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Choosing
Binoculars |
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Many sizes, shapes, and special capabilities are offered on today's binocular market. |
ANY OPTICAL AID will bring deeper views of the sky than the naked eye, and
any binoculars that happen to be available, no matter how poor or small, are
enough to launch a rewarding observing program. But some kinds are much better
for astronomy than others.
The variety of brands and models on the market can be bewildering. Prism
binoculars of the same basic type made today have been sold commercially for 100
years -- so manufacturers have long since discovered and incorporated
every easy improvement that is possible. This is a very mature technology.
Therefore, when a particular model offers special advantages, you can expect
these to be offset by corresponding disadvantages, either in performance,
convenience, or price. Choosing the right instrument for your purpose is a
matter of choosing where to compromise. The following guidelines will help.
Power. Every binocular has a two-number designation, such as 6×30 or 8×50. The
first number is the magnifying power. The second is the diameter of the
objective (front) lenses in millimeters.
Beginners usually assume that the higher the power the better. Higher powers are
indeed generally preferable; they penetrate light pollution more effectively and
are especially desirable for double stars, star clusters, and certain other
objects such as the moons of Jupiter. But high power also narrows the field of
view (making it harder to find your way among the stars), and, worst of all,
magnifies the dancing of the stars when the instrument is held in the hands. For
this last reason, 10 power is the maximum usually recommended for hand-held
binoculars.
Aperture. The bigger the objective lenses the brighter the stars, and here the
astronomer should compromise least. Most astronomical objects are hard to see
not because they are small and need more magnification, but because they are
faint and need more aperture. A pair of 8×50s collects twice as much light as
all-purpose 8×35s, and hence makes everything appear about 0.7 magnitude
brighter. The corresponding disadvantage of the 8×50s (aside from higher price)
is that they are bigger and heavier, making them less appropriate for prolonged
daytime use. For hikes or birdwatching the 8×35s would be the better choice --
or even 7×30s or 6×24s, sacrificing both power and aperture for light weight
and convenience.
Focusing. Most binoculars are "center-focus," meaning you turn a knob
in the center to focus both eyes at once. The right-hand eyepiece is also
individually focusable so you can correct for differences between your eyes; in
theory this only has to be done once. Center-focus binoculars are convenient for
birdwatchers and others whose targets often shift from near to far.
But astronomers don't need this feature. Everything in the sky is at the same
"infinity" distance as far as focusing is concerned. So you can save
both money and mechanical complication (with its increased likelihood of
problems) by choosing individual focus binoculars. With these you focus each
eyepiece separately.
Quality vs. price. Suppose you've decided on 8×50s -- a fine all-around choice
for astronomy. You may find three similar-looking instruments offered for $49,
$180, and $1,000. Do these prices really reflect the range of value?
This is a matter of opinion, though it's certainly true that a pair costing 20
times more than another won't show 20 times as much. Away from the price
extremes, say in the $75 to $400 range, you basically get what you pay for.
Some manufacturers offer different lines of binoculars having poor, moderate,
and good quality (in sales talk: "good," "better," and
"best") to provide a selection of prices and values. A cheap
instrument may be the best buy for a casual user. But quality is very important
in the stringent applications of astronomy, so the amateur should consider the
better grades. However, having decided on a make and model, you may get a
bargain on it by checking with discount stores and dealers.
Used binoculars can be bought at huge savings at yard sales, second-hand stores,
and pawn shops, but you risk getting stuck with a lemon. The following tests,
which can be done in less time than it takes to read them, will enable you to
judge the value of any binoculars, new or used.
Testing Binoculars
1. Pick up the instrument and compare its overall workmanship with other brands;
some will seem better made than others. Hold the two barrels and try to twist
them slightly. If there is any play in the joints or anything rattles, reject
the pair. Move the barrels together and apart; the hinges should work smoothly,
with steady resistance. So should the focusing motions for both eyepieces. On
center-focus binoculars, the eyepiece frame should not tilt back and forth when
you turn the focus in and out.
2. Next, look into the large objective lenses with a light shining over your
shoulder so the inside of the barrel is illuminated. Reject the pair if a film
of dirt or mildew is visible on any glass surface. (Dust on the outside is not a
problem.) Look at the two reflections of the light from the front and back of
the objective lens, which will appear to float a little above and behind it. If
the lens is antireflection coated -- as it should be -- both reflections will
have a blue, purple, amber, or greenish cast, instead of white. Move the
binoculars around until you see a third reflection deep inside, from the first
surface of the prisms. This too should be colored, not white. Then, still
looking in the front, aim the eyepiece at a nearby light bulb and move the
glasses around to view a row of internal reflections. The ratio of colored to
white images suggests the percentage of coated to uncoated surfaces.
The coatings increase light transmission and contrast, both of which are
especially important in astronomy. "Multicoating" is the best kind. In
top-notch models, all glass-to-air surfaces are multicoated.
Don't take vague advertising terms such as "fully coated" too
literally; this could mean one lens is "fully" coated and the rest are
not.
3. Turn the binoculars around and repeat your examination of lenses and coatings
from the eye end.
Then, holding the glasses a foot or so in front of you, aim them at the sky or a
bright wall. Look at the little disks of light seen floating just outside the
eyepieces. These are the exit pupils. If they have four shadowy edges, rendering
them squarish instead of round, the prisms are not the best and are cutting off
some light. In good binoculars the exit pupils are uniformly bright to their
round edges. Also, they should be surrounded by darkness, not by reflections
from inside the barrels.
4. Finally, look through the binoculars. Adjust the separation of the barrels to
match the separation of your eyes, then focus each side separately. A noticeably
filmy or gray image indicates an unacceptable contrast problem. If you have to
wear glasses to correct for astigmatism, make sure you can get your eyes close
enough to view the full field with the glasses on. If your glasses do not
correct for astigmatism, you can take them off.
Each barrel should point in the same direction! If you see a double image or
feel eyestrain as your eyes compensate for the binoculars' misalignment, you
have a reject. The eyestrain would soon become a real headache.
For a better test, first make sure the barrels are adjusted exactly for the
separation between your eyes, then look at something distant through the
binoculars. Slowly move them a few inches out from your eyes while still viewing
the object. It should not become double. This test is a bit tricky because your
eyes will automatically try to fuse a double image. At the same time, even a
correctly aligned pair of images will look double for a brief moment before your
eyes get them into register.
Misalignment due to flimsy prism supports is the worst problem of cheap
binoculars; even a small knock can render a working pair worthless. More
expensive instruments should survive minor accidents better.
Notice the size of the field of view: the wider the better. But the edges of a
wide field usually have poor optical quality. Sweep the field at right angles
across a straight line, such as a door frame or telephone wire. Watch whether
the line bows in or out near the edges. This distortion should be slight.
Look at sharp lines dividing light and dark, such as dark tree limbs or the edge
of a building against a bright sky. Do they have red or blue fringes? No
instrument is perfectly free of this chromatic aberration, but some are better
than others.
A star at night is the most stringent test of optical quality, so try the
binoculars on real stars if you get a chance. If not, look for an
"artificial star" such as sunlight glinting off a distant piece of
shiny metal. Center it in the field of view. Looking with one eye at a time, can
you bring it to a perfect point focus? Or, as you turn the knob, do tiny rays
start growing in one direction before they have shrunk all the way in the
direction at right angles? This astigmatism is especially bothersome when
viewing stars, and binoculars that are completely free of it can be forgiven
some other faults.
