From PRODIGY:

Board: SCIENCE & ENVIRONMENT

Topic: ASTRONOMY

Subject: BUYING ADVICE

To: ALL Date: 09/02/97

From: BVRM74A MICHAEL WHITING Time: 3:56 PM

Hi. I am posting a long article that I got offering advice
and information to beginners needing telescope buying
advice. It is a re-print from ASTRONOMY magazine. While a
little dated, it still offers much useful information.
Buying your first telescope can seem like a complicated
affair. There are many models to choose from and many
technical terms to contend with. To help you make the
right choice, here are answers to 30 of the most-asked
questions we get from prospective telescope buyers. We
trust you'll find answers to your questions among them.

If not, please call us at ASTRONOMY magazine (414) 796-8776
Monday to Friday, 8:30 a.m. to 5:00 p.m. Central Time], or
see your local telescope dealer.

1. How much does the telescope magnify?

Beware of any telescope advertised as "500x" or"high-power".
Some manufacturers make it sound as if the more magnification
a telescope offers, the better it is. This is not true.
Contrary to the claims of department-store catalogs,
magnification is not important. Any telescope can be made to
magnify any amount. However, the highest power that will still
give you a clear view is about 50x per inch of aperture,
making the upper limit for a 3-inch telescope 150x, and for a
4-inch telescope 200x. Beyond this limit, the image will be
faint, fuzzy, and disappointing.

2. How, then, do I select the best telescope?

The key characteristic of a telescope is its aperture - the
diameter of the main lens or mirror.The larger the aperture,
the more light the telescope gathers and focuses into the
image. This in turn makes for a brighter, and usually
clearer, image. Brighter images make it easier to see faint
objects like nebulae and galaxies.

3. Which are better - refractors or reflectors?

A refractor uses a lens mounted at the front of the telescope to
gather and focus light. A reflector (sometimes called a Newtonian)
uses a concave or bowl-shaped mirror mounted at the back of the
telescope. Both work well; each has its advantages. Reflectors
generally offer more aperture for the money. (A 4-inch reflector
costs $400 to $500; 4-inch refractors start at $1,000 or more.)
However, refractors usually provide slightly sharper images than
reflectors of similar aperture. Amateur astronomers who like to
view fine details on planets often prefer refractors; those who
like to look at faint deep-sky objects use reflectors. For most
first-time buyers on a budget, either an 80mm or 90mm refractor
or a 4.5-inch reflector is a good choice. Both cost $400 to $600
and have comparable performance.

4. What are Schmidt-Cassegrain telescopes?

A third type of telescope system, called a catadioptric, uses a
combination of mirrors and a refractive corrector lens at the
front. The most popular of these hybrid models is the 8-inch
Schmidt-Cassegrain (prices start at $1,200). It folds a long
focal length into a compact tube, making this type of telescope
very portable and convenient to use for its aperture. It is also
a good general pupose telescope suitable for observing all
classes of celestial targets.

The two main manufacturers of this type of telescope are Celestron
International and Meade Instruments Corporation. These two
companies are very competitive and offer a similar range of
Schmidt-Cassegrains, from basic no-frills units to feature-laden
computer-controlled models. Which company makes better telescopes?
Over the years we have never found a consistent winner between the
two, either in optical or mechanical quality.

5. What are apochromatic refractors?

One of the principal problems with conventional refractors over
80mm aperture is spurious color around bright objects caused by the
inability of the lens to bring all colors to the same point of focus.
To greatly reduce this "chromatic aberration," manufacturers have
introduced refractors that use 3-element lens systems or special
fluorite or "ED" lenses. Called apochromatics, these high-end
refractors are among the finest optical systems you can buy and
have become popular with telescope aficionados. Models are available
from Astro-Physics, Celestron, Meade, Takahashi, and TeleVue.

However, a 4-inch "apo" refractor can cost $2,500 to $5,000, more
than what many people wish to spend on a first telescope.

