make a big mirror than to make a big lens, which is what refractors use, and so it is relatively simple to make a reflector. Reflectors are ideal for viewing everything from nebulae to galaxies, but are not recommended for land viewing because they create upside-down or rotated images. This isn’t as much of a concern for astronomy.

Cassegrains

Cassegrain telescopes come in several forms, the most commonly used being the Maksutov-Cassegrain and Schmidt-Cassegrain designs. Cassegrain scopes have compact tubes, making them more portable than refractors or reflectors, even those with the same size aperture. Cassegrains have primary and secondary mirrors that direct light out the back of the telescope and into an eyepiece. In the front is a “corrector” lens, which is curved in the Maksutov and flat in the Schmidt. The “closed tube” design keeps the inner workings clean. Maksutov scopes work both for space and land viewing; they also create sharp images, and are ideal for viewing everything from the Moon to deep-sky objects. Schmidt-Cassegrains are especially suited to viewing fainter, distant objects like star clusters and galaxies. They’re also easy to attach to cameras, and so can be used for astrophotography.