Move the star from the center of the field to the edge. It will go out of focus
unless you have a perfectly flat field and freedom from various other
aberrations. As a rule of thumb, no degradation should be visible until the star
is at least halfway to the edge of the field.
After running through these tests with several binoculars, you will have an
excellent idea of their relative value.
One last word: Don't be discouraged if you can't find (or can't afford)
perfection. Success in amateur astronomy depends more on attitudes than
instruments. This was driven home to me some years ago after I moved into
downtown New Haven, Connecticut. The sky seemed hopelessly light polluted, my
pair of 7×50s was mediocre, and there was no place to use them except through a
plastic bubble skylight in the roof of my apartment. The plastic turned star
images into shapes I felt no proper amateur would deign to look at. But they
were there, all right, and I was so intrigued at being able to see them at all
under such conditions that I kept at it. It turned out that stars could be
detected down to 8th magnitude, and I wound up spending nearly a year following
variable stars, hunting clusters and doubles, comparing stellar colors, and
becoming more familiar with the sky than ever before. So take what you've got
and enjoy it.
Alan MacRobert is an Associate Editor of Sky & Telescope magazine and an
avid backyard astronomer.
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Buying
the Best Telescope |
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There are many different types of telescopes to choose
from. Before you rush out and buy, it helps to consider what instrument best suits your needs. All Sky & Telescope photographs in this article are by Chuck Baker. |
CONTRARY TO WHAT YOU MIGHT THINK, your best choice in a telescope isn't
buying the most powerful one you can afford, nor the biggest, nor the one with
the most features. The best telescope is the one you will use most often.
A telescope that can be easily carried and set up in moments is one you'll enjoy
for many years. Happily satisfied, you'll stay in the hobby and perhaps move to
a bigger, fancier instrument later on.
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| Astronomical telescopes come in all shapes and sizes. This article will help you decide which type is right for you. Shown here, from left to right, are a 6" Newtonian reflector from Meade, a 5" Schmidt-Cassegrain from Celestron, and an 80-mm short-focus refractor from Orion. | ||
A first telescope should combine simplicity, portability, and ease of use
with sharp optics and a steady mount. What fits the bill?
The 6" Dobsonian
For most first-time buyers on a budget the choice is simple. The ideal starter
scope is a 6" (15-centimeter) reflector on a Dobsonian mount. The
"6"" refers to the diameter of the main, or primary, mirror. In a
Dobsonian design, the telescope tube usually rests in a wooden cradle and turns
on bearings made of Teflon pads. Sky & Telescope test reports have
demonstrated the fine value of these entry-level models.
The 6" optics gather a generous amount of light, yielding bright, sharp
images. Planets appear reasonably crisply defined, and dozens of galaxies and
nebulae are bright enough to show as more than dim smudges. While larger
instruments will surpass a 6" for image brightness, they aren't as
portable. An 8" Dobsonian is a tempting alternative, but its tube and mount
will be bigger and weigh about 30 percent more.
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| Three Dobs in a row. The Orion Deep Space Explorer, Celestron Star Hopper, and Meade Starfinder Dobsonian (from left to right) offer bright, sharp views, ease of use, and good portability at a relatively low price. | ||
A Dobsonian mount takes little effort to set up. Put the base on the ground and
drop the tube into the base. There's no polar alignment necessary and, once the
mirrors are collimated, nothing else to adjust. To move and aim the scope, just
grab the tube and swing it across the sky. Its light weight and relatively
compact size make it a snap to move around a backyard or slide into the back
seat of a car. In short, a 6" Dobsonian reflector is a fine starter scope,
far outperforming the smaller, flimsier, yet comparably priced beginners'
telescopes that proliferate in department stores at Christmas.
It's Too Big!
There's no perfect telescope and sure enough, a 6" Dobsonian isn't for
everyone. Perhaps its 4-foot-long tube won't fit into your car and still leave
room for the family. Perhaps you live in an apartment and like to observe from
your balcony. The long tube, wide swing, and eyepiece height only two to three
feet above the ground may not be practical. Or maybe you want a telescope you
can easily bring on airline trips.
For ultimate portability there are several choices, all involving some sacrifice
of aperture. Compact Schmidt-Cassegrain or Maksutov-Cassegrain telescopes offer
3½- to 5" (90- to 125-mm) apertures and optical systems that fold a
48" focal length into a tube no more than 12 inches long. The entire
telescope will fit under an aircraft seat.
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Small is beautiful. Meade's ETX (left) and Celestron's C5 (right) incorporate decent apertures in a compact package |
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Alternatively, short-focus refractors are available in 2.7- to 3.1" (70- to
80-mm) apertures with tubes short enough to fit into a camera case. Primarily
designed for low-power, wide-field views of the stars, these refractors also
work well for moderate-power views of the planets. All require a sturdy tripod
at extra cost.
Is There Anything Less Expensive?
For many, $400 on up is too much to spend on a hobby that may be a passing
fancy. This is especially true for those buying telescopes for children. A few
manufacturers sell simple 3- and 4" (75- and 100-mm) Dobsonian reflectors.
Compared to 2.4" (60-mm) refractors, the usual choice of parents, these
small reflectors will provide better views of favorite targets such as planets.
Its size, construction, and ease of setup make a small Dobsonian ideal for a
child. The wooden mount provides a more stable platform (and therefore a
steadier image) than the wobbly tripods of many entry-level refractors. Wood
also lasts longer than the flimsy plastic parts found increasingly on imported
2- and 2.4" (50- and 60-mm) refractors, many of which are no more than
toys.
A unique and portable product is Edmund Scientific's Astroscan 2001, a 4¼"
(11 cm) Newtonian reflector in a sealed, ball-shaped tube.
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Perfect portablity. Edmund Scientific's Astroscan 2001 houses a 4¼" f/4.2 parabolic mirror within a molded plastic sphere. |
Those on a very tight budget might want to bypass a telescope altogether. In
its place consider binoculars. A 7 x 42, 8 x 50, or 10 x 50 model is best.
Binoculars show a surprising amount and, coupled with a good set of star charts,
can help a newcomer learn the constellations and how to locate scores of
interesting objects. This is essential knowledge for using a telescope.
Approximate prices: $250 to $350 for a 3- or 4" Dobsonian reflector. $350
for the Edmund Astroscan. $100 to $150 for a good pair of binoculars. $12 to $45
for a star atlas.
I Want a Telescope I Won't Outgrow
On the other hand, our basic $400 6" reflector may not be enough for you.
If you're serious about the hobby and willing to invest more, there's a universe
of choices.
For 25 years the 8" (20-cm) Schmidt-Cassegrain has remained the most
popular instrument for serious amateurs. These models offer generous aperture in
a compact package. The optical system provides excellent views of every class of
celestial object, from planets to distant galaxies. All manner of accessories
are available, allowing owners to expand their interest into fields such as
astrophotography and CCD imaging.
Cost-conscious buyers can take heart that even the most expensive
Schmidt-Cassegrain uses the same optics as the no-frills unit. Objects won't
look any better in the fancier models! But the high-end units offer features
such as computerized pointing.