6. How much more will I see with a bigger telescope?

Bigger telescopes can show fainter objects and resolve finer
details in bright objects. For example, a 2.4-inch (60mm)
refractor will easily show the cloudbelts of Jupiter, a
4-inch will show structure within the cloudbelts, and an
8-inch will resolve even smaller details. A 4-inch will show
globular star clusters as fuzzy-edged spheres of light, a
6-inch will resolve many globulars into myriad faint stars,
and a 12-inch will provide views of these clusters that
surpass any photograph. While a 4-inch will reveal a spiral
galaxy as a round glow, an 8-inch will begin to reveal the
galaxy's spiral arms.

7. Will I be happy with a smaller scope?

The fact that bigger telescopes usually show more details
and fainter objects leads many people to believe small
scopes aren't worth buying. But even an 80mm refractor can
show you enough of the universe to keep you entertained for
years. For many people it's all the telescope they ever need.

We warn people against buying a telescope that is too large-
yes, there is such a thing. A big telescope, though exciting
at first, can quickly become a burden to carry out to the
yard or car and to set up. The best telescope is not the
biggest, or even the one with the best optics, but the one
that you will use most often. Portability and convenience
are factors we urge you to consider when selecting a
telescope that you'll have fun using.

8. I live in the city (country) where the skies are
terrible (great). What type of telescope is best for me?

A large-aperture telescope can be useful at any site, but
faint deep-sky objects (the kind big scopes are well-suited
for) won't show up well under urban skies, no matter what
size the scope. City observers often spend more time looking
at the Moon and planets, for which a 3- to 8-inch telescope
is sufficient. Telescopes in that size range are also very
portable, important for city observers who need to transport
their scopes to better skies. For most buyers, we feel that
a 5-inch refractor, a 6-inch equatorial reflector, an 8-inch
Schmidt-Cassegrain, or a 10-inch Dobsonian reflector (see
question 16) are the largest telescopes of their types that
are conveniently portable. Only if you live under dark
skies, or really don't mind lugging a big, heavy scope
around, should you consider anything larger for a first
telescope.

9. What does focal length mean? and f/ratio?

The focal length of a telescope is the length of the light
path from the main lens or mirror to the eyepiece. In most
refractors or reflectors, the focal length is roughly the
length of the tube.In telescopes such as Schmidt-Cassegrains
in which the light path is bounced back and forth inside the
tube several times, the length of the tube is much shorter
than the focal length. Focal lengths of telescopes, as with
camera lenses, are usually measured in millimeters.

The f/ratio of a telescope is the focal length divided by
the aperture. For example, a 100mm-aperture telescope with
a 900mm focal length is an f/9 telescope. A 200mm telescope
(an 8-inch) with a focal length of 1,800mm is also an f/9.

10. What focal length is best?

The focal length of the telescope is not a critical
specification. Shorter focal lengths (400 to 700mm) will
give lower powers and wider fields of view with any given
eyepiece than will telescopes with moderate (800 to 1,200mm)
or long focal lengths (1,300 to 3,000mm). For this reason,
short focal lengths are often preferred for low-power
viewing of deep-sky targets and Milky Way starfields. On
the other hand, a long-focal-length scope will give a higher
power with any given eyepiece. Since the planets require
higher powers (100x to 200x), planetary fans often prefer
long-focal-length scopes.But with the use of the appropriate
eyepieces, most telescopes can be used at both low and high
powers.

11. Is a faster telescope better?

Sometimes manufacturers give the impression that a "faster"
telescope (one with an f/ratio of f/4 to f/6) is better
than a "slow" telescope (f/7 to f/16). After all, in many
situations faster is better. But in this case it isn't. The
term "faster" comes from photography where an f/4 lens will
record an image with a faster exposure time than an f/16
lens. And for those intending to take long-exposure photos
through a telescope, faster scopes can be better. But when
looking through a telescope, a faster telescope is not any
brighter than a slower scope. For example, as long as both
are operating at the same power, the image in an 8-inch f/6
telescope will appear as bright as the image in an 8-inch
f/10 scope. The difference is that with the same eyepiece
the f/6 telescope will give a lower power and a wider field
of view than will the f/10, making faster scopes preferred
for deep-sky observing where wide fields are desirable.

