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February 16, 2004
Understanding Telescopes
By Chuck Fuller
Introduction
The primary goals of this article are to explain
how telescopes work, what the major types and categories are,
and how you can best choose a telescope for yourself or a
budding young astronomer in your midst. We’ll look at some
baseline principles, the major types of optical systems, mountings,
manufacturers, and of course, what you can actually see and
do with any given telescope.
I think it is important to point out some things
at the outset: while astronomy can be a casual hobby,
it tends not to be. It rapidly engenders passion, and when
astro-geeks get together, the passion reinforces itself. The
planets, stars, clusters, nebulae, and space itself are profound
things, an experience waiting to happen. When it happens to
you, be prepared for your life and daily perspective to be
altered by the general nature of the cosmos. When you fully
understand the physical scale of the stars and galaxies, and
the role that light (AKA “electromagnetic radiation”) plays
in our understanding, you will be changed.
When you have the experience of knowing that
an individual photon traveled from the sun for several hours
(at the speed of light), struck an ice crystal in the rings
of Saturn, and then reflected back for several more hours,
passing through your telescope’s optical system, through the
eyepiece, and onto your retina, you will truly be awed. You
have just experienced “primary source” perception, not a photograph
on the Web or TV, but the real deal.
Once this bug bites you, you may need counseling
to prevent you from selling everything you own in order to
get a bigger telescope. You have been warned.
Rules of Engagement
Before we look at the equipment and principles
in detail, there are a few widespread myths that need clarification
and correction.
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Don’t buy a “department store” telescope:
While the price may seem right, and the pictures on the
box look compelling, small telescopes found in retail
stores are of consistently poor quality. The optical components
are often plastic, the mounts are wobbly and impossible
to point, and there is no “upgrade path” or ability to
add accessories.
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It’s not about magnification: Magnification
is the most over-hyped aspect used to lure uninformed
buyers. It is actually one of the least important aspects,
and is something you control based on your choice of eyepieces.
Your most-used magnification will be a low-power eyepiece
with a wide field of view. Magnification not only magnifies
the object, but also the telescope’s vibrations, its optical
flaws, and the rotation of the earth (making tracking
difficult). Far more important than magnification is light-gathering
power. This is a measure of how many photons your
scope collects, and how many make it to your retina. The
larger the diameter of the primary optical element (lens
or mirror) of the telescope, the more light-gathering
power it has, and the fainter objects you will be able
to see. More on that later. Lastly, the resolution
of your telescope is also more important than magnification.
Resolution is a measure of the ability of your optical
system to discern and separate features that are close
together, such as splitting double-stars, or seeing detail
in the belts of Jupiter. Although theoretical resolution
is determined by the diameter of your primary optical
element (lens or mirror), it turns out that the atmosphere,
and even your own eye, can be far more important. More
on that later, too.
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Computer pointing is not necessary:
In the past several years, advanced mounts with GPS and
computer pointing and tracking systems have come of age.
These systems increase the cost of the telescope significantly,
and don’t add much value for beginners. In fact, they
can be detrimental. Part of the reward of this hobby is
to develop an intimate relationship with the sky – learning
the constellations, individual stars and their names,
the movement of the planets, and the locations of numerous
interesting deep-sky objects. For technology junkies with
laptops sporting observation-planning software, the computer
pointing mounts can be fun. But don’t consider it a critical
buying decision for a first telescope.
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If you’re just curious: Don’t rush
out and buy a telescope. There are many ways to become
more familiar with the hobby, including local observatory
public observing sessions, local star parties put on by
astronomy clubs, and friends-of-friends who may already
be immersed in the hobby. Check out these resources, and
the Web, before deciding if you should spend hundreds
of dollars obtaining a telescope.
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Optical Systems
Telescopes work by focusing light from distant objects
to form an image. An eyepiece then magnifies that image for your
eye. There are two primary ways to form an image:refracting light
through a lens, or reflecting light off of a mirror. Some optical
systems employ a combination of these approaches.
Refractors (above) use a lens to focus light
into an image, and typically are the long, thin tubes most people
think of when they imagine a telescope.
Reflectors use a concave mirror to focus light.
Catadioptrics use a combination of lenses and
mirrors to form an image.