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| For a given light grasp, no other telescope
is quite as compact and portable as the Schmidt-Cassegrain. Meade's LX10 (left) and Celestron's Celestar 8 (middle) both cost around $1,000. Both manufacturers offer more expensive models with "bells and whistles" such as beefier mountings and computer-controlled drives. As an alternative, consider the Astro-Physics 5.1" apochromatic refractor (right), which combines premium optics with a precision mounting. |
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A popular alternative to the Schmidt-Cassegrain is the apochromatic
refractor. Yes, their apertures are "only" 4 to 7 inches (10 to 18
cm), but these telescopes boast the sharpest optics on the market. Aficionados
prize them for their outstanding views of planets, pinpoint stellar images, and
high-contrast views of deep-sky objects. As with Schmidt-Cassegrains,
manufacturers offer a wide range of accessories. However, per inch of aperture,
apos are the most expensive telescopes on the market.
Approximate prices: $1,200 for a basic Schmidt-Cassegrain with tripod; $2,000
and up for a premium model with an advanced electronic drive or computer. $2,500
for a 4" apochromatic refractor with altazimuth mount; $4,000 and up for a
5" or larger apo refractor.
I Want to Take Photographs
Our 6" Dobsonian is out of the running here. Snapshot photos of the Moon
are possible, but for all other forms of astrophotography with a telescope, an
equatorial mount is essential. Unfortunately most popular 6- to 10" (15- to
25-cm) equatorial Newtonian reflectors lack drives and mounts accurate enough
for long exposures. Nor do they readily accept accessories such as piggybacked
guidescopes. They are designed primarily for convenient visual use.
| Don't try this at home! Astrophotography requires a solid equatorial mount, a precise drive motor, and other accessories. |  |
If astrophotography is in your future consider nothing less than an 8"
Schmidt-Cassegrain or 4" apochromatic refractor. Either way, purchase the
model with the best mount you can afford. A solid mount is essential for sharp
photos. The mount should be equipped with an electronically controllable drive
motor on each axis.
Approximate prices: $2,000 and up for an 8" Schmidt-Cassegrain with
heavy-duty mount, wedge, and tripod. $3,500 and up for a 4" apochromatic
refractor on a German equatorial mount. Budget an extra $400 to $1,000 for
camera adapters, a guiding eyepiece, guidescope or off-axis-guider, a
declination motor, and electronic motor controls.
I Want to Look at Birds Too
Some buyers have an eclectic interest. They'd like an instrument to serve double
duty as a daytime spotting scope to watch backyard birds or visiting wildlife.
The popular Newtonian reflector, however mounted, is not well suited for daytime
use. Images will be upside down and cannot be easily turned right-side up. A
better choice would be a 2.7- to 4" refractor or a 3.5- to 5"
Schmidt-Cassegrain or Maksutov-Cassegrain. All will accept optional image
erector prisms.
Avoid dedicated spotting scopes. Their fixed or zoom eyepieces work well for
daytime applications but not for the more rigorous demands of nighttime
astronomy.
Approximate prices: $600 to $1,200 for a 3.5- to 5" Maksutov- or
Schmidt-Cassegrain telescope. $400 to $2,000 for a 2.7- to 4" refractor
with altazimuth mount and tripod.
I'm Citybound
Urbanites may think they need a special telescope. In fact, our first
recommendation of a 6" reflector still applies. However, if you expect to
observe mostly the Moon and planets -- the objects that show best through
light-polluted skies -- then consider an equatorial mount instead of a
Dobsonian.
Though harder to set up than a Dobsonian, an equatorial mount offers a key
advantage: equipped with a motor drive, it will track an object across the sky
as the Earth turns. This is a real plus for high-power viewing of the Moon and
planets, which otherwise requires re-aiming several times a minute. Objects stay
centered in the eyepiece for vibration-free, hands-off viewing, making it easier
to study and see fine details.
A 6" reflector on an equatorial mount with a motor drive costs about twice
as much as a Dobsonian reflector with identical optics. But the added
convenience may be worth it.
Equatorially mounted versions of smaller 4.5" (115-mm) Newtonians and 3.1-
to 3.5" (80- to 90-mm) refractors offer decent views of planets at lower
cost, though with less brightness and resolution than a 6" reflector. These
two categories contain a wide range of models -- shop for ones with the features
listed under "What Do I Look For?" below.
At higher cost, 7- and 8" Maksutov- and Schmidt-Cassegrains offer
equatorial mounts, built-in motor drives, and more aperture for fine lunar and
planetary views.
Approximate prices: $650 to $850 for a 6" equatorial Newtonian. $400 to
$600 for a motor-equipped equatorial 3.1" or 3.5" refractor or
4.5" reflector. $1,200 and up for a 7- or 8" Maksutov- or
Schmidt-Cassegrain.
I Live in the Country
If you have ready access to dark skies, you may want a telescope that will take
special advantage of those skies. That means more aperture! Compared to our
base-level instrument, the 6" reflector, an 8" reflector provides
images 77 percent brighter (0.7 stellar magnitudes brighter). Images in a
10" are 56 percent (0.5 magnitude) brighter than in an 8". Bright
nebulae reveal more details. Globular clusters resolve into thousands of stars.
Definite shape appears in many galaxies. Faint galaxy clusters invisible in
smaller telescopes appear scattered across the eyepiece. If the sky is dark.
For deep-sky observing, there is no substitute for aperture. An 8- to 16"
Dobsonian will show more objects than you can possibly exhaust in years of
viewing. These instruments can be big and unwieldy, but a number of
manufacturers offer break-apart truss-tube models that allow even 12- to
16" telescopes to fit into a car or minivan. As a rule, cleverly designed
portable models tend to be more expensive than solid-tube instruments of the
same aperture.
Approximate price: $600 for an 8" Dobsonian with accessories; $3,000 for a
truss-tube 12- to 16" Dobsonian reflector.
I Can't Find Anything!
Even under dark skies, locating targets is the single biggest challenge most
owners face. Enter the computerized telescope.
Built-in computers programmed with the positions of thousands of objects are
most popular in Schmidt-Cassegrain models, but many other top-of-the-line
instruments now offer them. You need to do some preliminary setup when you take
out the scope. Then at the touch of a "Go To" button, the computer
slews the telescope across the sky to the correct location.
Other telescopes can be equipped with add-on digital setting circles that
provide many of the same functions, but without the Go To capability. You move
the telescope manually until the computer flashes that the object has been
found.
Approximate prices: $2,500 and up for a computerized 8" Schmidt-Cassegrain.
$300 and up for add-on digital setting circles.
I have Limited Mobility
Everyone can enjoy astronomy. Those with limited mobility, however, find many
telescopes awkward or impossible to use -- the eyepiece is too high or too low
or swings through too great an arc.
For viewing from a fixed seated position, consider a 3.5"
Maksutov-Cassegrain or 5" Schmidt-Cassegrain. When aimed at the southern
sky and overhead, the eyepiece stays at about the same position. These
telescopes can be placed on a table, allowing a wheelchair user to slide
underneath and sit close to the eyepiece.
Approximate prices: $600 to $1,200.
I Can't Wait! I Need a Telescope Now!
For those living outside large cities and far from local telescope dealers, many
telescopes are available through mail order only. This includes our favorite
6" Dobsonian. Also, high-demand models like these are often in short
supply, with long delivery times. Buyers wanting an instrument right away for
Christmas may need to make another choice.
A suitable alternative in the $300 to $600 price range is a 4.5" reflector
on a solid equatorial mount, or a 3.1- or 3.5" refractor on either a good
altazimuth or equatorial mount. These popular telescopes are available in a
range of models from many suppliers. At lower cost ($120 to $250) a 2.4"
refractor can serve as a modest starter scope if expectations are low -- don't
expect to see many details on the planets -- and if you are careful to select a
model with features listed under "What Do I Look For?"
