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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
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Refractor
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Schmidt-Cassegrain
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Maksutov-Cassegrain
Recommended Mount
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
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Newtonian
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Schmidt-Cassegrain
Recommended Mount
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
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Refractor
-
Schmidt-Cassegrain
Recommended Mount
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.
Equipment
Basics Page
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