So Just How Do I Choose a Telescope?
There’s no question at all that the sky is a wonderful place – there
is just so much to see. But unless you know where to look and have at least some
understanding of what it is you are looking at, an aspiring astronomer can very
quickly lose interest. So our first important message is that it matters
quite a bit just how you get your first experience of the sky. Some
people’s first real look is just with their eyes or through binoculars, while
others might be lucky enough to be able to look through a telescope. Both
experiences can capture (or lose you) as an astronomer forever, depending on a
number of things which might seem irrelevant at the time, but which can
seriously impact how you enjoy your "first time". What you are
looking at, what you are using to look at it with, and the knowledge level of
the person who is helping you (if indeed you have anyone to help you) are all
relevant, and contrary to what you might expect, there are some objects which
actually look better through binoculars than through a telescope. The most
important thing is to try to share your first experience with someone who at
least partly knows the sky, and who can point out things which otherwise you
might miss. From the first moment you glimpse that clear, crisp image of
Saturn’s rings, or see a close-up of the Moon’s craters, or the majestic
glow of the Great Orion Nebula, or the cloud bands on Jupiter, you will know
whether or not you are destined to be an astronomer.
Astronomy appeals to different people for different reasons. Some people enjoy "discovering" other worlds with their own eyes, and for others astronomy offers a relaxing escape from the day to day routine of "normal" life. It’s also a great family activity, allowing parents and children to explore new frontiers together, to learn together. If you are the sort of person who enjoys teaching others, then there is tremendous scope for this, while at the same time enjoying your hobby. We are volunteer workers at the George Observatory in south west Houston, traveling there most Saturday nights and using our own telescopes and the telescopes at that facility to show families and visitors from all parts of the world the mysteries of the night sky. One of the most rewarding parts of these evenings is to see the look of wonder in the eyes of young and old alike as they see for the first time the almost unreal images of Saturn and it’s rings, or Jupiter and its moons, looking as if I have placed a color slide in front of the telescope. Fathers bring children, and usually end up more entranced than the siblings they have brought. Mothers bring mothers and grandmothers, and we end up speaking French, Russian, German and lots of other languages. All languages are the language of astronomy. If you become really interested, consider joining an astronomy club. There are amateur astronomy clubs all over the world, and most of them organize regular star parties where you can mix with other enthusiasts and learn from them.
If you live in the United States and are ever near to the Houston area, we would strongly recommend that you make a trip to the Brazos Bend State Park in south west Houston. There you will find the George Observatory, and every weekend (Saturday nights are best) the Fort Bend Astronomy Club, of which we are members, puts on a star party for the public. The facility has three large telescopes in domes, and numerous club members like us also turn up with their own telescopes to let the public look at the stars and planets and to answer questions on astronomy. It's well worth a visit, and if you want more information on the George Observatory, click here.
So you've thought about it and have decided that you definitely want to buy a
telescope. Where do you start?
How To Choose the Right Telescope
- OverviewLike choosing a house, a car or a wife/husband, buying a first telescope is a highly subjective decision. There is no "best" telescope for everyone. The one that's right for you will depend on your lifestyle, where you plan to use it, where you are able to store it, and also on what you wish to do with it in your astronomy. Let's look at some of the most important considerations that might influence your choice of a scope.
Forget about "power." It's
diameter that counts.
You will see many telescope advertisements which brag about the
magnifying power. Ignore them! A telescope's most important feature is not
its magnifying power, but rather its ability to gather light and to display the
image in a sharp, easy to see manner. Light gathering power is determined
by the diameter (aperture) of the main lens or mirror. The more light
gathered, the better you will be able to see the objects when they are
magnified. Telescopes advertised on the basis of high magnification
("450x!") are virtually always of inferior quality. In all
telescopes, power is adjusted by changing eyepieces, lower powers providing
brighter, sharper images. So you can boost the power as high as you like
on any telescope, but all this does is make the image larger. If the
optical system quality is poor - as it will be in a cheapo telescope - then you
will have a poor quality, fuzzy image to start with, and magnifying it will only
make it more and more fuzzy. So remember, the sharpness and clarity of an
image is usually most dependent on the quality of the optical system. The
worse the alignment and quality of the various lenses and mirrors, the worse the
image will be.
How big a scope do you need?
Well, that depends on what you want to do. For viewing
craters on the Moon, the rings of Saturn, and Jupiter with its four bright
moons, a 60mm (2.5 inch) or 70mm (3 inch) refractor or a 3 to 6 inch reflector
will do the job. A slightly larger instrument – perhaps an 80mm (3.5
inch) to 100mm (4 inch) refractor or 8 inch to 10 inch reflector will show more
planetary and lunar detail as well as glowing nebulae and sparkling star
clusters. Going bigger still, under dark, non-light-polluted skies, a
scope of 10 inches or more diameter can produce magnificent images of fainter
clusters and nebulae, and start to show some detail in galaxies.
