By Dyer, Alan
Magazine: SKY & TELESCOPE; DECEMBER 01, 1997
BUYING THE BEST TELESCOPE
-------------------------
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.
A first telescope should combine simplicity, portability, and
ease of use with sharp optics and a
steady mount. What fits the bill?
The 6-inch Dobsonian
For most first-time buyers on a budget the choice is simple. The
ideal starter scope is a 6-inch
(15-centimeter) reflector on a Dobsonian mount. The "6-inch"
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-inch 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-inch for image
brightness, they aren't as portable. An 8-inch Dobsonian is a
tempting alternative, but its tube
and mount will be bigger and weigh about 30 percent more.
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-inch Dobsonian
reflector is a fine starter scope,
far outperforming the smaller, flimsier, yet comparably priced
beginners' telescopes that
proliferate in department stores at Christmas.
Approximate price: About $350, but count on spending about $50
to $100 more for
recommended options such as a good finder-scope, an extra eyepiece
or two, and star charts.
It's Too Big!
There's no perfect telescope and, sure enough, a 6-inch 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, and all involve
some sacrifice of aperture.
Compact Schmidt-Cassegrain or Maksutov-Cassegrain telescopes
offer 3 1/2- to 5-inch (90- to
125-mm) apertures and optical systems that fold a 48-inch focal
length into a tube no more
than 12 inches long. The entire telescope will fit under an aircraft
seat.
Alternatively, short-focus refractors are available in 2.7- to
3.1-inch (70- to 80-mm) apertures
with tubes short enough to fit into a camera case. Primarily
designed for lowpower, wide-field
views of the stars, these refractors also work well for moderate-power
views of the planets. All
require a sturdy tripod at additional cost.
Approximate prices: $600 to $1,200 for a 3 1/2- to 5-inch Maksutov-
or Schmidt-Cassegrain
telescope. $300 to $800 for a short-focus refractor without tripod.
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-inch (75- and 100-mm) Dobsonian reflectors. Compared to 2.4-inch
(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-inch (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 4 -inch (11 cm)
Newtonian reflector in a sealed, ball-shaped tube.
Those on a very tight budget might want to bypass a telescope
altogether. In its place consider
binoculars. A 7x42, 8x50, or 10x50 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-inch 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-inch 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-inch (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.
A popular alternative to the Schmidt-Cassegrain is the apochromatic
refractor. Yes, their
apertures are "only" 4 to 7 inches (10 to 18 crn), 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: $1,000 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-inch
apochromatic refractor with altazimuth mount; $4,000 and up for
a 5-inch or larger apo
refractor.
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-inch refractor or a
3.5- to 5-inch 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-inch Maksutov-
or Schmidt-Cassegrain
telescope. $400 to $2,000 for a 2.7- to 4-inch 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-inch
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-inch 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-inch (115-mm) Newtonians
and 3.1- to 3.5-inch
(80- to 90mm) refractors offer decent views of planets at lower
cost, though with less
brightness and resolution than a 6-inch reflector. These two
categories contain a wide range of
models -shop for ones with the features listed under "What Do
I Look For?" in the sidebar
below.
At higher cost, 7- and 8-inch 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-inch equatorial Newtonian.
$400 to $600 for a
motor-equipped equatorial 3.1-inch or 3.5-inch refractor or 4.5-inch
reflector. $1,000 and up for
a 7- or 8-inch Maksutovor 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-inch reflector, an 8-inch reflector provides images 77
percent brighter (or 0.7 stellar
magnitudes deeper). Images in a 10-inch are 56 percent (0.5 magnitude)
brighter than in an
8-inch. 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-inch 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-inch 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-inch Dobsonian with accessories;
$3,000 for a truss-tube 12-
to 16-inch Dobsonian reflector.
I Want to Take Photographs
Our 6-inch 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-inch (15- to 25-cm) equatorial
Newtonian reflectors lack
drives and mounts accurate enough for long exposures. Nor do
they readily accept accessories
such as piggy-backed guidescopes. They are designed primarily
for convenient visual use.
If astrophotography is in your future consider nothing less than
an 8-inch Schmidt-Cassegrain
or 4-inch apochromatic refractor. Either way, purchase the model
with the best mount you can
afford. A solid mount is essential for sharp photographs. The
mount should be equipped with an
electronically controllable drive motor on each axis.