Concepts
Before we look at various types of refractors and
reflectors, there are some useful concepts that help in overall
understanding:
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Focal Length: the distance from the primary
lens or mirror to the focal plane.
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Aperture: a fancy word for the diameter
of the primary.
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Focal Ratio: the ratio of the focal length
divided by the aperture of the primary. If you are familiar
with camera lenses, you know about F/2. 8, F/4, F/11, etc. These
are focal ratios, which, in camera lenses, are changed by adjusting
the “F-stop”. The F-stop is an adjustable iris within the lens
that modifies the aperture (while the focal length is constant).
Low F-ratios are called “fast”, while large F-ratios are “slow”.
This is a measure of the amount of light hitting the film (or
your eye) compared with the focal length.
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Effective Focal Length: for compound optical
systems (employing an active secondary element), the effective
focal length of the optical system is typically much larger
than the focal length of the primary. This is because the curvature
of the secondary has a multiplying effect on the primary, a
kind of optical “lever arm,” allowing you to fit a long focal
length optical system into a much shorter tube. This is an important
benefit of compound optical systems like the popular Schmidt-Cassegrain.
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Magnification: magnification is determined
by dividing the focal length of the primary (or the effective
focal length) by the focal length of the eyepiece.
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Field-of-View: there are two ways
to consider field of view (FOV). The actual FOV is the
angular measurement of the patch of sky you can see in the eyepiece.
The apparent FOV is the angular measurement of the field
that your eye sees in the eyepiece. An actual field of view
might be ½ of a degree at low power, while the apparent field
might be 50 degrees. Another way to calculate magnification
is to divide the apparent FOV by the actual FOV. This results
in exactly the same number as the focal length method described
above. While apparent FOVs are readily obtained from the specs
of a given eyepiece, the actual FOV is harder to come by. Most
folks calculate the magnification based on focal length, and
then calculate the actual FOV by taking the apparent FOV and
dividing it by the magnification. For an apparent FOV of 50
degrees at 100X, the actual field is ½ degree (about the size
of the moon).
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Collimation: collimation refers to the
alignment of the overall optical system, making sure thing are
properly aligned, and the light is forming an ideal focus. Good
collimation is critical to getting good images in the eyepiece.
Different telescope designs have various strengths and weaknesses
with respect to collimation.
Types of Refractors
 You may wonder, “Why are there different types of refractors?”
The reason is because of an optical phenomena known as “chromatic
aberration”. Chromatic means color, and the aberration is due to
the fact that light, when passing through certain mediums like glass,
undergoes “dispersion.” Dispersion is a measure of how different
wavelengths of light are refracted by different amounts. The classic
effect of dispersion is the action of a prism or crystal creating
rainbows on the wall. As the different wavelengths of light are
refracted by different amounts, the (white) light spreads out, forming
the rainbow.
Unfortunately, this phenomena also affects lenses
in telescopes. The earliest telescopes, used by Galileo, Cassini,
and the like, were simple, single-element lens systems that suffered
from chromatic aberration. The problem is that blue light comes
to a focus at one location (distance from the primary), while red
light comes to a focus at a different location. The result is that
if you focus an object at the blue focus, it will have a red “halo”
around it. The only way known at the time to reduce this problem
is to make the focal length of the telescope very long, perhaps
F/30 or F/60. The telescope used by Cassini when he discovered Cassini’s
Division in Saturn’s rings was over 60 feet long!
In the 1700’s, Chester Moor Hall exploited the fact
that different types of glass have differing amounts of dispersion,
measured by their index of refraction. He combined two lens elements,
one of flint glass and another of crown, to create the first “achromatic”
lens. Achromatic means “without color”. By using two types of glass
with different indices of refraction, and having four surface curvatures
to manipulate, he produced a vast improvement in the optical performance
of refractors. They no longer had to be massively long instruments,
and subsequent developments over the centuries further refined the
technique and performance.
While the achromat greatly reduced false color in
the image, it did not eliminate it completely. The design can bring
the red and blue focal planes together, but the other colors of
the spectrum are still slightly out of focus. Now the problem is
purple/yellow halos. Again, making the f-ratio long (like F/15 or
so), helps dramatically. But that is still a long “slow” instrument.
Even a 3” F/15 achromat has a tube around 50” long.