Have realistic expectations and do not be
fooled by advertising photographs showing bright galaxies, as presumably seen
through a 3 inch telescope. With the exception of some nebulae and the
brighter planets, most astronomical objects are very faint. Generally, the
larger the telescope, the brighter will be the images and the more detail you
will see, but be warned that even in my 14 inch telescope, most galaxies show up
as nothing more than faint, fuzzy objects. But don't bite off more size
than you can chew. Don’t forget that you must be able to comfortably
lift and transport it, and you will also need adequate space to store it
somewhere safe and dry. Many people have chosen a large scope, only to
find that they can’t be bothered to haul it out to use, or that it doesn’t
fit in the car or truck, or their aching back prevents them from lifting and
carrying it around, or that it is too complicated to put together. So
these scopes collect dust in a cupboard until the day that the owner decides to
sell them. The correct choice is very important, and a good pair of
binoculars might well have been more useful in these cases.
On the other hand, be careful not to buy something small and portable, only
to be disappointed by what you can see through it. Like I said before in
this section, have realistic expectations before you buy.
Refractor, Reflector, or Catadioptric?
People do not normally know that there are several different types
of telescope:
Refractors
Reflectors
Catadioptrics
The choice of which type to buy will to some extent be dictated by the choice
factors discussed above – size, weight, ease of handling etc. The
different types of telescope all do the same thing: they collect and focus light,
it's the way they do it which differs. A refractor does it with a
multi-element glass lens, while a reflector uses a concave mirror. A
Catadioptric, or compound, telescope uses a mixture of both mirrors and lenses.
The Schmidt-Cassegrain is an example of one type of Catadioptric scope.
Reflector telescopes are designed
exclusively for astronomy. They deliver more light-gathering power for
the money than other telescope types. They produce an upside-down
image, which is fine for astronomy (since there's no "right way up" in
space) but is unsuitable for bird watching or other terrestrial observing.
Refractors and Catadioptrics work well for
both astronomical and terrestrial viewing. Refractors provide good image
quality but larger sizes start to get quite expensive compared to reflectors,
because of the difficulty of making large diameter, high quality glass lenses.
Mirrors are much easier and cheaper to make in larger sizes. But because
of the way they work, reflectors can be rather long and bulky at larger mirror
sizes, which makes them tough to carry around. Schmidt-Cassegrain and other Catadioptric scopes were developed to get around this
problem. You can think of them as "folded" telescopes and they
work very well, but they are usually more expensive. As an example, my 14
inch Schmidt has a focal length of nearly 4 meters but it fits in the trunk
(boot) of our Taurus sedan. It cost more than $2000.
What about a mount or tripod?
Most telescopes come supplied with a way to hold them steady and
allow you to find objects in the sky. The mount holds the telescope at an
appropriate angle for you to look through, and the tripod holds the scope steady
and brings the eyepiece up to eye level. A point to watch for is that the
tripods supplied with some of the "cheaper" telescopes can be quite
unstable, allowing the telescope to shake and wobble about in the wind or if
touched. There are sturdy, inexpensive tripods around and you should check
how stable is the one you are being offered before you buy. You should
also check how easily the telescope can be moved left and right and up and down.
If it’s difficult in the store in the daylight, imagine what it would be like
in the dark, perhaps in a field.
I said "up and down" in the previous paragraph. The simplest type of mount to use, what is called the "altazimuth", does indeed provide for up-down (altitude), left-right (azimuth) motions. It performs well for terrestrial or casual astronomical viewing.
The more complex equatorial mount is designed solely for astronomy. It has
the advantage of letting the user "track" the motion of celestial
objects with a single manual control or an electronic drive. Although you
can fit an electronic drive unit to a simple altaz mount, the resultant "up
and down only" tracking prevents any use for astrophotography (see the
section on imaging). Many equatorial mounts also allow objects to be
located by their celestial latitude and longitude coordinates.
In Summary . . .
Choosing a telescope involves a trade-off between size and
convenience. Bigger-diameter scopes will always show you more, but smaller
scopes are easier to handle. The "best" telescope is the one you
will use most often.
For a first telescope, I suggest you look at a basic refractor of around 90 to
100mm aperture, or a Newtonian reflector of perhaps 6" aperture, unless
you're really sure of yourself. After you've learned the basics of
observing and have developed an appreciation for the hobby, you can then move up
to a bigger, fancier scope.
Now that you've read this summary, and feel ready for some more detailed information, then click here and read on!