Most dealers will stock all these models for off-the-shelf purchases or same-day
deliveries. But don't wait until a few days before Christmas to buy -- the
better models may be sold out!
Am I Ready to Buy a Telescope?
Few people ask themselves this, but most should. Can you identify the brightest
stars? Can you find the main constellations? Can you point to the Andromeda
Galaxy? M13? Jupiter? Saturn? Do you know what M13 is? If not, how will you find
these things with a telescope? A computerized telescope will still be useless if
you don't know an M-object from a planet, or if you don't know enough bright
stars to get the computerized mount properly set up at night
("initialized").
Before spending hundreds or thousands of dollars on a telescope, spend $150 on
good star charts, an astronomy guidebook, and a pair of binoculars. Spend time
getting to know the sky. Learn where things are and what things are.
What Can I See?
Even the most modest optical aid will provide startling views of the Moon.
However, those 2.4" refractors with poor-quality eyepieces and wobbly
mounts will show little else well.
The better 2.4"ers and certainly a good 3- to 4" reflector or
refractor can capture all the popular targets: Saturn's rings, Jupiter's moons
and cloud belts, the polar caps of Mars (when Mars is close enough), as well as
bright star clusters and nebulae. The larger the aperture of the telescope, the
more you'll see.
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| Many companies sell small refractors on
altitude-azimuth or equatorial mounts, including (from upper left)
TeleVue, Meade, Celestron, and Orion. The higher the optical quality
and the sturdier the mount, the happier you'll be with your purchase
-- especially for such small-aperture instruments. |
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However, even a large telescope will not show you:
The flags on the Moon -- they're too small!
Stars appearing as disks -- they're points in even the largest telescopes.
Nebulae in technicolor -- the Orion Nebula looks greenish, but most nebulae and
galaxies are so dim that they appear gray visually.
Is This 400X Telescope Any Good?
The most important specification of any telescope is its aperture, the diameter
of the main lens or mirror. Forget power. Forget magnification. If you switch
eyepieces any telescope can be made to magnify at almost any power.
The maximum useful magnification is 50 times the telescope's aperture in inches.
Under that rule the top power of a 2.4" is 120x, of a 4", 200x. Beyond
those limits images break down into dim, fuzzy blurs. Even at that, seeing most
objects well rarely requires more than 150x. And the clearest, sharpest views
will be at a telescope's lowest power, around 8x per inch of aperture. Quality
telescopes are equipped not with high-power eyepieces but with good low- and
medium-power ones.
Above all, avoid any telescope advertised primarily by its magnification -- for
example, "Powerful 400x model!" That's a sure sign of poor quality
disguised to lure impulse buyers who don't know any better. But now you do!
What Do I Look For?
In a word, aperture. But the following features are also hallmarks of a quality
telescope:
A 1¼" focuser. Telescopes that accept only the smaller, 0.965"
eyepieces are almost always inferior. The small eyepieces certainly are,
especially ones marked H, HM, or SR. Better telescopes come with Kellner (K),
Modified Achromat (MA), or better yet, Plössl eyepieces.
A true 6 x 30 finderscope (6 power with a 30-mm front lens that is not
"stopped down" by an internal baffle to hide poor optical quality).
The finder should be mounted in a bracket with six adjustment points rather than
three -- it's much more solid.
All wood-and-metal construction with minimal use of plastic, especially in
moving parts such as the focuser.
A mount and tripod combination that doesn't flex and shake at every touch. Does
the telescope move smoothly and precisely over small distances and stay firmly
in place when you let go?
Slow-motion controls on both axes (unless it is a Dobsonian mount).
On low-cost telescopes, ignore high-tech-looking features such as dials and
setting circles. You won't use them.
The manufacturers and dealers who advertise in Sky & Telescope cater to the
knowledgeable amateur-astronomy market. Staying with their products will net you
a telescope of far better quality than the models made for the impulse buyer.
So I Buy the Biggest Telescope I Can?
No. Spending lots of money on the fanciest or biggest telescope doesn't
necessarily get the best telescope. Too many dream scopes end up decorating
living rooms or cluttering up garages. Why? A telescope has to be moved. It has
to be carried out to the backyard or packed into a car and transported to a
rural observing site, then reassembled. Many people's first telescopes, superb
as they are in optics and features, are just too big and heavy.
Think twice about buying any instrument weighing more than 60 to 75 pounds. The
novelty will soon wear off and excuses will replace enthusiasm.
A Refractor is Better, Right?
Refractors use a lens (actually a matched pair of lenses) to gather and focus
light. They are most popular in 2.4" (60-mm) to 4" (10 cm) apertures.
Premium "apochromatic" models in 4" and larger sizes use lenses
made from special glasses to eliminate false color fringing around bright
objects. Refractors provide a rugged instrument requiring no adjustments to the
optics.
Reflectors, on the other hand, use a large primary mirror, which requires
occasional collimation adjustments. Newtonian reflectors (named after their
inventor, Isaac Newton) employ a small, flat secondary mirror to deflect the
light through the side of the tube to the eyepiece.
Schmidt-Cassegrain and Maksutov telescopes are also reflectors, but with an
added lenslike correcting plate.
As a rule, refractors provide sharper views than do reflectors of similar
aperture. However, reflectors, especially Newtonians, offer far more aperture
for the money than refractors. So which is better? A good refractor offers
performance; a good reflector offers value.
Don't I Need an Equatorial Mount?
German equatorial mounts are popular on long-tube telescopes such as refractors
and reflectors. Schmidt-Cassegrain and Maksutovs often use an equatorial fork
mount. When equipped with a motor and polar aligned properly either style of
mount can automatically track the sky. This is convenient for visual use and
essential for photography.
However, equatorial mounts can be heavy, expensive, and confusing for beginners
to set up and operate. Worse still, equatorial mounts supplied with many
low-cost telescopes look high tech but are wobbly, ruining the view.
An altazimuth mount may be far steadier and less expensive. Models supplied with
entry-level refractors provide up-down and side-to-side motions. The best units
have slow-motion controls on both axes for making fine pointing adjustments.
The Dobsonian (named after amateur John Dobson, who popularized the design) is a
form of altazimuth mount. These wooden mounts are popular on Newtonian
reflectors of all sizes from 3" to 36". They are simple to set up,
move smoothly even without slow-motion controls, and provide outstanding value.
For most beginners, a sturdy Dobsonian mount is far superior to an equatorial
mount that is lightweight but shaky, or one that is solid but heavy and costly.
Where Can I Find Out More?
Contact manufacturers and dealers to get all the literature you can, and visit
local dealers. Of course they'd like to sell you a telescope, but they're also
good sources of information. Keep in mind, dealers hate spending lots of time
with a prospect only to have that person buy from another supplier just to save
a few percent. If a dealer helps you, give him your business. You may need his
personal after-sale service. For a list of venders of binoculars, telescopes,
and accessories, see SKY Online's Marketplace.
Local astronomy clubs often host star parties where you can see various
telescopes in action and talk to their owners. See S&T's Astro Directory for
a list of clubs in North America and Europe.
Your local planetarium or science center probably has someone on staff
proficient in telescopes who can provide recommendations. S&T's Astro
Directory (see previous item) lists planetariums and science centers too!
Online services such as CompuServe and America Online, as well as Internet
newsgroups, have forums geared to amateur astronomers. Frequently Asked
Questions files contain useful tips.
Read, read, read! Go to your local library. Test reports in Sky & Telescope
contain detailed information on specific models. Check SKY Online's Test Reports
Page for a selection of past reviews.
