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Home / Science lessons / Selecting a Telescope
  • Selecting a Telescope

    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:

    • Telescope Features
    • Telescope Selection

    Telescope Features

    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:

    Aperture Diameter

    Light Gathering Ability

    60 mm

    70 times the unaided human eye

    70 mm

    100 times the unaided human eye

    90 mm

    165 times the unaided human eye

    114 mm

    260 times the unaided human eye

    130 mm

    335 times the unaided human eye

    6-inch

    475 times the unaided human eye

    8-inch

    840 times the unaided human eye

    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.

    • Altazimuth tripod mounts are often used on less advanced telescopes. These mounts are easy and natural to use. You just rotate the telescope on the tripod horizontally and pivot it up or down vertically until it points at the desired object. Then lock the mount to hold the telescope in place. They work well for casual viewing of the night sky.
    • The Dobsonian mount is a very stable, easy-to-use altazimuth design used on large reflector telescopes. The telescope simply rotates on a swivel base (like a lazy susan) for horizontal movement and pivots on a cradle for vertical movement. It eliminates the tracking problem of tripod altazimuths by using tension springs to hold vertical position and the weight of the telescope to hold horizontal position on the base. Slight pressure on the telescope tube allows easy positioning and tracking of night sky objects. These mounts are so natural that even young children can learn to use them.
    • Equatorial tripod mounts are more expensive and more difficult to learn to use initially. However, they are very stable and provide the ability to quickly locate specific stars by the right ascension and declination of the star address published in a star atlas. This is a real benefit for serious astronomers. In addition, the right ascension control allows easy tracking of objects as the Earth rotates.

    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.

    Magnification = 1200mm aperture f/l / 25mm eyepiece f/l

    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.

    Eyepiece f/l

    450mm f/l Telescope

    700mm f/l Telescope

    900mm f/l Telescope

    1200mm f/l Telescope

    1300mm f/l Telescope

    6mm

    75x

    117x

    150x

    200x

    2170x

    10mm

    45x

    70x

    90x

    120x

    130x

    17mm

    26x

    41x

    53x

    70x

    76x

    25mm

    18x

    28x

    36x

    48x

    52x

    32mm

    14x

    22x

    28x

    38x

    41x

    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:

    Aperture Diameter

    Maximum Magnification

    60 mm

    144x

    70 mm 168x
    90 mm 216x

    114mm

    274x

    130mm

    312x

    6-inch

    360x

    8-inch

    480x

    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.

    • Super Ramsden (SR) and Huygenian (H) eyepieces are the least expensive and lowest quality. Their simple two-lens design is reliable with acceptable image quality but a narrow field of view. These eyepieces are fine for beginners and are commonly provided on small, inexpensive telescopes. The 4mm and 6mm eyepieces are rarely functional. We do not currently sell telescope with these lower quality eyepieces.
    • Kellner (K) and Modified Achromat (MA) eyepieces have a better quality three-lens design that produces a wider field of view with less distortion. They also provide good eye relief, which is important for people with glasses. These eyepieces provide a good balance between performance and economy and are often provided on intermediate level telescopes. The Orion Explorer II eyepieces that come with some of the less expensive Orion telescopes are good-quality 3- and 4-element fully-coated eyepieces of this design with a 50 ° apparent field of view.
    • Plössl (P) eyepieces have a complex four-lens design that provides an even wider field of view along with better image quality and contrast. They provide great good eye relief and include folding rubber eyecups to block stray light. Although more expensive, they are very popular with amateur astronomers and are usually provided on better intermediate and advanced telescopes. The Plössl multi-coated eyepieces included with most Sky-Watcher and Orion telescopes provide clear, sharp images with impressively high contrast and a 52° field of view.

    Telescope Selection

    Telescope Design, Size, and Cost
    Now that you have an understanding of basic telescope features, it is time to consider design, size, and cost. These factors are addressed in the following descriptions of the three most common and affordable telescope designs. This information combined with what you have already learned will help you choose the best telescope for your family.

    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:
    Orion Observer 70mm Refractor Telescope

    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:
    Orion StarBlast 4.5' (114mm) EQ Reflector Telescope
    Sky-Watcher 130mm with Equatorial Mount

    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.

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