Telescope Buying Guide

A telescope is your front-row ticket to the dazzling celestial show that plays out overhead every night. Due to their scientific roots and obscure vocabulary, telescopes are extremely difficult pieces of equipment to buy, but armed with a few definitions and an understanding of how a telescope works, you'll be ready to explore the heavens.

Basic Telescope Choices
The type of telescope you should choose depends on what exactly you want to study. If you want to check out moons and planets, a refractor telescope is the way to go. If you'd rather check out deep space objects, such as nebulae and distant systems, reflector telescopes are perfect for the job.

A telescope can be tough to move, so you'll need to balance portability with magnification. If you live in a suburban area subject to light pollution, you'll want a telescope that you can pack up and take with you to the dark back country for better observation. If you already live in a rural area and have a dedicated space for your telescope, buying a large telescope is a more viable option.

Telescope Types
Telescopes come in three basic designs: refractors, reflectors and catadioptric telescopes. A refractor telescope is a long tapered tube with the objective, or main lens on the far end of the telescope to gather light and a secondary lens to focus the light to the eyepiece. These telescopes provide the most detailed views, but due to potential errors in the lens, many display visual aberrations.

The most common chromatic aberration in a refractor telescope is a coma, or a colored blur surrounding light objects like stars or planets. Telescope manufacturers address this issue with coatings and lens systems. Achromatic lenses include a series of lens elements to bend the light, reducing the effects of visual aberrations. An apochromatic telescope goes one step further, combining multiple lens elements with special materials, like extra-low-dispersion (ED) glass.

Refractors tend to be larger and more expensive than the second option: reflectors. Reflector telescopes, the most common of which is the Newtonian Reflector, contain a mirror at the top of the telescope that reflects the light down to an objective mirror at the bottom, which sends light through the eyepiece. Because reflectors don't bend the light, they are not subject to chromatic aberration. Images viewed in a reflective telescope experience a loss of contrast and scattering of light, due to the central obstruction of the secondary mirror.

The mirror coatings in a reflector telescope break down over time, so they must be recoated every several years to ensure continued use. Transporting a reflector telescope can cause the mirrors to wobble and require frequent collimation, or adjustment of the mirrors.

The best of both worlds is the catadioptric telescope. These telescopes, often named for their component parts, such as Schmidt-Cassegrain, use both lenses and mirrors to get the best possible picture in a reasonably sized telescope.

Magnifying Power
Compare the focal length and the aperture size to accurately measure the magnifying power of a telescope. Don't rely solely on manufacturer specifications, as they often describe only the raw magnifying power of the elements without consideration of the way they work together.

The focal length of a telescope is the length of the telescope from the main lens to the point where light converges. To accurately measure the magnifying power of a telescope, divide the focal length of the telescope by the focal length of the eyepiece. If you have a 1,000mm tube and a 25mm eyepiece, the magnification is 40X.

Focal ratio is a slightly different way to express telescope magnification. A focal ratio uses the focal length and the aperture size. The aperture is the size of the large objective lens or mirror, typically ranging from 2.4 to 10 inches in consumer models. To determine the focal ratio, divide the aperture size into the focal length of the telescope. Say, for example, that you have a 2.5-inch aperture and a 25-inch focal length. The focal ratio of that telescope is 10, displayed as f10. Low-magnification eyepieces require less light to focus, so they function best for dim objects. High-magnification eyepieces provide more details but also require more light.

Telescope Mounts
A telescope is useless without a mount to secure it during observation. The most common mounts are alt-azimuth, also known as altazi, altazimuth or just plain azimuth, which refers to the vertical and horizontal motion of the mount. A common type of alt-azimuth mount is the Dobsonian-you can simply push the telescope up, down, left or right without making extensive adjustments. Some mounts offer motorized adjustments, such as a Driven Alt-Azimuth, which helps move the telescope and track objects. An Equatorial mount follows the Earth's movement to keep an object in the field of sight.

Useful Telescope Features
Having trouble finding things? Some telescope mounts feature optional Digital Setting Circles, also known as DSCs, which are astronomical coordinate systems that track objects and include catalogs of objects. To use this type of system, you calibrate it by focusing on known objects and then look up an item in the catalog, following coordinates until the display reaches zero and the desired object is centered in the field.

Go-to-drives are DSCs combined with motors that point the telescope automatically to an object from a preset catalog. If you don't have a DCS or Go-to-drive, the most basic feature to use your telescope successfully is a scope. A scope has a much wider field of view than the telescope, and you use it to locate the item you want to view so that you can focus on it with the telescope. Filters that enhance or block certain types of light are also useful. Manufacturers sell filters that make it safe to view solar objects, or filters to block ambient light so you can view dim and distant nebulae.

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