In recent decades, scientists have created exotic
new types of glass that have extra-low dispersion. These glasses,
known collectively as “ED”, greatly reducing false color. Fluorite
(which is actually a crystal) has virtually no dispersion and is
used extensively in small to medium sized instruments, though at
very great cost. Finally, advanced optics employing three or more
elements are now available. These systems give the optical designer
more freedom, having 6 surfaces to manipulate, as well as possibly
three indices of refraction. The result is that more wavelengths
of light can be brought to the same focus, almost completely eliminating
false color. These groups of lens systems are known as “apochromats”,
which means, “without color, and we really mean it this time”. The
short hand for apochromatic lenses is “APO”. Refracting telescope
designs using APOs are now able to achieve low focal ratios (F/5
to F/8) with excellent optical performance and no false color; however,
be prepared to spend 5 to 10 times the amount of money that would
buy the same diameter achromat.
A final note about refractors is that they use a “closed-tube”
design, helping to minimize convection currents (which can degrade
images), and offering a system that rarely needs alignment. Unpack
it, set it up, and you’re ready to go.
Types of Reflectors
The main advantage of the reflecting telescope design
is that it does not suffer from false color – a mirror is intrinsically
achromatic. However, if you look at the above diagram for the reflector,
you will note that the focal plane is directly in front of the primary
mirror. If you place an eyepiece there (along with your head), it
will interfere with the incoming light.
The first useful design for a reflector, and still
most popular, was invented by Sir Isaac Newton, now called the “Newtonian”
reflector. Newton placed a small, flat mirror at a 45 degree angle
to deflect the light cone to the side of the optical tube, allowing
the eyepiece and the observer to remain outside the optical path.
The secondary diagonal mirror still interferes with the incoming
light, but only minimally.
Herchel constructed several large reflectors that
used the technique of “off-axis” focal planes, that is, diverting
the light cone from the primary to one side where the eyepiece and
observer could operate without interfering with the incoming light.
This technique works, but only for long f-ratios, as we will see
in a minute.
While the mirror conquered the color problem, it has
some interesting problems of its own. Focusing parallel rays of
light onto a focal plane requires a parabolic shape on the primary
mirror. It turns out that parabolas are rather difficult to generate,
compared to the ease of generating a sphere. Pure spherical optics
suffer from the phenomena of “spherical aberration”, basically,
a blurring of the images in the focal plane because they are not
parabolas. However, if the f-ratio of the system is sufficiently
long (more than about F/11), the difference between the shape of
the sphere and parabola is smaller than a fraction of the wavelength
of light. Herchel built long focal length instruments that could
take advantage of the ease of generating spheres, and use the off-axis
design for observing. Unfortunately, this meant his telescopes were
rather huge, and he spent many hours observing on a 40-foot ladder.
Several inventors created additional “compound” reflectors,
employing a secondary to pass the light back through a hole in the
primary mirror. Some of these types are the Gregorian, the Cassegrain,
the Dall-Kirkham, and the Ritchey-Cretchien. All of these are folded
optical systems, where the secondary plays an important role in
creating long effective focal lengths, and differ mainly in the
types of curvature employed on the primary and secondary. Some of
these designs are still favored for professional observatory instruments,
but very few are available commercially for the amateur astronomer
today.
The presence of a secondary mirror is an important
aspect of Newtonians, and indeed almost all reflector and catadioptric
designs. First, the secondary itself obstructs a small part of the
available aperture. Second, something must hold the secondary in
place. In pure reflecting designs, this is usually accomplished
with the use of thin vanes of metal in a cross, called a “spider”.
These are made as thin as possible to minimize obstruction. In catadioptric
designs, the secondary is mounted on the corrector plate, and there
is therefore no spider involved. The small loss of light-gathering
power in these designs is of almost no consequence since inch-for-inch,
reflectors are less expensive than refractors, and you can afford
to purchase a slightly larger instrument. However, an effect called
“diffraction” is more important than the light-gathering power concern.
Diffraction is caused when light passes near edges of things on
its way to the primary, causing them to bend and change direction
slightly. Additionally, secondaries and spiders cause scattered
light – light coming in from off-axis (i. e. , not part of the patch
of sky you are viewing), and bounce off the structures and into
and around the optical system. The result of diffraction and scattering
is a small loss of contrast – the background sky is not as “black”
as it would be in the same size refractor (of equal optical quality).