12. What are the eyepieces for?

Eyepieces allow you to change magnification. To determine
the magnification an eyepiece gives, divide the focal length
of the telescope by the focal length of the eyepiece. For
example a 25mm focal length eyepiece used on a 2,000mm focal
length scope (such as an 8-inch f/10 scope) will give
2000/25 = 80x. The same eyepiece used on a 1,600mm scope
(such as an 8-inch f/6) will give 64x.

13. What is the small finderscope for?

Finderscopes are essential accessories. They provide a low
power (5x to 8x) and a wide field (3* to 5*) and allow you
to aim the telescope easily and center it on bright planets
and stars. Without a finderscope locating even the Moon can
be difficult.

14. Why are images in telescopes upside-down?

All astronomical telescopes present images that are either
upside-down or flipped left-to-right as in a mirror. To
flip the image right-side up would require extra lenses in
the light path that would dim the view of already faint
astronomical objects or add imperfections like flares and
ghost images.

15. Which is better - an altazimuth or equatorial mount?

Alt-azimuth mounts use simple up-down (altitude) and side-
to-side (azimuth) motions to aim the telescope. The best
of these mounts are equipped with slow-motion controls to
allow you to make fine adjustments to the position of the
scope. However, alt-az mounts cannot automatically follow
the stars as they appear to arc across the sky from east to
west. An equatorial mount is more complex. It can follow the
stars across the sky with a single motion around one axis.
If the telescope is equipped with a motor, the telescope
will automatically track the stars. This is a nice feature
because at magnifications of 100x or more, the apparent
motion of the sky will cause objects to drift out of the
field of view in less than a minute. Having to recenter the
image constantly can be distracting, inconvenient, and can
introduce vibration that shakes the image.

16. What about Dobsonian telescopes?

A Dobsonian is a Newtonian reflector. Its unique feature is
a simple wooden alt-azimuth mount that rides on Teflon pads.
The philosophy of the popularizer of this type of telescope,
John Dobson, was to keep the scope easy-to-build and
low-cost. The design also lends itself to relatively large
apertures. Dobsonians cannot track the stars automatically,
but their motions are very smooth - it's easy to nudge the
scope every so often to recenter the object.
As of 1992, only one manufacturer was selling low-cost
Dobsonian telescopes - Coulter Optical Company. Their models
offer big aperture for very little money (for example, a
10-inch reflector for under $400). People often wonder if
there's a catch. There are a few: Coulter scopes are in such
demand that you may have to wait several months to a year
for delivery. You must order directly from the factory and
pay shipping costs from their plant in California (which may
add up to $100 to the cost). You will also need to add a
finderscope. The fit and finish of the scopes is nothing
fancy - the mounts are painted chipboard, the tubes
cardboard. Our opinion? For the money the quality of
construction and optics can't be beat. The 8- and 10-inch
Coulter models make good starter scopes. The 13- and 17-inch
models are best for die-hard deep-sky observers.

17. I've heard you can make your own telescope.

The Dobsonian design lends itself to do-it-yourselfers.
Plywood for the mount and a cardboard tube like those used
for concrete forms are the main ingredients. Few people
make their own mirrors these days. It can be done, but
ready-made mirrors from suppliers such as Coulter, Meade,
Parks and other suppliers don't cost much more than mirror-
making kits. You'll also need a focuser and cells to hold
the main mirror and the small secondary mirror. For more
information about telescope making see the book Build
Your Own Telescope by Richard Berry, available from Kalmbach
Publishing.

18 What accessories do I need?

Some telescopes come with only one eyepiece. Additional
eyepieces for higher and lower powers are the first
accessories most first-telescope owners need to buy. An
accessory called a Barlow lens can double or triple the
power of each eyepiece, but the best Barlows (the only
ones worth buying, trust us!) cost $80 to $100.
Colored filters can enhance views of the planets slightly,
but the difference is subtle. They are not essential.
Nebula or light pollution filters can improve views of some
deep-sky objects like emission and planetary nebulae, but
they do little to improve star clusters and galaxies.
Contrary to what many beginning backyard astronomers believe
, these filters are not a cure-all for light-polluted skies.
Computerized digital readouts to aid in finding objects have
become popular telescope accessories in recent years. They
work well but are luxury options for those that can afford
their $500 to $1,000 price tags.