Alan Dyer is a contributing editor of Sky & Telescope. He is coauthor
with Terence Dickinson of The Backyard Astronomer's Guide.
|
How
to Choose a Telescope |
A pair of binoculars makes an excellent "first telescope" -- at
least up to a point. Binoculars were the only optical instrument I had for my
first year as a skywatcher, and this turned out to be exactly the right approach
to take. But during that time I was laboring toward a bigger goal: building a
6" reflector.
Making the telescope was the only way I could afford it. I later realized that
this constraint was a blessing in disguise. It kept me from getting the
telescope too soon -- before I knew what to do with it -- and led me to value it
like the crown jewels despite all the things a telescope of this fairly modest
size cannot do.
Sooner or later, every beginning amateur astronomer faces up to the question of
what to do about getting a telescope. This is the most critical decision you
will probably make in the hobby. Choose well, and the telescope will open up a
lifetime of pleasurable evenings exploring the sky. Choose poorly, and it is
liable to bring frustration and disillusionment and get sold off in the
classified ads, "mint condition, rarely used."
What makes for the right decision? This depends more on you than on the
telescope itself. If you live in a fifth-floor city apartment with tiny storage
closets and are fascinated by the Moon and planets, you should get an entirely
different telescope than if you live on a farm in Vermont with a nice empty shed
and your true love is galaxies. The money you can spend, the weight you can
lift, and the amount of observing you've already done with the naked eye and
binoculars are also crucial.
A telescope's most important characteristic is its aperture. This is the
diameter of the main lens or mirror. The aperture determines the brightness and
sharpness of everything you will see. A 3"-aperture telescope can never
show stars as faint, or detail as fine, as a well made 6". The 6" in
turn can never match a good 10".
Power, or magnification, is not something to consider when purchasing. You can
make any telescope magnify at essentially any power you want by using different
eyepieces. An eyepiece is the little removable lens assembly you look into. Most
telescopes come with several of them, and more can be bought separately. But
it's pointless to use too high a power on a small-aperture telescope. You'll see
nothing but highly magnified fuzz. Only a large-aperture scope (on a sturdy
mounting!) can show a worthwhile image at 200x or more. In any case, the lowest
powers are the easiest to use and provide the most pleasing views. You'll be
using low powers the most often.
The rule of thumb is that the maximum useful power, even under ideal sky
conditions, is 50x per inch of aperture. This limits you to 300x on a 6",
and even that's usually pushing it way too far.
Shun any telescope that is promoted for its high magnification. If you see a
2.4" (60-millimeter) department-store telescope advertised as "475
power!!!", you know that the manufacturer thinks you are ignorant and
gullible. With that attitude he probably cut a lot of other corners too.
Exaggerated emphasis on high magnification is the surest tipoff of a junky toy
scope.
Since aperture is so important, you might think choosing a telescope is easy --
get the biggest aperture you can afford! But in practice it's not so simple. If
a scope is too massive to lug outdoors easily and too time-consuming to set up,
you'll rarely use it. Even among telescopes with the same aperture, some designs
are more portable, others give somewhat sharper images, and others are more
economical. The following advice will help you juggle all factors to make the
best decision.
Telescope Types
There are three basic kinds of telescope to choose from: the refractor, the
reflector, and the catadioptric. Each has its strengths and weaknesses, which
you should match to your lifestyle and observing desires.
 |
These 3 telescopes all have an 8" aperture, and therefore the same light-gathering properties, despite their differences in size and weight. The tubes are cut-away to show the light paths. |
Refractors have long, relatively thin tubes with an objective lens up front
that collects and focuses the light. Refractors range from the very worst
telescopes to the finest. "Department-store" refractors of the kind
mass-marketed to the public are generally the worst. Their optical quality may
be low, and their mountings are often so wobbly that the telescope can hardly be
aimed at anything. If your astronomy budget limits you to this price range,
stick with binoculars.
You say you already got a scope like this? Well, take heart; Galileo would have
been overjoyed with it. Keep your expectations low, your patience intact, and
don't blame yourself if it gives trouble. Attitude is everything. Lots of
amateurs have started off successfully with department-store refractors. For
bright, easily found objects (try the Moon), it may work pretty well.
Very good refractors, on the other hand, are also on the market if you are
willing to hunt them out and pay for them. New, complex lens designs offered by
a few companies have created some of the most superb -- and expensive --
telescopes anywhere. These lens designs are called "apochromatic," not
to be confused with the ordinary "achromatic" refractor. With so much
invested in the main lens, the makers generally produce a smoothly working,
high-quality mounting as well.
Advantages.
Refractors of all kinds are rugged, require little or no maintenance, and have
sealed tubes that keep out dust and reduce image-degrading air currents. If the
lenses are good, a refractor provides very crisp, high-contrast images for a
given size aperture; this is especially desirable for the Moon and planets.
Disadvantages.
Refractors generally have small apertures, typically 3 to 5 inches. For many
astronomical purposes this is just too small; faint objects such as galaxies and
nebulae will appear very dim when you can detect them at all. A refractor
usually requires a right-angle mirror (a "star diagonal") at the
eyepiece for comfortable viewing. This mirror flips the image right-for-left,
making it hard to compare with charts. And good-quality refractors cost more per
inch of aperture than any other kind of telescope.
Reflectors use a large, heavy, concave mirror instead of a lens to gather and
focus the light. You look through an eyepiece on the side of the tube up near
the top.
For decades the reflector was the undisputed king in amateur astronomy. Some
would say it still is. From the beginner's standpoint, "reflector"
means the Newtonian design illustrated above.
Advantages.
A reflector offers the most telescope for the money. It is simple enough so the
do-it-yourselfer can build it from scratch or tinker with a ready-made one. The
optical quality can be very high. A reflector contains an even number of mirrors
(two), so you see a "correct" image, not a mirror image. Dew is
unlikely to condense on the optics in the night chill, a common annoyance with
other designs. The mounting can be stubby and low to the ground, providing
stability while the eyepiece is still at a convenient height.
Disadvantages.
Reflectors may require a little more care and maintenance. The tube is open to
the air, which means dust on the optics even if the tube is kept under wraps in
storage (though a moderate amount of dust has zero effect on performance). The
mirrors need occasional adjustment to keep them lined up exactly right, a simple
but slightly tedious procedure of turning nuts on the mirror mounts. During
observing, air currents in the tube are likely to fuzz up the image until the
telescope comes to the same temperature as the surrounding air -- unless the
tube is very well ventilated.
Different f/ratios.
All telescopes, but reflectors especially, perform differently at different
f/ratios. In general, the higher the f/ratio the better.
|
About f/numbers What does it mean to say a telescope has an f/ratio of f/6 or f/15? The number is simply the focal length (the distance from the objective to the eyepiece) divided by the aperture. For example, a 4" telescope with a focal length of 40 inches is called f/10. In complex telescopes such as Cassegrains, the "effective focal length" is used instead of actual distances between parts, but the principle is the same. A 5" Cassegrain with an effective focal length of 100 inches is an f/20. |
Lower than f/6 or f/5 a reflector's secondary mirror has to be relatively
large, and this slightly reduces image harpness. Distortions become more
apparent near the edge of the field of view, and the entire optical system is
much more sensitive to tiny misalignments. A low f/ratio mirror is harder to
manufacture to high quality. Also, with a low f/ratio you have to use better,
more expensive eyepieces to get sharp views anywhere but at the center of the
view. For all these reasons an f/4 reflector will almost never
quite match an equally
well-made f/8.