Not to worry – it takes a highly seasoned observer to even notice
the difference, and then it is only noticeable under ideal circumstances.
Types of Catadioptrics
One of the problems with pure reflecting optical
designs is spherical aberration, as noted above. The design goal
of catadioptrics is to take advantage of the ease of generating
spherical optics, but fix the problem of spherical aberration with
a corrector plate – a lens, subtly curved (and therefore generating
minimal chromatic aberration), to correct the problem. There are
two popular designs that achieve this goal:the Schmidt-Cassegrain,
and the Maksutov. Schmidt-Cassegrains (or “SCs”) are perhaps the
most popular type of compound telescope today. However, Russian
manufacturers have, in the past few years, made significant inroads
with various “Mak” designs, including folded optical systems and
a Newtonian variant – the “Mak-Newt”. The beauty of the folded Mak
design is that all the surfaces are spherical, and the secondary
is formed by merely aluminizing a spot on the back of the corrector.
It has a long effective focal length in a very small package, and
is a preferred design for planetary observing. The Mak-Newt can
achieve fairly fast focal ratios (F/5 or F/6) using spherical optics,
without the need for the (by-hand) optical figuring needed for parabolas.
Finally, both Mak designs result in closed-tubes, minimizing convection
currents and dust gathering on the primaries.
Types of Eyepieces
There are more eyepiece designs than there are telescope
designs. The most important thing to keep in mind is that the eyepiece
is half of your optical system. Some eyepieces cost as much
as a small telescope, and generally, they’re worth it. The past
two decades have witnessed the emergence of a variety of advanced
eyepiece designs, and there are many considerations to make in choosing
an appropriate design for your telescope, your uses, and your budget.
There are three major format standards for telescope
eyepieces:0. 956”, 1. 25”, and 2”. These refer to the eyepiece barrel
diameters, and the type of focuser they fit into. The smallest 0.
965” format is most commonly found on Asian-imported beginner telescopes
found in retail chains. These are generally of low quality, and
when it comes time to upgrade your system, you’re out of luck. Don’t
buy a department-store telescope!. The other two formats are
the preferred system in use today by the majority of amateur astronomers
worldwide. Most intermediate or advanced telescopes come with a
2” focuser and a simple adapter that also accepts 1. 25” eyepieces.
If you anticipate getting a modest size telescope and taking it
to dark skies to observe nebulae and clusters, you’re going to want
some of the better 2” eyepieces, and you should make sure you get
a 2” focuser.
Eyepieces are constructed of lenses, and thus we have
the same issue of chromatic aberration that we had in the case of
the refractor. Eyepiece design has evolved over the centuries in
step with the overall advancements of optics and glass. Modern eyepiece
designs use achromats (“doublets”) and more advanced designs (involving
“triplets” and more), along with ED glass to maximize their performance.
One of the original optical designs came from Christian
Huygens in the 1700’s that used two simple (non-achromatic) lenses.
Later, the Kellner employed a doublet and a simple lens. This design
is still popular in low cost, beginner telescopes. The Orthoscopic
was a popular design throughout the 1900’s, using dual doublets,
and is still favored by hard-core planetary observers.
In the past two decades, exploiting advances in glass,
optical design, and ray-tracing software, manufactures have introduced
a wide variety of new designs, most all of which try to maximize
the apparent field of view (which also increases the actual field
of view at a given magnification). Eyepieces before this were limited
to 45 or 50 degrees apparent FOV.
The first and foremost of these is the “Nagler” (designed
by Al Nagler of TeleVue), which is also dubbed the “Space-Walk”
eyepiece. It provides an apparent FOV of over 82 degrees, giving
the feeling of immersion. The FOV is actually larger than what your
eye can take in during any one glance. The result is that you must
actually “look around” to see everything in the field. Numerous
other manufactures have produced similar, very wide field eyepieces
in only the last five years varying from 60 degrees to 75 degrees
in apparent FOV. Many of these offer an excellent value, and produce
a far better experience for casual observers than the low-end designs
that come bundled with most beginner telescopes (where the feeling
is like looking through a wrapping paper tube).
