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1 inch = 2.54 cm =25.4mm
Choosing a telescope for astronomy
Given the large number of telescopes on the
market, how does an enthusiastic but inexperienced astronomer choose the right
one? To answer this question we will explain the differences between specific
telescope types, it is however important first to understand some very basic
concepts about astronomical telescopes in general.
Aperture is the most important factor
The single most important specification for any astronomical telescope is its aperture.
This term refers to the diameter of the telescope’s main optical element, be
it a lens or a mirror. A telescope’s aperture relates directly to the two
vital aspects of the scope’s performance: its light-gathering ability (which
determines how bright objects viewed in the scope will appear), and its maximum
resolving power (how much fine detail it can reveal). There are other criteria
to be considered in selecting a telescope, but if you learn only one thing from
this article, let it be this: the larger a telescope’s aperture, the more you
will see.
Power is not the big question
Unfortunately, the first question most beginners ask is “What is its
magnifying power?” The truth is, any telescope can be made to provide almost
any magnification, depending on what eyepiece is used. The factor that limits
the highest power that can be used effectively on a given scope is its aperture. As magnification is increased, and the image in the scope
grows larger, the light gathered by the telescope is spread over a larger area,
so the image is dimmed. There is also an absolute limit, determined by the
physical properties of light, to the resolution that is possible with any given
aperture. As the magnification is pushed beyond that limit the image fails to
reveal any additional detail and gradually breaks down into a dim, fuzzy blob.
The maximum useful magnification for
any telescope is about 50 times the aperture in inches, or 2 times the
aperture in millimeters. This equates to about 100x to 120x with the smallest
telescopes, which is enough to see such wonders as the rings of Saturn and cloud
bands on Jupiter. The 2x per millimeter figure is a rule of thumb, and can vary
up or down somewhat depending on the optical quality of the scope in question
and the vision of the individual observer. Experienced observers usually use
much less power; 0.5x to 1x per millimeter is more appropriate for most objects.
Beware, any manufacturer claiming that their 60mm scope can provide good views
at 450x (7.5 times the aperture in millimeters) is selling you air.
Bigger is better, but...
While aperture is the most important specification of any telescope, there are
exceptions to the rule that “bigger is better.” One reason is obvious: the need for
portability. The largest amateur telescopes are very big indeed, and demand
either housing in a permanent observatory or considerable effort to get them to
the observing site! There is a line
to be drawn between performance and portability, and where it will be drawn
varies with the individual and his or her capacity for storage and movement of
scope.
Beginners are encouraged to start out with a scope of sufficient aperture to
feed their interest, but of a size that they can manage easily. Avoid succumbing
to “aperture fever.” Those infected choose
the largest telescope they can afford without regard to portability. Their heavy
scopes soon gather dust in the garage, unused for being too
heavy and bulky, while the once enthusiastic would-be stargazers end up frustrated
.
The sky is the limit, well actually the
atmosphere and weather are the limit,
The second limitation on very large telescopes is less obvious, but becomes
apparent after the first couple of viewing sessions: the Earth’s atmosphere
limits how much we can see. Stars and planets viewed through a telescope appear
to shimmer or wiggle, as their light passes through the air and is distorted.
This effect is known to astronomers as seeing, and becomes more
noticeable and bothersome as telescope aperture increases. It especially affects
observations of the Moon and planets, where high power applied to reveal fine
details also magnifies the air turbulence.
The amount of distortion due to seeing varies,
depending upon the behavior of air currents in the upper atmosphere, and to a
lesser extent upon the altitude and topography of the observing site. But on an
average night, at an average site, air turbulence will limit useful
magnification to 250x or 300x, and prevent telescopes larger than about 8"
or 10" aperture from achieving their full potential for high-powered
viewing. Telescopes larger than 10" are most often chosen by observers who
want to gather as much light as possible for viewing dim galaxies, nebulas, and
star clusters. These “deep sky” objects are most often viewed at much lower power than the planets, so
seeing is less of a problem.