On the other hand, the f/4 is much more handy and portable. It's only half as
long! A 10" reflector at f/4 is less than four feet long and will go in the
back seat of a car for jaunts to dark sites. A 10" f/8 is about seven feet
long and a major logistical problem to transport. Everything's a tradeoff.
Catadioptric or compound telescopes use both lenses and mirrors. The most
popular design is the Schmidt-Cassegrain, which burst onto the market in the
1970s and quickly gained a place for itself alongside refractors and reflectors,
which had been around for centuries. The following comments apply primarily to
Schmidt-Cassegrains.
Advantages.
The pluses of the "Schmidt-Cass" are in portability, convenience, and
special options such as advanced tracking and electronics -- not visual
performance. While most people can haul an 8" reflector in and out of
doors, it is awkward and heavy. Most 8" Schmidt-Cassegrains come in a
padded footlocker that can be hoisted with one hand. (The tripod is separate.)
The footlocker can be stowed in a car trunk or closet like a large piece of
luggage, whereas a reflector tends to displace everything around it.
A Schmidt-Cassegrain's relatively short tube allows a motorized mounting to
track the stars more reliably, making astronomical photography less difficult
(it's never easy). These are excellent photographic telescopes. Elaborate
electronic drive controls are available as options on Schmidt-Cass mountings for
photographers and CCD camera users. Some can be bought with robotic computerized
pointing capabilities.
Disadvantages.
The image formed by a Schmidt-Cassegrain will probably be a touch less sharp
than the image formed by a good reflector of the same aperture. This is most
noticeable when observing planets. The cost is higher than for a reflector of
the same aperture. A right-angle mirror (star diagonal) is generally used at the
eyepiece to provide a comfortable viewing position, and this means your view is
mirror-reversed. The focusing mechanism can be a bit sloppy and imprecise. You
can't take the scope apart yourself; major adjustments mean shipping it back to
the factory.
Special options.
The newest options on Schmidt-Cassegrains, and on some high-performance
refractors too, are robotic aiming motors controlled by an on-board computer
with a database of celestial objects. Once you "initialize" the
computer by aiming at a couple of known stars and doing some other setup, these
telescopes will swing around to point automatically, by magic, at whatever you
specify. This is supposed to make astronomy easy; you don't have to know the
sky.
Computerized scopes are clearly the wave of the future, but opinions about them
remain divergent. Some longtime observers rave about them, saying a computerized
scope finally lets them spend more time looking at objects than looking for
them. Others, including me, think a computerized scope for beginners is an
expensive crutch that impedes learning and takes too much of the fun out. After
all, you wouldn't want to learn to fly an airplane by being told just to push a
button labeled "autopilot." And what would you do if anything went
wrong?
Mountings
The best telescope is worthless if it is on a poor mounting. The tiniest wobble
will be magnified into an earthquake by whatever power you are using. You can't
see much in a view that's having earthquakes.
Unfortunately, almost all telescope mountings have an unpleasant amount of
wiggle. Usually this is due to designers' oversights (or manufacturers'
cost-cutting) at one or more key stress points. But to some degree it's the
inevitable result of making any mount that's light enough to carry without a
forklift.
There are two basic telescope mountings: the equatorial and the altazimuth.
An equatorial mount is designed so you can easily track the motion of the sky as
the Earth turns. Otherwise, the Earth's rotation carries things out of the field
of view fairly quickly -- in just a minute or so at 75 or 100 power. Most
equatorial mounts come with an electric "clock drive" to follow the
motion of the sky automatically. Another way in which an equatorial mount is
useful is that its motions indicate celestial north-south and east-west in the
eyepiece. This is a great help when you're trying to find your way among the
stars with a map.
An equatorial mounting must be aligned on the north celestial pole at the start
of each observing session for these features to work. Fortunately, this doesn't
need to be done accurately for visual observing. Just plunk the mount down so
that its polar axis is aimed more or less at Polaris by eyeball judgment.
Altazimuth mounts are simpler. They just swing up-down and left-right. You have
to nudge the scope along every so often to follow the stars. An altazimuth mount
is both cheaper and lighter for the same degree of stability, advantages that
are exploited to the utmost in the Dobsonian mount design for giant, low-cost
reflectors. Large altazimuth telescopes, however, require the user to be a
skilled pathfinder among the stars. The really big Dobsonians are best for
experienced deep-sky observers hungry for aperture.
Whatever mount you get, don't compromise on its size and strength. Nothing can
kill your enthusiasm like a perpetually shaky view, but a solidly mounted
telescope -- one that wiggles hardly at all when you touch it and focus it -- is
a joy to use.
Your Interests
Planets, the Moon, and close double stars require high power, good contrast, and
sharp resolution, and if these objects are your main interest, a refractor or
high-f/ratio reflector is probably the best bet.
Very faint objects like galaxies and nebulae need aperture, aperture, aperture.
A big reflector is the logical choice if this will be your specialty.
If you haven't specialized and don't intend to, an all-purpose midrange
telescope should serve best -- perhaps a 6- or 8" reflector with a focal
ratio of f/8 or f/6, or an 8" Schmidt-Cassegrain.
One factor may force your choice of interests: light pollution. The Moon and
major planets shine through even the worst light pollution unhindered. But faint
objects such as galaxies and nebulae are devastated by it. The fifth-floor city
dweller could clamp a small refractor to the rail of a fire escape and enjoy as
fine a view of the Moon as the Vermont farmer. But most deep-sky objects would
be invisible.
Your Living Situation
You don't just look through a telescope. You have to store it and carry it. You
have to set it up and take it down at the end of a long day when most people are
ready for bed. If this is a difficult chore, you won't observe very often no
matter how burning your enthusiasm may be right now. Too many novices forget
this and buy massive "white elephants" they end up hardly ever using.
Before drooling over the ads for giant telescopes, remember that a lowly 3"
scope will show more of the universe than a 16" if you use it more.
The best scope for you is the one you'll use the most. How much fun you have,
and how good an astronomer you become, depend on how much time you spend
observing -- not the size of your aperture.
Figure out where you'll use the telescope and where you'll store it. The farther
apart these two places are, the smaller and lighter the instrument you should
get. Does the route between them involve stairs? Then think carefully before
getting a reflector bigger than a 6".
An enclosed, unheated porch or a well-ventilated, dry shed are excellent for
storage. Not only will the telescope be close to where you'll use it, it will
already be at the outdoor temperature when you set it up. This will save
problems with the image-blurring "tube currents" of warm air that
plague telescopes brought from a warm house into the cold night -- and the
massive dewing that can drench a cold scope brought back indoors.
If the scope is stored in a cold location, however, be on the lookout for
moisture condensing on or inside it during changes of weather. Never put the
caps on a telescope unless the inside of it is dry. If there's any tendency for
condensation to form on things at your storage site, leave the eyepiece tube
open so the inside stays aired out, and wrap the whole scope in a blanket.
Will you have to tote the telescope around to avoid trees and lights? If you
have one permanent observing site, consider installing a pier rather than
lugging the tripod in and out. A large, sand-filled pipe or a cut-off telephone
pole planted deep in the ground will be steadier than the most expensive tripod.
The ideal solution is a shelter or observatory around the entire telescope right
where it will be used.
