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As you might imagine, there is no simple answer to this question.  The first thing we ask someone when they come into Starizona to shop for a telescope is "What do you want to do with the telescope?"  Deciding what you want to be able to do with a scope can narrow your choices considerably.  Thinning out the herd is a good idea since there are around 400 telescope models on the market!  Once you know what type of telescope you need, the biggest factors become features and budgetary considerations.  Sometimes the biggest or most expensive telescope is not the best choice.  The page below will walk you through the process of determining the best scope for your interests and budget.  The first section describes the basics of telescopes and their various designs.  The next section describes the different features of each type of telescope.  The final section points you in the right direction based on your observing priorities and budget.  For more details on the specific telescope designs, see the Telescopes page.

 

Choosing a Telescope

The decision of which telescope is best should be based largely on what you want to do with a telescope, as well as what you think you will want to do in a few years.  You might not be interested in photography now, but if you are considering it down the road, it might be best to buy a telescope with photographic capability now and avoid having to upgrade later.  Also, keep ease of use and portability in mind.  Be realistic about how much telescope you really want to carry around.  More than a few people get 12" SCTs during a bout of aperture fever, only to regret it later every time they have to lift 80 pounds in the middle of the night.  Simplicity is important as well, especially for beginners.  There is no reason to get a complicated telescope that will frustrate you while you should be enjoying the wonders of the universe.  Keep these things in mind when shopping for a telescope and you will end up with the right instrument to keep you stargazing for years.

 

Telescope Terms

A check of the Glossary on this website will show that there are many specialized terms in amateur astronomy.  However, for the purposes of selecting a telescope, there are only three terms that describe just about everything you need to know about an optical system:  aperture, focal length, and focal ratio.

 

Aperture

The diameter of the lens or mirror in a telescope--the aperture--is the single most important factor for stargazing.  The bigger the aperture, the more light the telescope gathers.  Ultimately this is the main purpose of a telescope:  to gather as much light as possible and funnel it into your eye.  Since the light-gathering ability of a telescope is determined by the area of the lens or mirror, doubling the aperture quadruples the light-gathering ability.  A small difference in aperture makes a big difference in what you see.  A look at the picture below shows the difference between the area of an 8" telescope mirror and the typical 7mm opening of the human eye.  An 8" telescope gathers more than 800 times as much light as the unaided eye.

Above:  Comparison of the relative areas (and light-gathering ability) of an 8" mirror and the human eye

Bottom Line:  When choosing a telescope, aperture is the most important thing to consider.  The usual recommendation is to buy as much aperture as you can afford (or can lift, since large-aperture telescopes get quite heavy).  It's as simple as this:  The bigger a scope is, the more you will see.

 

Focal Length

Less important than aperture, but still helpful to know, is the focal length of an instrument.  The focal length is simply the effective distance from the lens or mirror to the focal point, where an eyepiece or camera would go.  In refractors and Newtonian telescopes (described more below), the focal length is the actual distance from the lens or mirror to the focal point, as shown below.

Above:  Focal length of a telescope mirror

In some telescopes, such as the popular Schmidt-Cassegrain telescopes (SCTs), the optical path is folded, bouncing the light off two mirrors.  In this case, the distance from the mirror to the focal plane is about twice the length of the telescope tube.  However, due to the curvature of the mirrors, the effective focal length is around 5 times the length of the tube.  The diagram below shows how this occurs.

Above:  How a 2-mirror telescope can create a long focal length in a short tube

The importance of focal length is in determining magnification.  The longer the focal length, the greater the magnification with a given eyepiece.  This does not mean you can get more magnification with a longer focal length (maximum magnification is limited by the aperture), but only that you can get more power with a given eyepiece.  Magnification is determined simply by dividing the focal length of the telescope by the focal length of an eyepiece.  For example, a telescope with a 1000mm focal length gives a magnification of 40x with a 25mm eyepiece.  (1000mm/25mm = 40.)

For photography, focal length does determine magnification.  The longer the focal length, the greater the magnification and the smaller the field of view.  This is why long-focal-length telescopes such as SCTs are used for imaging small objects like galaxies, while short-focal-length telescopes such as small refractors are used for wide-field imaging of large subjects like nebulae.

