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Selecting a telescope can be a challenging task. This article will help you understand the differences in telescope features so you can make the best decision for a telescope that meets your needs. Important telescope considerations are aperture diameter, mount type, magnification, eyepiece quality, and telescope design, size, and cost.
This article addresses the following:
The aperture, which is the light gathering lens or mirror that points at the sky, is the key to observing more stars and greater detail. A larger aperture gathers more light to see fainter objects and provide brighter, more detailed images. Telescopes gather many times the light of the unaided human eye, allowing us to see incredible views of the night sky. The light gathering ability of some common telescope apertures are as follows:
The aperture diameter is so important that serious amateur astronomers usually try to get the largest aperture telescope that meets their size limitations and budget. In general, the larger the aperture the bulkier and more expensive the telescope.
Mount types are commonly altazimuth or equatorial. Economical altazimuth mounts provide only simple vertical and horizontal controls. More expensive equatorial mounts are designed to locate and track stars in the night sky with declination and right ascension controls.
Magnification is determined by dividing the aperture focal length (f/l) by the eyepiece focal length. For example, using a 25mm eyepiece with a telescope that has a 1200mm focal length produces 48x magnification.
Telescopes with a longer focal length provide greater magnification using any given eyepiece. The magnification produced by various eyepieces with standard telescope focal lengths is shown below.
Magnification can also be increased with a Barlow lens. A Barlow lens mounts between the eyepiece and the focuser. It acts to increase the focal length of the telescope and thus increase the magnification for any given eyepiece. A very common Barlow lens increases magnification by 2x, but Barlows that increase magnification 1.5x, 2.5x, and 3x are also available.
The aperture diameter or light gathering ability limits the effective magnification of a telescope. A basic rule of thumb is the maximum magnification is 2.4 times the aperture diameter in mm or 60 times the aperture diameter in inches. However, atmospheric conditions usually limit the effective magnification to 200x or less. Many inexpensive small aperture telescopes are packaged with short 4mm f/l eyepieces and a 3x Barlow that provide very high, magnification in the 500-700x range. This magnification is useless as it produces a very poor image. However, these telescopes are often a good value--just use the longer f/l eyepieces without the Barlow lens. The maximum effective magnification for common telescope apertures is as follows:
Eyepiece designs and sizes also affect image quality and field of view. Small diameter 0.96" eyepieces are of poor optical quality and included with the cheapest telescopes--they should always be avoided. Standard 1.25" diameter eyepieces are large enough to provide a very good working field of view. They are commonly provided on many intermediate and advanced level telescopes. Large 2" diameter eyepieces are provided on larger advanced telescopes and produce extra-wide views of the sky. Some common eyepiece designs are listed below in order of increasing quality and cost.
Telescope Design, Size, and Cost
Refractor telescopes are what most of us picture in our mind when we think of a telescope. They have a long tube that you look through one end and point the other end at the night sky. The straight-through optical design results in claar sharp images with minimal distortion. Refractors are popular because their simple design is affordable, reliable, lightweight, and easy to use. The sealed optical tube requires little maintenance and functions well under various atmospheric conditions. Most inexpensive refractors have a 60mm objective lens. However, a 70 mm refractor (which collects 36% more light than a 60mm telescope) is considered by many amateur astronomers to be the minimum size for a good quality beginner refractor telescope. It is acceptable for observing bright objects like lunar details, planets, star clusters, and bright double stars. A good 70mm refractor telescope with 2 eyepieces, a finderscope, diagonal mirror, and an altazimuth mount costs $130-200. A more versatile but more difficult to use equatorial mount increases the cost about $50. A 70 mm refractor is often the best choice for families desiring a basic telescope with compact design for occasional sky observation. Choose a refractor with 1.25" better quality eyepieces for best performance.
You might like this refractor model:
Newtonian reflector telescopes have a large concave mirror that collects incoming light and focuses it on a diagonal mirror. The diagonal mirror then transmits the image to the eyepiece. These telescopes are less expensive per aperture diameter since large mirrors are cheaper to manufacture than lenses. The light gathering ability of the larger apertures make them better suited for observing faint objects like star clusters, nebulae, and more distant galaxies. However, they are bulkier and the open optical tube requires more care to maintain. Short-tube reflectors provide all the benefits of a larger aperture telescope with almost same portability as a 60mm refractor. A Newtonian reflector with a large 114mm aperture, two good 1.25" eyepieces, a finderscope, Barlow lens, and equatorial mount costs $200-300. This is a good intermediate telescope for families planning to spend more time together studying the heavens. A 130mm refractor provides even more light-gathering capacity allowing you to see more night sky detail.
We recommend these models:
Dobsonian reflectors have a Newtonian style optical tube that uses a large concave mirror for light gathering and focusing. However, they have a unique, economical, easy-to-use altazimuth mount design. This heavy-duty mount is stable for large aperture and long optical tube telescopes. As a result you can buy more aperture for your money to observe much fainter sky objects. Popular Dobsonian telescopes have 6", 8", or 10" aperture diameters with a long 1200mm focal length. These telescopes out-perform smaller telescopes because of their significantly greater light gathering ability. However, these same features make for a bulky telescope--they are over 50" tall with a 24" diameter base when the telescope is in a full vertical position. A Dobsonian with 1-2 Plössl eyepieces and a good finderscope will cost between $300-700, depending on the aperture. These advanced telescopes are an excellent option for families who want to spend significant time in star study and have a place to store such a large instrument. For families, the Orion StarBlast 6" Astro Telescope is a good choice—it has a Dobsonian-style mount but is still highly portable.
A Maksutov-Cassegrain telescope uses two mirrors and a front correcting lens to "fold" the light. The result is a long focal length telescope (ideal for high magnification) in an incredibly compact tube that produces sharp, detailed images. It is ideal for detailed lunar and planetary observation at high magnification and for bright deep-sky objects at low magnification. These typically have a good quality equatorial mount and can range in size from 90mm to over 11" in diameter! These telescope are for the more advanced user who wants a high-quality, long focal length, large diameter telescope in a portable package. These can be considerably more expensive than a Dobsonian reflector per aperture diameter.
Note: For helpful tips on using your telescope to study the night sky, we recommend this beginner's article from Sky & Telescope.