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ne of the most common questions asked
of amateur astronomers is advice on purchasing a telescope.
Many considerations are necessary to choose a proper
instrument. A few of those considerations are detailed
here. First, decide what the interest level is (or will
be) of the potential telescope user:
Casual
This includes a child who has completed
a science unit on astronomy in school or an adult who
has a passing interest and wants to learn a little more.
Do not confuse an interest in "space" with an interest
in astronomy. If the interest wanes, for what else can
the telescope be used?
Special
A bright comet (e.g., Halley), lunar
or solar eclipse, close approach of the planet Mars
or other transient event might focus an otherwise casual
interest. Consider the requirements of the special interest
and potential uses of the telescope after the event
is past.
General
The user has some knowledge of the
breadth of astronomy as a science, maybe from a college
or adult education class. There has been some contact
with active amateur astronomers, perhaps through a local
astronomy club. While there is no specific area of particular
interest, the user has had an opportunity to look at
astronomical objects through a telescope.
Critical
You have a telescope now, but want
another instead/besides that is more appropriate to
a particular area of study. Such areas may include deep
sky, planetary, solar, photography, photometry or others.
If you are in this category, you should know just what
you need.
The Powers of a Telescope
The objective of a telescope
is its light gathering element. It may be a concave
mirror (reflecting telescope), a lens (refracting telescope)
or a combination of the two (catadioptic telescope).
The size of the objective is the major characteristic
to consider when deciding which telescope to buy.
Light Gathering Power:
Most astronomical objects are quite faint. A major purpose
for a telescope is to collect as much light as possible
and concentrate the light so the object appears brighter.
Light gathering power depends on the area of the objective
or the square of the objective's diameter.
Resolving Power:
The resolution of a telescope provides the ability to
see fine detail. Although many factors influence resolution,
resolving power depends on the diameter of the objective.
Magnifying Power:
This is the commonly associated ability to make distant
(small) objects appear nearer (larger). Many astronomical
applications use relatively low magnifications. Theoretically,
magnification above 50x per inch of objective diameter
is useless. Magnifications in the range of 15x - 30x
per inch are more commonly used. Therefore, usable
magnification depends on the diameter of the objective.
Since all three "powers" depend on
the diameter of the objective, the general rule for
astronomical use is: Buy the largest diameter
objective you can afford.
Telescope Selection
Binoculars
Often overlooked for astronomical
use, binoculars provide an opportunity to explore the
night sky with low magnification and wide field of view.
They are especially suitable for large nebulae, open
star clusters and bright comets. Low magnification prohibits
their use for planet study. The relatively small size
and weight provide excellent portability. You may already
own binoculars, purchased for another purpose -- use
them for astronomy! The "7x50" models make excellent
astronomical instruments and are available in the $50
- $100 range.
Spotting Scopes
Most commonly found with 20x-25x magnification,
higher priced models may include variable magnification.
60 - 80mm objective diameters are readily available.
Even 20x will require a tripod to steady the image for
observing. The table-top tripod usually sold with spotting
scopes seldom permit tilting high enough to observe
objects in the sky. Plan on a sturdy camera tripod with
an adapter to hold the scope. 20x will allow observing
lunar craters, phases of Venus, Jupiter's four bright
moons, Saturn's ring and its brightest moon, Titan.
Do not expect to see detail on planet disks. Some star
clusters and nebulae are spectacular in spotting scopes.
If astronomical interest wanes, the scope can be used
for bird watching, sporting events or hunting. Prices
start about $75 and go up to several hundred. Plan on
another $50 - $100 for the camera tripod.
Small Refractors (60mm typical)
Widely available in discount and department
stores, these telescopes are an amazing value for the
money. Since they are designed for astronomical use,
their utility for general purpose observing is limited.
Images are usually inverted (upside down and/or backward)
but this is not detrimental for astronomy. Look for:
interchangeable eyepieces to change magnifications;
a separate "finder telescope" with "cross hairs" for
locating objects; a "star diagonal" for comfortable
overhead viewing; a sturdy tripod; objective
lens at least 60mm diameter. Beware of:
a draw-tube for changing magnification; a non-optical
finder (sometimes just a peep sight or a long plastic
tube); a flimsy tripod; claim of high magnification
(anything over 100x generally is useless for these scopes).
Be especially cautious of solar filters which
attach to the eyepiece: if the scope has one of these,
throw it away -- it is not safe. At magnifications
up to 100x more lunar detail can be seen, some planetary
features are visible (Mars' polar caps, Jupiter's main
cloud belts, gaps in Saturn's ring), many double stars
are "split" and a host of variable stars can be monitored.
Prices start around $100 and go up, depending on the
accessories offered.
Small, Japanese Reflectors
Also found in many discount stores,
avoid them. Although usually available with larger objective
mirrors than the objective lenses of their refracting
cousins, the reflectors are seldom well made. In particular,
they are prone to alignment problems and need constant
adjustment. They can be so frustrating to use that they
often discourage would-be astronomers.
Equatorial Mounting
As the Earth rotates on its axis,
the stars (and Sun and moon) rise in the east, pass
generally overhead and set in the west. A telescope's
mounting must permit movement so the desired object
can be tracked as it drifts by. The alt-azimuth and
equatorial mountings are available. The alt-azimuth
moves up and down in altitude and left or right in azimuth.
The equatorial has one axis tilted to match the Earth's
axis of rotation and makes tracking of astronomical
objects easier. At magnifications below 100x, the alt-azimuth
is satisfactory. Higher magnifications usually require
an equatorial mounting and it is a must for long exposure
astronomical photography. While it may be available
for scopes discussed so far, the equatorial mount is
usually not necessary for the beginning observer and
it may add up to $100 to the telescope cost.
Mirror-Lens Telescopes
Also called catadioptic telescopes,
these instruments are mainly for someone with a known
interest in astronomy. Objectives range from 3.5 to
14 inches. Generally, the 8-inch size is the best value.
The smaller sizes are very compact and may be useful
as a travel scope. The larger sizes almost require a
permanent observatory for their practical use. If you
are considering one of these scopes, you should know
what you want and why. Prices for a 8-inch size range
from $1000 - $2500, depending on the accessories offered.
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