Telescope mounts
The last important topic to cover before diving into optical designs is that of
mounts. Telescopes are offered on either altitude-azimuth (or alt/az)
mounts, which move up-down (altitude), left-right (azimuth), or equatorial
mounts, which are tilted to align with the rotational (polar) axis of the Earth.
Alt/az mounts are generally lighter and simpler
to use, and are preferred if the telescope is to be used both for astronomy and
daytime (terrestrial) observing . The better ones offer
slow-motion controls to aid in moving the scope by small increments, and are
useful for powers up to about 150x. The Dobsonian mount is a variation on the A
mount. It employs unconventional (for telescopes) materials like plywood
and Teflon in a compact mounting that moves easily, is extremely stable, and can
adequately support large telescopes at a very low cost. Though there are no
mechanical slow-motions or electric drives on a Dobsonian, a well-made example
glides so smoothly on the Teflon bearings that with a little practice it is
quite easy to track objects manually at 200x or more!
Equatorial mounts are designed specially for
astronomy, and are not recommended for terrestrial viewing. Their advantage is
that they allow easier tracking of the stars across the sky. This motion can be
achieved with either a single manual slow-motion control or an electric motor
drive (or clock drive). The easier viewing they provide at high power makes
equatorials preferred by observers who want to track an object for a length of
time. Also, you’ll need an equatorial mount if you want to do
astrophotography.
Find a scope that suits you
Now that we understand these basic points of telescope performance and mounting,
we can discuss the three basic optical designs of telescopes: the refractor, the
reflector, and the compound (or Catadioptric) telescope.
 A refractor is what most
non-astronomers think of when they hear the word
“telescope.” Its tube is most often long and skinny, mounted on a tripod,
with a lens at one end and the eyepiece at the other. Refractors were the first
type of telescope invented, and the finest refractors still provide the best
images of any design for a given aperture. They are often chosen by observers
with a dominant interest in the planets and Moon, because they can provide
sharp, high-contrast views at high magnification and are less bothered by
atmospheric “seeing” than the other designs. They also require less
maintenance than reflectors or compound scopes, and are therefore popular with
beginners. The refractor’s good performance at high power and relative
insensitivity to light pollution makes it a good choice for a city-based
observer, as the design performs best on the objects that are most easily seen
from urban or suburban locations.
These advantages do not come without a price
— literally: refractors are the most expensive telescopes per inch of
aperture. Big refractors can cost several thousand dollars, and still are
considered too small in aperture for serious deep-sky observing. The long focal
length of most refractors restricts the field of view, making it difficult to
take in large extended objects like some clusters of stars. And the long tube,
with the eyepiece located at the back end, requires a tall tripod, which, if
poorly made, can allow the scope to shake and shimmy in the breeze, rendering
high-powered observing difficult.
The reflector uses a mirror, rather than a lens, to gather and focus
light. By far the most common design is the Newtonian reflector, which places a
concave (dish-shaped, parabolic) primary mirror at the bottom end of the telescope tube. A
small secondary mirror at the other end directs the focused light out the side
of the tube and into the eyepiece. Newtonians offer the largest aperture
available at given price, and when well made, they can provide sharp,
high-contrast
views that rival all but the finest refractors. A Newtonian’s low center of
gravity and eyepiece location at the top of the tube allow for comfortable
viewing with a more compact mounting, which can be made stable with much less
bulk and cost than the tall mounting required by a refractor of similar
aperture.
Big reflectors of 10” aperture and larger on
Dobsonian mountings are the most popular telescopes for astronomers who seek to
gather “buckets of light” for deep-sky observing. These giant scopes perform
best at remote dark sky sites, away from the glare of city lights. The value and
versatility of the smaller 4.5” to 8” Newtonians, mounted either
equatorially or as Dobsonians, makes them a fine choice for the beginner with
general interests.
Newtonian reflectors require occasional
maintenance. Unlike the lenses in a refractor, the mirrors in a reflector need
periodic alignment, or collimation, for best performance. While many
beginners seem intimidated by collimation, it’s really not difficult, and
takes only a few minutes once you get the hang of it. A reflector’s tube is
also more open to air and humidity than that of a refractor, and if left
uncovered the mirrors can accumulate dust and grime, which necessitates
occasional cleaning. While these maintenance concerns are often overstated, a
Newtonian may not be the right choice for someone who finds the prospect of
occasional tinkering with the telescope unappealing.