Buying Advice
Having narrowed your choices -- perhaps to a 6- or 8" reflector or an
8" catadioptric -- get all the manufacturers' catalogs and compare details,
paying careful attention to size and weight. Sky & Telescope magazine has
ads for nearly every serious astronomical telescope. Call different dealers for
the best price, but also ask the dealers' policies on returns and repairs (an
unfortunate necessity for some brand-new scopes).
Insist on being told a definite delivery date -- which in certain cases might
turn out to be a year from receipt of your order! The law says a mail-ordered
product must be delivered within 30 days of payment. After that the seller must
either refund your money or get your agreement for a later delivery date.
In my opinion, good servicing and prompt delivery are worth more than finding
the rock-bottom price.
|
An ugly telescope can outperform a pretty one; quality of optics, firmness of mount, and ease of use are all that matter. This excellent 12" reflector was built by Luc Secretan of Riverdale, Maryland, who named it the Ugly Duckling. It has an equatorial mount of the rigid English yoke design and includes many conveniences, such as electric motions on both axes, a very smooth clock drive, and a rotating secondary mirror that allows the eyepiece to be used on whichever of the four sides of the wooden tube is most convenient. A weatherproof shelter encloses the telescope and folds apart to provide an observing platform. |
Consider building a reflector yourself from parts -- an activity that many
astronomy clubs support. If you buy the mirror rather than grinding and
polishing it yourself, the most complicated tool you'll need is an electric
drill. You may save money and you'll end up knowing know your telescope
literally inside out.
Optical Quality
If possible, star-test a telescope before buying. This is especially important
when considering a used one not covered by warranty. If you can't test before
you buy, do so right after. Optical quality can vary quite a lot even among
identical-looking instruments from the same assembly line. The reason is that
large optical parts cannot be mass-produced with reliable quality. Each one has
to be hand-finished individually by a (hopefully) skilled worker. This means you
never know for sure what you're getting till you test it.
Here is a simple but very stringent test. With the optics properly aligned or
"collimated" (read the instructions that come with the scope), and
after the telescope has come to the same temperature as the night air (up to an
hour or two after it emerges from a warm house), focus on a 2nd- or
3rd-magnitude star using very high power. Polaris is a good choice because it
doesn't move.
Turn the focus knob slightly to one side, then slightly to the other side, of
best focus. The star's fuzzy, shimmering, out-of-focus diffraction rings should
look the same on both sides of best focus. That is, they should be the same
shape and have the same distribution of light inside them.
Poor atmospheric "seeing" -- the quivering and blurring caused by the
Earth's unsteady atmosphere -- may make this test difficult. Keep trying on
subsequent nights until you hit a spell of good seeing.
This test is so sensitive that very few telescopes pass it perfectly. If the
out-of-focus star image is almost the same on either side of focus, you still
have a good scope. If it's obviously quite different, however, something is
wrong. Before leaping to conclusions, try again on other nights and remember
about giving the scope plenty of time to cool. A telescope that
fails the star test won't ever focus very sharply. At high power, the star will
seem to gradually ooze or "shmush" through best focus as you turn the
knob, compared to a fine scope where the star "snaps" through focus.
The "snap test" is more of a judgment call that the
either-side-of-focus star test, which is quite exact. All stars are rendered
shmushy to some degree by atmospheric seeing. But if you get a chance to test a
good scope and a poor one side by side, the shmush-versus-snap effect is plain.
Do this at high power, because uncorrectable imperfections in your own eye often
make a star shmush through focus at low power no matter how good the telescope
and eyepiece may be.
If a scope is definitely bad, be assertive about returning it for repair or
refund. The better makers have excellent reputations for fixing problems. No
matter who made the scope you have a moral right to this treatment, so act
accordingly.
The best advice when considering telescopes is to seek the opinions of other
amateurs. Members of your local astronomy club will be glad to offer help and
frank opinions. With luck you may even get to try out a variety of their
telescopes, which will help you decide whether twice the aperture is really
worth four times the cost and six times the weight. The addresses and phone
numbers of some 400 clubs in the United States and Canada (and many e-mail
addresses) are listed in Sky & Telescope's Astronomical Directory.
Happy hunting!
Alan MacRobert is an Associate Editor of Sky & Telescope magazine and an
avid backyard astronomer.
|
The
Art of Using a Telescope |
Getting Started
Once you've obtained an astronomical telescope, what can you expect of it? Both less and more than many new owners realize.
One of the most fun parts of being an amateur astronomer is showing off the heavens to others. The "oohs" and "aahs" at a public star party as people get their first good look at the Moon or Saturn are a pleasant reward for the proud telescope owner. Naturally, you will have aimed the scope at the most spectacular object above the horizon. Sometimes there's a temptation to show people more typical objects -- ghostly, barely visible apparitions with obscure catalog numbers -- "to give them an idea of real astronomy." The reactions then are not so encouraging, even when viewers are told they're looking at a recently recovered comet or a galaxy 40 million light-years away.
The truth is, most of the thousands of objects visible in amateur instruments are not the least bit spectacular. Anyone who gets a telescope expecting dramatic visual thrills is in the wrong hobby.
The riches that astronomy offers are of a different sort. Visual observing outdoors in the dark usually means working to detect something that's extremely faint, tiny, hard to find, or all three. The more difficult the task, however, the greater the rewards of success. The excitement lies in finding and seeing first-hand remote marvels far beyond our planet -- and in gaining skills and knowledge as an amateur scientist.
Too many people buy a telescope as if it were a TV, expecting it to show pictures all by itself. It's more like a piano, which gives back only as much value as the work you put into it. Learning to use a telescope well is a lot easier than learning a musical instrument, however. If you're reasonably persistent and careful and are willing to practice the techniques described here, you'll soon master the skies.
Know Your Equipment
Naturally, everyone first tries out a new telescope in the daytime. This is when to become familiar with its motions, pointing, focusing, different eyepieces, and magnifying powers, so you can then do everything in the dark.
The Finder
Most telescopes have a finderscope attached to the side to help aim it. You need a finder because the main telescope has such a tiny field of view -- that is, it shows such a tiny piece of sky -- that you can't tell exactly where it's pointed just by looking.
The higher the power, the smaller the field of view. For example, at 50 power you're looking at a magnified piece of sky about as small as your little fingernail covers when held at arm's length. An 8× finderscope, on the other hand, displays about as much sky as a golf ball covers at arm's length.
This is big enough to aim at something you see with the naked eye and get it in the finderscope's view. Once it's there, you center it in the finder's crosshairs. That should be a precise enough aim for the object to appear in the view of the main telescope.
First things first: you'll need to adjust the finder's mounting screws so it's aimed parallel to the main telescope. In daylight, point the main scope at something at least several hundred feet away using the lowest-power eyepiece. (But not the Sun! Never look through a telescope that might get aimed at the Sun or you could blind yourself.) A distant treetop is ideal. Center it in your view. Never mind if it appears upside down.
Now look in the finder. See the treetop? Is it centered in the crosshairs? Adjust the screws holding the finder until the crosshairs line up on the target. Now check back in the main telescope to make sure it hasn't moved. Then switch to a high-power eyepiece in the main telescope, and repeat the operation until the finder is locked in position with perfect aim.
And why, you ask, is the treetop upside down or oriented at some other weird angle? The answer is that this is an astronomical telescope, and after all, there's no up or down in space. So it doesn't matter how the field is oriented. Turning the image right-side up would require extra optical parts, adding to the expense and complication of the instrument and probably degrading its performance slightly. Therefore, "image erecting" lenses are used only in terrestrial telescopes, those intended for looking at things on Earth.