Bottom Line:  For most visual observing, focal length is not a deciding factor when picking out a telescope.  Sometimes manufacturers offer the same aperture telescope in different focal lengths.  In smaller-sized telescopes, it is usually better to select the longer-focal-length version as it will have better optics and will still be a manageable size.  In larger apertures, a longer focal length is often impractical, so a shorter focal length is a good choice.  For photography, focal length is a more important consideration, based on what you want to image.

 

Focal Ratio

For most applications, the focal ratio is the least important factor of the three listed here.  However, it does come into play for certain applications and can tell you about the optical design of a telescope.  Focal ratio is simply the ratio between the focal length of a telescope and its aperture.  For example, an 8" (200mm) aperture telescope with a 1000mm focal length has a focal ratio of f/5.  (1000mm/200mm =  5.)

Visually, the focal ratio is not tremendously important.  Only when the focal ratio is very small does it begin to significantly affect the image quality.  This is not normally seen in commercial telescopes.  All things being equal it is easier to make a telescope with a larger focal ratio, and a larger-focal-ratio telescope will have fewer inherent aberrations.  In practice, this difference is normally not noticeable.

Focal ratio is very important photographically because it determines how quickly a picture can be taken.  For this reason, smaller focal ratios are called faster, while larger focal ratios are termed slower.  Thus an f/5 telescope is faster than an f/10 telescope.  In fact, for photography, focal ratio is more important than aperture.  This surprises most new photographers.  Consider the image below, which was taken with a 3" telescope.

Above:  Image of the Orion Nebula taken with a 3" refracting telescope

Bottom Line:  For visual observing, focal ratio is pretty unimportant.  Aperture is always the most important consideration.  For photography, though, a fast focal ratio can mean the difference between pictures that require hours versus minutes to capture.  Part of the reason for the popularity of SCTs is the ease with which they can be converted from f/10 to f/6.3 to even f/1.8 for certain models, making them ideal for a variety of applications.

 

Optical Designs

There are three basic telescope designs.  Which you choose depends on the observing priorities you have.  Each is suited best to a different application, and some are more versatile than others.  The basic designs are refractors, reflectors, and catadioptric telescopes (which include SCTs).  The section below details the basics of each design and how it applies to choosing a new telescope.  For more details on each design, please see the Telescopes section of the website.

 

Refractors

Refracting telescopes use lenses to gather light.  The primary advantages of a refractor are that it is easy to maintain, easy to use, and is capable of terrestrial observing as well as stargazing.  It also has the distinction of looking like a telescope.  This seems trivial, but many a reluctant spouse has been sold on a telescope that looks nice sitting in the living room. 

There are two basic categories of refractors, achromatic and apochromatic.  Easier to remember might be inexpensive and expensive.  Even inexpensive refractors cost more than reflecting telescopes of the same size, and do not generally provide better images.  The primary advantage of an inexpensive refractor is the ability to view terrestrial objects.  The image in a refractor is correct, while in a reflector it is upside-down.  This allows nature-watching, birdwatching, spying on your neighbors, whatever you're into.

An expensive apochromatic refractor--called an apo--can give the best possible image quality and is exceptional for observing the planets, large deep-sky objects, and excels at photography.  It is also the most expensive type of telescope.  Even relatively small apos in the 4" to 6" range can cost thousands of dollars.

Bottom Line:  For a beginner who wants to look at wildlife as well as stars, a refractor is a good choice.  For stargazing only, a reflector will always get you more bang for the buck, so if you can do without terrestrial viewing, skip the refractor.  For advanced observers who want the best possible image quality for both visual observing and photography, and who bought Google stock when it was cheap, a high-quality apochromatic refractor cannot be beat.

 

Reflectors

Reflecting telescopes use mirrors instead of lenses.  While there are a variety of reflecting telescope designs, the most common is the Newtonian.  Reflectors do not suffer from the chromatic (color) aberration that plagues inexpensive refractors.  This means they generally have better optical quality.  And as if that were not enough, reflecting telescopes are less expensive.  That means more bang for the buck.  the main drawback versus a refractor is that the image through a Newtonian is inverted, making it poorly suited to terrestrial observing.  But for strictly stargazing, a Newtonian is tough to beat.

A Newtonian telescope does require a little more maintenance, since the optics need to be occasionally (but infrequently) aligned to provide the best optical quality.  Depending on the type of mount used (see below), the eyepiece may need to be adjusted more often to provide a convenient observing position.  Large Newtonian telescopes become fairly cumbersome, and for photography they require very large and difficult-to-transport mounts.  For this reason, large Newtonians are rarely used for photography.