The most modern of the three common designs for amateur telescopes is the
compound, or Catadioptric type, which uses a combination of lenses and
mirrors to gather and focus light. The greatest advantage of this design is its
compactness: the lenses and mirrors “fold up” the light path inside the
telescope, reducing large-aperture scopes to a manageable size. If an equatorial
mounting is desired, the smaller tube can be carried on lighter and more
economical mounts than that required by a Newtonian of the same size. Compound
telescopes are most popular with observers who desire both generous aperture and
an equatorial mounting in a transportable package.
The names Schmidt-Cassegrain and
Maksutov-Cassegrain refer to specific designs of compound telescopes, which use
differently shaped lenses and mirrors to achieve a similar result. The Maksutov
is often cited as offering better image quality, there are many different
opinions about this. Most probably the Maksutov has
developed its reputation as the superior Catadioptric design because its
spherical optical surfaces are easier to make to very high precision than the
more complex shapes demanded by the Schmidt. As a result, the “average”
Maksutov will outperform their “average” Schmidt. In top-quality telescopes
from careful manufacturers, both designs can yield excellent images.
There are a few drawbacks to all compound
designs. As in any telescope that employs mirrors, occasional alignment is
required for peak performance. The cost of a compound is higher than that of a
Newtonian of the same aperture, though still lower than the cost of a comparably
sized refractor. Most significantly for the planetary observer, the secondary
mirror in a compound is much larger than that in a Newtonian, and its presence
in the light path of the scope reduces contrast for
viewing. In general, astronomers who desire a highly capable, easily
transportable telescope find these worthwhile compromises, and have made the
compound scopes very popular. Schmidt Cassegrains are the king of
the photographic world, this is what they excel at. Price is a consideration
Budget is a factor in almost every telescope purchase decision, but there are at
least three major price-related pitfalls to be avoided.
- Don’t buy a flimsy, "department
store scope" with the intention of getting a taste of the sky and upgrading
later. Many of those scopes are so poor-quality and frustrating that they
can turn budding stargazers off of astronomy for good!
- On the other hand, don’t give up on
astronomy if the scope of your dreams is financially out of reach at this
moment. There are many reasonably priced, high-quality beginner’s scopes
that can reveal incredible wonders, while helping a novice define his or her
particular observing interest. Another option is to consider binoculars.
- Finally, if price does not concern
you, think twice before buying the
biggest, most expensive telescope . Many of the large, fully
featured scopes favored by experienced observers are also the most
complicated, and are too much to grasp for someone still trying to find
Orion!
What about astrophotography?
Before concluding, here’s a quick word for the beginner who wants to jump
right into astrophotography through their new telescope: Don’t! At least, not
until you have taken some time to learn the sky and become familiar with
operating your scope. Photography of the sky can be a wonderfully rewarding
pastime, but is a combination of art and science with a steep learning curve
that can discourage beginners who try to take on too much at once. Of course, if
astrophotography is a primary interest there is nothing wrong with selecting a
first scope based on its easy adaptability to camera work in the future. While
most telescopes can be used for picture-taking (with varying prospects for
success), the most important qualifications for a photographic instrument are a
rock-solid equatorial mounting, and ease of attaching a camera so that it can be
focused. For a variety of technical and economic reasons, compound telescopes of
8” aperture and larger are most popular for photography. They also make fine
instruments for general observing.
To summarize
Which, then, is the right telescope? That’s a decision that must be made
individually, but the three best pieces of advice are:
1. The best telescope for you is the one you
will use most often. A huge, optically wonderful scope will bring no
joy if
it sits in the closet!
2. All else being equal, a
larger-aperture
(diameter) telescope will reveal more in the night sky than a smaller one.
Our advice is to select a well-made
telescope, of a design matched as well as possible to your primary observing
interest and most frequent observing site. Make sure it’s a size that can be
handled easily (by your standards and no one else’s) and used often,
and you will enjoy a lifetime of nights under the stars!
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