A final consideration in eyepiece selection is “eye
relief”. Eye relief refers to the distance your eye must be from
the lens of the eyepiece to be able to see the entire apparent FOV.
One of the drawbacks of the designs such as the Kellner and Orthoscopic
is limited eye relief, sometimes as small as 5mm. This doesn’t usually
bother folks with normal eyesight, or those who are simply near-sighted
or far-sighted, because they can remove their glasses and use the
telescope to focus ideally for their vision. But for some people
with astigmatism, their glasses cannot be simply removed, and this
introduces the need to accommodate the extra distance required by
their glasses and still allow them to see the entire field. Typically,
eye relief of more than 16mm is adequate for most eyeglass wearers.
Many of the new, wide-field designs sport an eye relief of 20mm
or more. Again, the eyepiece is half of your optical system. Make
sure you match your eyepiece selection to the overall quality of
your optics, and to your needs as an individual observer.
Popular Telescope Designs
Achromatic refractors are popular in the F/9 to F/15
range, with apertures from 2” to 5” at reasonable cost. There are
several fast achromats (F/5) offered as “rich-field” telescopes
because they give wide fields of view at low power, ideal for sweeping
the Milky Way. These designs will show substantial false color on
the moon and bright planets, but this will not be noticeable on
deep-sky objects. To get both fast optics and no false color, you
must go with an APO design at considerable cost. APOs are available
from select manufactures (often with long waiting lists) in designs
from F/5 to F/8, in apertures from 70mm to 5” or 6”. The larger
ones are very expensive (more than $10,000) and are the domain of
the true fanatics in the hobby.
The popular Newtonian designs range from Richfield
4. 5” F/4’s to the classic 6” F/8, probably the most popular entry-level
telescope. Larger reflectors (8” F/6, 10” F/5, and so on) are gaining
wide popularity because of the low cost and portability of the “Dobsonian”
mount (more on that later) and increasing availability from numerous
manufacturers, including kit offerings. Large Newtonians tend to
have faster f-ratios to keep the tube length under control. Mak-Newts
are mostly found in the F/6 range.
The Schmidt-Cassegrain is probably the most popular
design with more advanced amateurs – the venerable 8” F/10 SC has
been a classic for 3 decades. Most SCs are F/10, although some F/6.
3’s are on the market. The trouble with fast SCs is that the secondary
needs to be significantly larger, obstructing 30% or more. Overall,
the F/10 design is ideal for a general mix of deep-sky observing
as well as planetary and lunar.
The up-and-coming Maksutovs are generally in the F/10
to F/15 range, making them somewhat slow optical systems that tend
not to be ideal for expansive Milky Way and deep sky viewing. However,
they are ideal systems for planetary and lunar observing, rivaling
far more expensive APOs of the same aperture.
Mounts
The telescope mount is definitely as important, if
not more important, than the optical system. The best optics are
worthless unless you can hold them steady, point them accurately,
and make fine adjustment in the pointing without undo vibrations
or backlash. There are a variety of mount designs, some optimized
for portability, with others optimized for motorized/computerized
tracking. There are two basic categories of mount designs:alti-azimuth,
and equatorial.
Alti-Azimuth
Alti-azimuth mounts have two axes of motion:up-and-down
(alti), and side-to-side (azimuth). A typical camera tripod head
is a kind of alti-azimuth mount. Many small refractors on the market
employ this design, and it has advantages of being convenient for
terrestrial viewing as well as sky viewing. Perhaps the most important
alti-azimuth mount is the “Dobsonian”, almost exclusively used for
medium to large Newtonian reflectors.
John Dobson is a legendary figure in the San Francisco
Sidewalk Astronomer community. Twenty years ago, John was seeking
a telescope design that was highly portable, and offered the ability
to bring fairly large instruments (12” to 20” apertures) out to
the public, literally on the sidewalks of San Francisco. His design
and construction techniques created a revolution in amateur astronomy.
“Big Dobs” are now one of the most popular telescope designs seen
at star parties all over the world. Most telescope vendors today
offer a line of Dobsonian designs. Before this, even an 10” reflector
on an equatorial mount was considered an “observatory” instrument
– you wouldn’t generally move it around due to the heavy mount.