19. Are enhanced coatings worth the extra cost?

Some telescopes are offered with special lens or mirror
coatings as optional extras (Celestron's Starbright(tm) and
Meade's MCOG, for example). These increase the light
transmission, lding images up to 15 percent brighter.
They are definitely worth the extra expense.

20. Can I use setting circles to find things?

Many equatorial mounts are equipped with graduated dials
called setting circles. Theoretically, these allow you to
find objects by moving the telescope so that the circles'
readings match the celestial coordinates (called right
ascension and declination) of the object you're looking
for. However, in our experience we have rarely seen a
novice amateur astronomer (nor many experienced ones!) who
have been able to make effective use of setting circles.
Poor alignment of the telescope mount, improperly calibrated
circles, and imprecise circle scales usually combine to make
circle readings inaccurate. The best method to find celestial
targets is to hop from star to star using a good star chart
as your guide. Plan on buying such a star chart as an
essential accessory.

21. Can I take pictures with this scope?

Anything you see through a telescope can be photographed,
but most objects require exposures of several seconds to an
hour or more. Keeping the object perfectly positioned on the
film during that time requires a solid equatorial mount and
a motor drive. These are essential features if you intend to
do astrophotography.

22. Can I use a spotting scope for astronomy?

Some spotting scopes (such as those sold for birding) have
only a fixed-power eyepiece or a variable zoom eyepiece.
These models are unsuitable for astronomy. Other models use
interchangeable eyepieces but must be placed on a solid
camera tripod. Because they lack fine slow-motion controls,
camera tripods are difficult to aim precisely, a problem at
high power.

23.But I also want to use my telescope for nature observing.

If your interests mix astronomical and terrestrial viewing,
we suggest an 80mm or 4-inch refractor, or a small 4-inch
Schmidt- Cassegrain. Don't buy a Newtonian reflector -
the position of the eyepiece makes a Newtonian awkward for
use as a spotting scope.

24. How much should I have to spend?

We feel that $400 to $500 is the minimum for a quality
starter scope such as an 80mm refractor or 4.5-inch
reflector. The next step up is to a 6-inch equatorially-
mounted reflector (such as the models from Celestron,
Meade, Parks Optical, or Pirate Instruments). These sell
for $600 to $900.

The next jump up many first-time buyers consider is to an 8-
inch Schmidt-Cassegrain ($1,200 to $2,500). No-frills
Dobsonian reflectors defy these price/aperture categories by
offering much more aperture for the money (see question 16).

25. What does "1/20th-wave optics" mean?

The deviation of an optical surface (lens or mirror) from
the ideal shape is often stated as a fraction of a
wavelength of light. The smaller the fraction, the better
the optics and the sharper the image. However, to be
meaningful for a complete telescope this deviation figure
should be provided for the final wavefront reaching the eye,
not just for individual lenses or mirrors. When measured in
this manner, a telescope with a total error on the final
wavefront of 1/4 wave is very good, 1/8 or 1/10 wave is
excellent, and 1/20 wave is outstanding but seldom acheived.
Manufacturers have no agreed-upon standard for measuring
these values - one company's 1/20 wave may be the same as
another company's 1/10 wave.

26. What does "diffraction limited" mean?

This is another freely used term in telescope advertising.
It means that the optics are so good they are limited only
by the wave nature of light and not by any flaws in the
surface accuracy of the lenses or mirrors. Specifically,
it means the final wavefront error is better than 1/4 wave,
a figure known as Rayleigh's Criterion. Again, few
manufacturers have the technical equipment to quantitatively
support this claim. Most test telescope quality by ensuring
units form good star images. Although this is a very
sensitive test that will detect small flaws in the optics,
it cannot guarantee a numerical specification like 1/4 wave.

 
27. Where can I buy a good telescope?

We suggest shopping at a local telescope dealer if there is
one near you (check the Yellow Pages under "Telescopes").
If he is doing his job right he will check each scope he
sells, provide good service, answer your technical
questions, and perhaps allow you to take home a scope on a
trial basis. You can at least see what you're getting before
you buy it. This peace of mind is worth any extra cost
involved.