Bottom Line:  For a beginner, a Newtonian is hard to beat.  If you don't need to view terrestrial subjects, and do not plan on doing much photography, there is no better telescope for the money.

 

Catadioptrics

Catadioptric telescopes include any telescope which incorporates both lenses and mirrors.  By far the most popular telescopes in this category are the Schmidt-Cassegrain telescopes (SCTs).  The versatility and portability of these instruments make them among the most popular for amateur astronomers of all levels.  A similar, but less versatile, design is the Maksutov-Cassegrain.

By folding the light path, the SCT provides a long focal length is a very compact tube.  This makes even fairly large-aperture SCTs portable.  An SCT is less than half the length of a typical comparable Newtonian, yet has a longer focal length.  Another advantage of the SCT is that, unlike most other designs, it can easily be converted to a variety of focal ratios for different photographic applications for both narrow- and wide-field imaging.

SCTs tend to be priced between Newtonians and the very expensive apochromatic refractors, while being more versatile than either.  Like a refractor, an SCT can be used for terrestrial viewing as well as stargazing.

Bottom Line:  For the observer who wants a little of everything, from deep-sky to planetary viewing and imaging, plus portability, an SCT is an excellent choice.  It is the one telescope that can be used for almost any application.  That and the moderate price makes the SCT one of the most popular telescopes on the market today.

 

Telescope Mounts

There are two basic types of telescope mounts:  equatorial and alt-azimuth.  Which you choose depends on the application of the telescope, as well as portability and setup considerations.

 

Equatorial Mounts

Equatorial mounts allow a telescope to track the sky as Earth rotates.  They do this with motor drives but do not necessarily require a computer system to track, although some equatorial mounts are computerized for finding objects.  Most equatorial mounts are German equatorial mounts (seen on the left above), which use counterweights to balance the telescope.  This type of mount is versatile and breaks down into smaller pieces, making even large telescopes portable.  Fork-mounted telescopes (such as the one on the right above) can also be mounted equatorially, although this is done almost exclusively for photography.  Using an equatorially mounted telescope is more cumbersome than an alt-azimuth telescope (described below).

Bottom Line:  A German equatorial mount is a good choice for large instruments which must be transported since it can break down into manageable parts and because it is very stable for photography.  It is also necessary for tracking without a computer system.  However, it is harder to use an equatorial mount, so they are not as ideal for beginners.

 

Alt-Azimuth Mounts

Alt-azimuth mounts move up-down and left-right, making them very intuitive to use.  However, an alt-azimuth mount must be computerized to track, since the rate of tracking varies across the sky in this orientation.  Many telescopes are computerized now and an alt-azimuth mount makes sense for many observers because of its convenience.  Setting up an alt-azimuth mount is extremely easy compared to an equatorial mount.  Fork-mounted telescopes (above left) may need to be mounted equatorially for photography, but this is easily done, and there is the advantage with this type of system of being able to change back and forth between the two types of mount.  This allows equatorial mounting when necessary but can be converted to the easier-to-use alt-azimuth mount for visual observing.  Dobsonian telescopes are alt-azimuth mounted Newtonian telescopes.  This design is very popular thanks to its ease of use and large aperture for the price.  Dobs are among the best choice for beginners.  While they do not track automatically (in most cases), they are extremely simple to operate and some models can be computerized to find objects.

Bottom Line:  For both beginners and advanced observers, alt-azimuth mounts are becoming more popular because of the ease of use.  For beginners especially, Dobsonians are hard to beat.  The computerized fork-mounted SCT telescopes are probably the most common choice for all types of observers due to their versatility and portability.

 

Features

This section describes some of the features observers look for in a telescope.  Recommendations are made for the designs that would be most appropriate for someone looking to have a certain feature as a priority.

 

Terrestrial/Celestial Observing

Viewing objects on land as well as in the sky requires a telescope with a correct (non-inverted) image.  This rules out Newtonian telescopes.  The most popular remaining designs are refractors and Schmidt-Cassegrain telescopes (SCTs).  Also available are the similar Maksutov-Cassegrains, although these are usually only available in smaller sizes.  If your priority is terrestrial viewing, and you might only occasionally look up at the stars, a small refractor or Cassegrain on an alt-azimuth mount is ideal.  As soon as your priorities favor stargazing over birdwatching, it is better to have more aperture and a tracking mount.  An alt-azimuth configuration is still recommended, so a computerized SCT is a good choice.  In smaller sizes (6" or less) these are still highly portable.