Generally, alti-azimuth designs are smaller and lighter
than equatorial mounts offering the same level of stability. However,
to track objects as the Earth rotates requires motion on two axes
of the mount instead of just one as for equatorial designs. With
the advent of computer control, many vendors now offer alti-azimuth
mounts that can track the stars, with some caveats. A 2-axis mount
suffers from “field rotation” over long periods of tracking, meaning
that this design is not suitable for astrophotography.
Equatorial
Equatorial mounts also have two axes, but one of the
axes (the “polar” axis) is aligned with the axis of rotation of
the Earth. The other axis is called the “declination” axis, and
is at right angles to the polar axis. The key benefit of this approach
is that the mount can track objects in the sky by rotating only
the polar axis, simplifying tracking, and avoiding the problem of
field rotation. Equatorial mounts are fairly mandatory for astrophotography
and imaging efforts. Equatorial mounts must also be “aligned” to
the Earth’s polar axis when they are set up, making their use somewhat
less convenient than alti-azimuth designs.
There are several types of equatorial mounts:
German Equatorial: the most popular design
for small to medium sized scopes, offering great stability, but
requiring counterweights to balance the telescope around the polar
axis.
Fork mounts: popular design for Schmidt-Cassegrains,
with the base of the fork being the polar axis, and the arms of
the fork being declination. No counterweights are needed. Fork designs
can work well, but are usually large compared with the telescope;
small fork designs suffer from vibration and flexure. Fork designs
have difficulty pointing near the north celestial pole.
Yoke mounts: similar to the fork design, but
the forks continue past the telescope, and join together above the
telescope in a second polar bearing, offering improved stability
over the fork, but resulting in a fairly massive structure. Yoke
designs were used in many of the world’s great observatories in
the 1800’s and 1900’s.
Horseshoe mounts: a variant of the Yoke mount,
but employing a very large polar bearing with a U-shaped opening
at the top end, allowing the telescope tube to point to the north
celestial pole. This is the design used on the Hale 200” telescope
at Mt. Palomar.
Key Considerations for Mounts
A stated, the telescope’s mount is a critical part
of the overall system. When choosing a telescope, mounting considerations
play an important role in your ability and willingness to use it,
and ultimately governs the types of activities you can undertake
(such as astrophotography). Below are some of the key considerations
you should make.
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Portability: assuming you don’t have a
backyard observatory, you will be moving and transporting your
telescope out to an observing site. If you have dark skies with
minimal light-pollution where you live, this may only mean moving
the telescope from the closet or garage into the back yard.
If you have substantial light pollution, you will want to take
your scope to a dark-sky site, preferably on a mountain top
somewhere. This implies transporting the scope in your car.
A large, heavy mount can make this a chore. Furthermore, if
astrophotography is not a main consideration, the task of setting
up and aligning an equatorial mount might not be worth the effort.
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Stability: the mount’s stability is measured
by the amount of vibration that the telescope experiences when
“nudged”, when focusing, changing eyepieces, or when a slight
breeze blows. The time it takes these vibrations to dampen out
should be around 1 second or so. Dobsonian mounts generally
have excellent stability. German equatorials and fork mounts,
when properly sized to the telescope, also exhibit good stability,
although they tend to weigh more than the telescope itself by
a significant margin.
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Pointing and Tracking: to really enjoy
observing, the telescope must be easy to point and aim, and
the mount should allow you to carefully track the object you
are observing, either by nudging the telescope, by using manual
slow-motion controls, or with a tracking motor (a “clock drive”).
The higher the magnification you are using (such as for planetary
observations or splitting double stars), the more critical the
tracking behavior of the mount. Backlash is one good measure
of the mount’s tracking ability:when you nudge or move the instrument
a slight bit, does it stay where you aimed it, or does it move
back slightly? Backlash can be a frustrating behavior of a mount,
and usually means the mount is either poorly manufactured, or
is too small for the telescope you have mounted.
It is difficult to get a feel for mount behavior from
a catalog or web site. If you can, go to a telescope store (there
aren’t very many) or a high-end camera dealership that carries major-brand
telescopes for a touch-and-feel evaluation. Additionally, there
are many resources, message boards, and reviews of equipment available
on the Web and in astronomy magazines. Perhaps the best form of
research is to attend a local star party held by your neighborhood
astronomy club where you can see a variety of telescopes, talk to
their owners, and have the opportunity to observe through them.