Next let's turn to the mounting. As noted in the article "How to Choose a Telescope," telescope mounts come in two basic types: equatorial and altazimuth.
An equatorial mount allows the telescope to swing only in the directions of celestial north-south and east-west. The altazimuth goes up-down (moving in altitude) and side to side (azimuth). An altazimuth mount at least has the virtue of simplicity. An equatorial mount is ultimately more helpful, but it takes some getting used to.
The Equatorial Mount
If this is what you have, find its polar axis (the rotating part that's more toward the base and maybe has a setting circle showing right ascension). Outdoors, place the telescope so the polar axis points roughly towards where Sigma Octantis will be located after dark (refer to StarCharts for the location). Take a line through the long axis of the Southern Cross and a line perpendicular to mid-way between Alpha and Beta Centauri. Where these lines intersect is approximately celestial South where the Equatorial mount must face. The telescope's motion around this axis now traces the paths taken by celestial bodies across the sky as the Earth turns.
Sweep the telescope around its polar axis from the eastern horizon across the sky to the west to visualize nightly star paths. At first the mount's motions will seem awkward and unpredictable. But remember that no matter where the telescope is pointed, it will move only toward or away from Polaris (celestial north-south) and at right angles to this direction (celestial east-west). The orientation of these varies in different parts of the sky, but with some practice swinging the telescope around in daytime you'll get used to them.
The challenge of astronomy is that we must view most of the universe from extremely far away. When you're trying to see something well on Earth your instinct is to move closer for a better look. But when it comes to distant stars and galaxies, we're stuck where we are. So, ever since the dawn of telescopic astronomy, the art of observing has been the art of using your eye to the utmost of its ability.
Viewing tips
When looking through the telescope, focus and refocus with care. A good observer is always fiddling with the focus, trying to get it just a hair sharper. Many people find it best to keep both eyes open, since squinting strains the working eye. You can cover the "off" eye with one hand.
Don't expect to see right away everything an astronomical object has to offer. The first look always shows less than comes out with continued scrutiny. This is true whether your subject is a dim galaxy that can hardly be told from the blackness of space, or detail on the Moon or a planet where the light is almost blindingly bright.
One reason it takes time to see detail is the unsteadiness of the Earth's atmosphere. Celestial objects constantly shimmer and boil when viewed at high power, due to weak but ever-present heat waves in the air around and above us. The severity of this shimmering -- called the atmospheric seeing -- varies from night to night and often from minute to minute.
As you watch an object quiver and churn, unsuspected detail will flicker into view during quick moments of stability when the view sharpens up, only to fade out again before you know it. The skilled observer learns to remember these good moments and ignore the rest. The quality of the atmospheric seeing is most important when viewing bright objects at high power, but it can influence the visibility of faint ones too.
The main reason it takes time to see detail, however, has to do not with the atmosphere but with the eye and mind. Wringing everything possible out of very distant views means learning new visual skills that involve active, concentrated effort.
You'll discover that the eye's picture of a difficult object builds up rather slowly. First one detail is noticed and fixed, and you think there's nothing more to be seen. But after a few minutes another detail becomes evident, then another.
To convince yourself of this, look at a piece of sky with the naked eye and try to spot faint stars. Some will be visible right away; others take a few seconds to come out. When no more appear, most people would quit trying. But keep at it for a few minutes. Chances are some more will glimmer into view in places you would have sworn were blank. After a while you're seeing at least half a magnitude fainter than at first.
The planet Mars is another classic example of this effect. For the beginner taking a first look with a small telescope, Mars ranks as the most disappointing object in the sky. It's just a tiny, featureless, orange fuzzball. The beginner steps aside to let an experienced Mars observer look in the eyepiece. Silence. "There's the north polar cap.... That big dark area in the south must be Mare Erythraeum. Okay, I've got Sinus Meridiani.... There's a cloud patch on the western limb...."
The beginner looks again. Nothing but a fuzzball. Well, maybe there is a bit of brightness at the north edge crawling around in the poor seeing, and the fuzziness isn't a perfectly uniform orange, but these hardly seem like things worth noticing. Nevertheless, the next time the beginner looks he or she won't be quite a beginner, and the bright spot and dark area will come into view more readily.
An excellent way to train yourself to see better is to make sketches. These don't have to be works of art; the idea is just to record details in your notebook more directly than you can with words. Star fields require no artistic talent whatsoever, but by sketching a field that contains a faint asteroid or outer planet, you can identify the intruder be checking back in the next few days or weeks and seeing which one changes position.
For practice sketching planets, try drawing the Moon with the naked eye. If you have reasonably sharp or well-corrected vision, the Moon shows much more detail to the naked eye than any planet will in a telescope! Make a semicircle a couple of inches in diameter by tracing some round object and then draw the terminator exactly as you see it on the Moon. Carefully add the major dark areas with pencil shading, then look for finer markings. By now you'll be seeing much more detail on the Moon's face than you ever thought possible without optical aid.
"The lesson is clear," wrote the British author James Muirden in The Amateur Astronomer's Handbook, long a classic: "No opportunity should be lost to train the eye to work with the telescope; to observe the same object with different powers so as to see the effect of magnification; to try to see faint stars; and to draw planetary markings. In the beginning, to be sure, this may all seem to be wasted effort; the observing book will fill up with valueless sketches and brief notes of failure. But this apparently empty labor is absolutely essential; for, as the weeks pass, a steady change will be taking place. Objects considered difficult or impossible to see will now be discerned at first glance, and fainter specters will have taken their place. Indeed, these former features will now be so glaringly obvious that the observer may suppose that some radical improvement has occurred in the observing conditions. But the credit belongs entirely to the eye."
Life's Little Comforts
Naturally, this sort of concentration will be spoiled by any undue discomfort or inconvenience at the telescope. You'll need a table right at hand to hold charts, red flashlight, eyepieces, notebook, pencil, and other gear. The perfect solution would be a folding camp table.
Nothing ruins your ability to see like having to twist and strain to look through the eyepiece. A rotating tube, which can turn in its cradle to orient the eyepiece more where you prefer, is therefore a nice plus in a small reflector and almost mandatory in a large equatorially mounted one. If you can find or make an adjustable-height observing chair, your telescope may start showing new worlds. I've used an assortment of seats from a milk crate to a stepladder.
Any jerkiness and backlash in the mount's motions can also spell doom, especially if you lack a clock drive. Make sure the telescope is balanced properly by adjusting any counterweights; it shouldn't move in one direction more easily than in another. Don't be afraid to take a mount apart and lubricate it, or return it to the manufacturer if it's truly unsatisfactory. The mount I bought for my 6-inch reflector years ago was originally quite jerky. After trying various lubricants, I settled on candle wax rubbed onto all the bearing surfaces. The mount's "clamps" were merely bolts that tightened head-on against the shafts; I epoxied small pieces of leather to the bolt ends, impregnated these with graphite powder and a little oil, and thus gained adjustable tension. The improvement was enormous. At high power I could follow the stars with a smooth, continuous motion just by touching the side of my nose against the eyepiece.
In wintertime, you can either heed the astronomer's standard advice to dress for 5 to10 degrees C colder than the actual temperature, or you can learn the hard way. As for the summer, it remains a mystery how successful observations were performed before the invention of mosquito repellent.
In short: Anything that makes your observing easier, surer, or more relaxed, no matter how much trouble it takes beforehand, is worth the effort.
Alan MacRobert is an Associate Editor of Sky & Telescope magazine and an avid backyard astronomer.