Approximate prices: $2,000 and up for an 8-inch Schmidt-Cassegrain
with heavy-duty mount,
wedge, and tripod. $3,500 and up for a 4-inch apochromatic refractor
on a German equatorial
mount. Budget an extra $400 to $1,000 for camera adapters, a
guiding eyepiece, guidescope or
offaxis-guider, a declination motor, and electronic motor controls.
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-inch 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-inch
Maksutov-Cassegrain or 5-inch
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 dose 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-inch Dobsonian. Also,
high-demand models like these are often in short supply, with
lengthy delivery times. Buyers
who want to purchase 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-inch
reflector on a solid equatorial
mount, or a 3.1- or 3.5-inch 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-inch 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!
PHOTO (COLOR): Above: 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.
PHOTO (COLOR): While a simple scope like Edmund Scientific's Astroscan
2001 is perfect for
portable observing, your needs may grow beyond it to bigger,
more powerful instruments.
PHOTO (COLOR): Small is beautiful. Meade's ETX and Celestron's
C5 incorporate decent
apertures in a compact package.
PHOTO (COLOR): 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.
PHOTO (COLOR): For a given light grasp, no other telescope is
quite as compact and portable
as the Schmidt-Cassegrain. The Meade LX10 (right) and Celestron's
Celestar 8 both cost
around $1,000.
PHOTO (COLOR): Don't try this at home! Astrophotography requires
a solid equatorial mount, a
precise drive motor, and other accessories.
PHOTO (COLOR): This Astro-Physics 5.1-inch apochromatic refractor
combines premium optics
with a precision mounting.
PHOTO (COLOR): Many companies sell small refractors including
Celestron, Meade, Orion, and
Tele Vue.
Further Reading
Harrington, Philip. Star Ware. New York, NY: John Wiley and Sons, Inc., 1995.
Kitchin, Christopher R. Telescopes and Techniques. London, U.K.: Springer-Verlag, 1996.
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. A selection
of sky maps and books
can be found in the 1998 Sky Publishing Catalog packaged with
this issue.
Take time getting to know the sky. Learn where things are and what things are.
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 2.4-inch is 120x, of a 4-inch, 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 kind of statement is a sure sign
of inferior quality disguised to lure
impulse buyers who don't know any better. But now you do!
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-inch (60-mm) to 4-inch (10-cm) apertures.
Premium "apochromatic"
models in 4-inch and large 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 telescope are also reflectors,
but with an added lenslike
correcting plate.
As a rule, refractors provide sharper views that 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.
DO 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 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, making it nearly impossible to obtain a steady
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 promoted the
design) is a form of
altazimuth mount. These wooden mounts are popular on Newtonian
reflectors of all sizes from
3-inch to 36-inch. 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.
WHAT DO I LOOK FOR?
In a word, aperture. But the following features are also hallmarks of a quality telescope:
o A 1 1/4-inch focuser. Telescopes that accept only the smaller,
0.965-inch 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, Plosel
eyepieces.
o A true 6x30 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 backet with six
adjustment points rather than three--it's much more solid.
o All wood-and-metal construction with minimal use of plastic,
especially in moving parts such
as the focuser.
o 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?
o Slow-motion controls on both axes (unless it is a Dobsonian mount).
o On low-cost telescopes, ignore high-tech 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 SCOPE 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.
WHAT CAN I SEE?
Even the most modest optical aid will provide startling views
of the Moon. However, those
2.4-inch refractors with poor-quality eyepieces and wobbly mounts
will show little else well.
The better 2.4-inchers and certainly a good 3- to 4-inch 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. However, even a large telescope
will not show you
o The flags on the moon -- they're too small!
o Stars appearing as disks -- they're points in even the largest telescopes.
o Nebulae in technicolor -- the Orion Nebula looks greenish, but
most nebulae and galaxies are
so dim that they appear gray visually.
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.
o Local astronomy clubs often host star parties where you can
see various telescopes in action
and talk to their owners. If you are on the Internet, go to
http://www.skypub.com/astrodir/astrodir.html
o Your local planetarium or science center probably has someone
on staff proficient in
telescopes who can provide recommendations.
o 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 at
http://www.skypub.com/testrept/testrept.shtml for past reports.
~~~~~~~~
By Alan Dyer
ALAN DYER is a contributing editor to Sky & Telescope. He
is coauthor with Terence Dickinson
of The Backyard Astronomer's Guide.