Help in locating these resources is provided in a later section.
Finder Scopes
Finder scopes are small telescopes or pointing devices
affixed to the main tube of your telescope to aid in locating objects
that are too faint to see with the naked eye (i. e. , almost all
of them). The field of view of your telescope is generally quite
small, about one or two diameters of the moon, depending on your
eyepiece and magnification. Generally, you use a low-power, wide-field
eyepiece first to locate an object (even bright ones), then change
eyepieces to higher magnifications as appropriate for the given
object.
Historically, finder scopes were always small refracting
telescopes, similar to a binocular, offering a wide field of view
(5 degrees or so) at low power (5X or 8X). In the past decade, a
new approach to pointing arose using LEDs to create “red-dot finders”
or illuminated reticle projection systems that project a dot or
grid onto the sky at no magnification. This approach is very popular
because it overcomes several use-difficulties of traditional finder
scopes.
Traditional finder scopes are difficult to use for
two main reasons:the image in the finder scope is inverted, making
it hard to correlate the naked-eye view of the star pattern with
what is seen in the finder, and also making it hard to make adjustments
left/right/up/down. Additionally, getting your eye to the eyepiece
of the finder can be challenging at times since it is fairly close
to the main telescope tube, and in many orientations, may have you
straining your neck in awkward positions. While it is true that
with practice, the orientation problem can be mitigated, and it
is also possible to purchase correct-image finder scopes (at increased
cost), the jury of the astronomical community has clearly spoken
– projection finders are easier to use and much less expensive.
Filters
The last part of the optical system to understand
is the use of filters. There are a wide variety of filter types
used for various observing needs. Filters are small disks mounted
in aluminum cells that thread into the standard eyepiece formats
(another reason for getting 1. 25” and 2” eyepiece, and not
a department store telescope!). Filters fall into these main categories:
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Color Filters: red, yellow, blue, and green
filters are useful for bringing out detail and features on planets
such as Mars, Jupiter, and Saturn.
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Neutral-Density Filters: most useful for
lunar observing. The moon is really bright, especially
when your eyes are dark-adapted. A typical neutral-density filter
cuts out 70% of the moon’s light, allowing you to see details
of craters and mountain ranges with less eye discomfort.
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Light-Pollution Filters: light pollution
is a pervasive problem, but there are ways to mitigate its effect
on your observing enjoyment. Some communities mandate Mercury-Sodium
vapor streetlights (especially near professional observatories)
because these types of lights emit light at only one or two
discreet wavelengths of light. Thus, it is easy to manufacture
a filter that eliminates only those wavelengths, and allows
the rest of the light to pass through to your retina. More generally,
both wide-band and narrow-band light-pollution filters are available
from major vendors that help substantially in the general case
of a light-polluted metro area.
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Nebula Filters: if your focus is on deep-sky
objects and nebula, other types of filters are available that
enhance the specific emission lines of these objects. Most famous
is the OIII (Oxygen-3) filter available from Lumicon. This filter
eliminates almost all the light at wavelengths other than the
Oxygen emission lines generated by many interstellar nebulae.
The Great Nebula in Orion (M42) and the Veil Nebula in Cygnus
take on an entirely new aspect when viewed through an OIII filter.
Other filters in this category include the H-beta filter (ideal
for the Horsehead nebula), and various other more general-purpose
“Deep Sky” filters that enhance contrast and bring out faint
detail in many objects, including globular clusters, planetary
nebula, and galaxies.
Observing
How to Observe: The most important aspect
of a quality observing session is dark skies. Once you have
experienced truly dark-sky observing, seeing the Milky Way appear
as storm clouds (until you look closely) you will never again complain
about loading up the vehicle and driving perhaps one or two hours
to get to a good site. The planets and moon can generally be observed
successfully from almost anywhere, but the majority of sky gems
require excellent observing conditions.
Even if you are only concentrating on the moon and
planets, your telescope must be set up in a dark location to minimize
stray, reflected light getting into your telescope. Avoid streetlights,
neighbor’s halogens, and shut off all the outdoor/indoor lights
you can.
Importantly, consider the dark-adaption of your own
eyes. Visual purple, a chemical responsible for increasing the acuity
of your eyes in low-light conditions, takes 15-30 minutes to develop,
but can be eliminated immediately by one good dose of bright light.