Mail-order companies that specialize in astronomy products
can also offer personal service (over the phone) and money-
back guarantees of satisfaction. We would caution you about
some (not all) mail-order firms - their prices may be
heavily discounted but at a sacrifice of expert personal
service. Some have limited guarantees and no after-sale
service - if there is a problem you can find yourself on
your own dealing directly with the manufacturer. Also,
watch the shipping and packing charges!

28. What about buying a used telescope?

If well-cared for, a used telescope should perform as well
as a new one. You can find telescopes in the classifieds in
local newspapers and "bargain finders." You should also
check with the local astronomy club or ASTRONOMY's Reader
Exchange. Two newsletters called The Starry Messenger and
The Cosmic Exchange (contact ASTRONOMY magazine for their
addresses) are devoted to ads for used telescopes.

29. What telescope would YOU buy?

This is impossible to answer. Someone who has been in the
hobby for a while and who has already owned several
telescopes would not select the same scope a first-time
buyer would. Some people prefer the solidness and precision
of a fine-quality refractor, others like the aperture and
versatility of a Schmidt-Cassegrain, while others prize the
light-gathering power and simplicity of a large Dobsonian
reflector. There is no single best telescope. In fact,
chances are the first telescope you buy will not be the
last. Many backyard astronomers happily own two or three
telescopes, each outstanding for a certain type of viewing.

30. I have a child interested in astronomy. What scope
should I buy? My budget is $200.

Avoid low-cost 500-power "department store" 50mm and 60mm
refractors. Their poor mounts, eyepieces, and finderscopes
will almost certainly make these telescopes a disappointment
. The better 60mm refractors on alt-azimuth or equatorial
mounts with slow-motion controls and a decent 6x30
finderscope can serve as starter scopes if your expectations
are well-tempered. Acceptable models are available from
astronomical dealers (such as those who advertise in
ASTRONOMY) and local telescope stores. But the truth of the
matter is that for $200 (a common budget of parents with
young astronomers), there are few telescopes on the market
we can endorse. Instead, we, and many astronomy educators,
usually recommend a pair of 7x50 binoculars combined with
a set of introductory books and star atlases, a package that
will cost $100 to $200. Binoculars can reveal a surprising
number of celestial objects (craters on the Moon, the moons
of Jupiter, deep-sky objects such as star clusters and
nebulae). A year spent exploring the sky with binoculars
and a star chart can teach any novice astronomer, young or
old, an immeasureable amount about the sky, the identity of
stars and constellations, and the locations of celestial
targets. If your prospective astronomer is still interested
in the hobby after a year of binocular stargazing, then
purchase a decent telescope for $400 to $500. At that point
you will be more confident that your money will be well-
spent.

WHAT CAN I SEE?

What you can see through the eyepiece of a good telescope is
enough to keep you exploring the sky for many years. Here's
a bit of advice: when you unpack your first telescope and
set it in the backyard on its premiere night under the stars
the first thing you should look at is the Moon. You won't
need high power - 50x will be just fine. Even at this low
power, you'll be amazed at the view, we guarantee it.
After observing the Moon, the first planet you should look
at is either Jupiter or Saturn. (If it's winter or spring
try for Jupiter; if it's summer or autumn try Saturn - the
monthly "Sky Almanac" section of ASTRONOMY will tell you
where to find them).

When you aim your scope at Saturn, be prepared for a
remarkable sight. Most people utter an exclamation of "Wow!"
when they first see Saturn and its picture-perfect rings.
Even a small 60mm aperture telescope will reveal the cloud
belts on Jupiter. Over the course of several nights, you'll
see Jupiter's four large moons as bright dots shuttling back
and forth from one side of Jupiter to the other.
A telescope also allows you to follow the changing phases of
Mercury and Venus, and to watch Mars grow and shrink in size
as we approach then recede from the Red Planet every two
years. And, yes, Mars really does look orange-red. When it
is closest to Earth, a magnification of 100x to 200x will
show its reddish disk, dark surface markings, and famous
polar caps. You can continue to tour the solar system by
tracking down Uranus and Neptune, although even in large
telescopes they appear only as tiny blue-green dots.
Starlike Pluto is within reach of backyard scopes but
requires at least an 8-inch aperture instrument and dark
skies.