 

Ease of Use

No one wants a telescope that is difficult to use, but beginners especially make ease-of-use a high priority for their first telescope.  The two easiest types of telescopes to use are Dobsonians and computerized, alt-azimuth-mounted SCTs.  Both are simple to set up and to learn.  The basic differences are versatility and price.  For the money, a Dobsonian is the most telescope you can get.  However, it is only suited to visual stargazing and cannot be used for terrestrial observing or photography.  The SCT does have these capabilities and is very compact and portable.

 

Automatic Tracking

For sharing the view with a number of people, such as at a school or other public star party, automatic tracking becomes essential.  It is also required for photography.  This rules out the popular Dobsonian telescope.  Just about any other telescope can be easily equipped with tracking.  For public viewing, a computerized system is helpful since finding objects is quicker, reducing the time people wait to look through the telescope.  For photography, it is not enough that a telescope track--it must track very precisely.  So not all tracking telescopes are capable of photography.  A sturdy mount and accurate drive motors are essential.  Don't underestimate the importance of the mount for photographic applications.

 

Computerized Telescopes

Many, if not most, telescope models are now computerized to help find objects.  Some, such as the computerized Dobsonians, do not have motors to automatically move the telescope to an object, but the computer will guide the user to point at the right place in the sky.  Many other models not only move the telescope to find an object, but then track it as well.  Refractors, Newtonians, SCTs and more are available in this style.  The SCTs are the most popular due to their versatility.  A computerized telescope can actually help a new astronomer learn the sky.  Instead of spending all night struggling to hunt down a few objects, the computer can take you to dozens of objects a night and point out exactly where in the sky the objects are, making learning easier and more enjoyable.

 

Picking the Right Telescope

Finally, a summary and some more specific recommendations depending on the features you want to have in a telescope.

 

Day/Night Viewing

Recommended Optical Designs

  • Refractor

  • Schmidt-Cassegrain

  • Maksutov-Cassegrain

Recommended Mount

  • Alt-azimuth

Look for a portable telescope, most likely in the 3" to 6" aperture range.  A short optical tube is ideal for transport as well as ease of use, especially when aiming the telescope at terrestrial objects.  Refractors are a better choice for general wide-field viewing, while Cassegrains make better narrow-field and planetary telescopes.

 

Stargazing

Recommended Optical Designs

  • Newtonian

  • Schmidt-Cassegrain

Recommended Mount

  • Alt-Azimuth

  • Equatorial

Aperture is the key for celestial observing.  6" and up is recommended, with 8" being the most popular as a good compromise between power and portability.  Alt-azimuth mounts are more convenient and are all that is required for visual observing.  SCTs are the most portable, while Dobsonians are the most bang for the buck.  Both are popular and highly recommended.

 

Photography

Recommended Optical Designs

  • Refractor

  • Schmidt-Cassegrain

Recommended Mount

  • Equatorial

For photography, the mount is one of the most important considerations.  While the mount normally needs to be an equatorial for deep-sky imaging, an alt-azimuth computerized SCT is a popular choice since it can be converted to an equatorial mount when needed.  Refractors make nice wide-field photographic instruments, but keep in mind that only apochromatic refractors work for astrophotography, since the inexpensive achromatic models will have color aberrations that will significantly degrade the image.  SCTs are quite versatile and can be easily converted to a variety of focal lengths for different applications.  Other designs work as well, such as Newtonians, Ritchey-Chrétiens, and other sophisticated optical designs, but for beginners, SCTs are hard to beat since they are good at a little of everything.

 

Budget Considerations

Unless you plan on cashing in your Google stock and driving your new Mercedes-Benz SL55 AMG out to your private observatory in the mountains housing your 40" telescope, budgetary constrains will likely limit your choices.  For under $500, you can't beat a 6-8" Dobsonian telescope for stargazing, or a 3-4" refractor for terrestrial/celestial viewing.  Up to about $1500 you have a lot of choices, although the computerized SCTs are the most popular and versatile.  SCTs up to 8" in aperture are available in this price bracket.  Also in this range are large (10-12") Dobsonians, and a variety of computerized Newtonians and refractors.  Above $2000 the larger-aperture (10"+) SCTs are most common, although nice 4" and larger apochromatic refractors become a possibility for those seeking the highest quality optics or a nice wide-field photographic instrument.

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