Beyond the solar system there are hundreds of star clusters,
nebulae, and galaxies within reach of 60mm- to 80mm-aperture
scopes. Larger scopes reveal these deep-sky objects in even
more detail and bring thousands more deep-sky wonders within
reach. However, don't expect to see the colors you see in
photographs - the colors of nebulae and galaxies are so
faint they show up only in long-exposure photographs. Most
deep-sky objects appear as misty, gray patches of light.

However, some stars show colors your unaided eye cannot see,
and many stars that appear as single to the naked eye
appear split into two or more stars through a telescope
- they are systems of alien suns orbiting each other.
If you want an idea of how far you can see with a telescope,
you can actually explore the most distant reaches of the
cosmos by hunting down elusive galaxies. Because of their
immense distance, these islands of stars are typically very
faint. Nevertheless, from a dark rural site a 3- or 4-inch
telescope will show many of these ghostly spots of light,
enabling you to probe tens of millions of light- years into
space. All with a telescope costing no more than a round-
trip air fare to Europe. It's a bargain price for a ticket
to the stars.

TELESCOPE PROS AND CONS

Achromatic Refractors

(60mm to 5-inch)

Advantages: Economical in smaller sizes; rugged; portable;
easy to aim; usually provide sharp images.

Disadvantages: Small apertures have limited light-
gathering power; larger apertures exhibit chromatic
aberration.

Apochromatic Refractors

(3- to 7-inch)

Advantages: Provide high-quality images near perfection;
excellent for lunar and planetary viewing; fast models
good for wide-field, deep-sky viewing and photography.

Disadvantages: Relatively expensive for the aperture;
light-gathering power cannot compete with that of larger
reflectors.

Equatorial Newtonian

(4- to 18-inch)

Advantages: Large aperture for the money; small sizes are
excellent scopes for serious beginner. In f/6 to f/8 designs
they are good all-purpose scopes.

Disadvantages: Can be very bulky and heavy in sizes over
8-inches; mirrors require adjustment; mirror surfaces are
exposed and can get dirty.

 
Dobsonian

(8- to 20-inch)

Advantages: Biggest aperture for least money; portable for
the aperture; superb for deep-sky observing, easy to set
up (no polar alignment); great for dark sky sites.

Disadvantages: Optical quality in low-cost models is a

compromise; mount does not track the stars; mirror
collimation critical in fast f/ratio models.

Schmidt-Cassegrain

(4- to 14-inch)

Advantages: Very portable for an equatorially-mounted scope;
easy to set up and aim; adaptable for astro-photography;
expandable systems with many accessories; excellent general-
purpose telescopes.

Disadvantages: Outperformed by specialized telescopes for
planetary (refractors and long-focus Newtonians) and deep-
sky viewing (large Dobsonians); corrector plates attract
dew.

BUYER'S CHECKLIST

Does the scope have sufficient aperture?

We suggest at least a 4-inch telescope for viewing for deep-
sky objects.

How good are the optics?
Will the dealer provide a guarantee of satisfaction?

How steady is the telescope?
After a light tap, vibrations should damp out in 1 to 2
seconds.

How portable is the telescope?
Can you carry it easily? Will it fit in your car?

How easy is it to set up?
Is the mount complicated? Heavy? Does it require tools?

Does it have a drive motor? Is it AC or DC?
DC drives can run directly from batteries and have a wider
range of speed controls.

Does the mount have slow-motion controls?
These make it easier to aim the scope and follow objects.

Does it have a separate finderscope?
Finders that sight through the main optics are usually very
poor.

How large is the finderscope?
A 25mm-aperture finder is poor, a 30mm OK, a 50mm apeture
finder such as an 8x50 is best.

What diameter eyepieces does it come with?
0.965-inch-diameter models are usually of poor quality.
1.25-inch diameter eyepieces are better and are available
in a wider range of focal lengths and designs.

How good are the eyepieces?
Orthoscopic and Ploessl eyepieces (often included as
standard equipment) are better than Kellner eyepieces which
in turn are better than the poor Huygenian eyepieces
included with many import scopes.

Does it come with a case?
It's useful if you will be transporting the scope.

How expandable is the telescope?
Is there a good array of accessories available?

Does it come with a warranty?
And who will honor the warranty with service?