That means another 15-30 minutes of adaption time. Besides avoiding
bright lights, astronomers use flashlights with deep red filters
to help navigate their surroundings, view start charts, check their
mount, change eyepieces, and so on. Red light does not destroy visual
purple like white light does. Many vendors sell red-light flashlights
for observing, but a simple piece of red cellophane over a small
flashlight works just fine.
In the absence of a computer-pointed telescope (and
even if you have one), obtain a quality star chart and learn the
constellations. This will make it abundantly clear which objects
are planets, and which are merely bright stars. It will also increase
your ability to locate interesting objects using the “star hopping”
method. For example, the supernova remnant known as the Crab Nebula
is just a smidgen away to the north from the left horn of Taurus
the Bull. Knowing the constellations is the key to unlocking the
vast array of wonders available to you and your telescope.
Finally, become familiar with the technique of using
“averted vision”. The human retina is composed of differing sensors
called cones and rods. The center of your vision, the fovea, is
mainly composed of cones that are most sensitive to bright, colored
light. The periphery of your vision is dominated by rods, which
are more sensitive to low light levels, with less color discrimination.
Averted vision concentrates the light from the eyepiece onto the
more sensitive part of your retina, and results in an ability to
discern fainter objects and greater detail.
What to Observe: A thorough treatment of the
types and locations of the objects in the sky is far beyond the
scope of this article. However, a brief introduction will be helpful
in navigating the various resources that will help you find these
spectacular objects.
The moon and planets are fairly obvious objects, once
you know the constellations and begin to understand the movement
of the planets in the “ecliptic” (the plane of our Solar system),
and the progression of the sky as the seasons pass by. More difficult
are the thousands of deep-sky objects – clusters, nebula, galaxies,
and so on.
In the 1700’s and 1800’, a comet hunter named Charles
Messier spent night after night searching the skies for new comets.
He kept running into faint smudges that did not move from night
to night, and so were not comets. For convenience, and to avoid
confusion, he constructed a catalog of these faint smudges. While
he did discover a handful of comets during his life, he is now famous
and best remembered for his catalog of over 100 deep sky objects.
These objects now bear their most-used designation stemming from
the Messier catalog. “M1” is the Crab Nebula, “M42” is the great
Orion nebula, “M31” is the Andromeda galaxy, etc. Finder cards and
books on the Messier objects are available from many publishers,
and are highly recommended if you have a modest telescope and dark
sky availability. Additionally, a new “Caldwell” catalog gathers
another 100 or so objects that are of similar brightness to the
M-objects, but were overlooked by Messier. These are ideal starting
places for the beginning deep-sky observer.
In the early half of the 20th century,
professional astronomers constructed the New Galactic Catalog, or
“NGC”. There are approximately 10,000 objects in this catalog, the
vast majority of which are accessible by modest amateur telescopes
in dark skies. There are several observing guides emphasizing the
most spectacular of these, and a high-quality star chart will show
thousands of NGC objects.
When you understand the vast array of objects up there,
from the galaxy clusters in Coma Berenices and Leo, to the emission
nebula in Sagittarius, to the range of globular clusters (like the
amazing M13 in Hercules) and planetary nebula (like M57, “the Ring
Nebula” in Lyra), you’ll begin to realize that every patch of sky
contains marvelous sights, if you know how to find them.
Imaging
Like the observing section, a treatment of imaging,
astrophotography, and video-astronomy is far beyond the scope of
this article. However, it is important to understand some of the
basics in this area to help you make an informed decision about
which type of telescope and mounting system is right for you.
The simplest form of astrophotography is to capture
“star trails”. Set a camera with a typical lens on a tripod, point
it at a star field, and expose the film for 10 to 100 minutes. As
the earth rotates, the stars leave “trails” on the film depicting
the rotation of the sky. These can be very beautiful in color, and
especially if pointed toward Polaris (the “north star”) showing
how the entire sky rotates around it.
There are now several types of approaches to imaging
astronomical objects, thanks to the advent of CCDs, digital cameras
and camcorders, and continuing advances in film techniques. In any
of these cases, an equatorial mount is required for accurate tracking.
In fact, the best astrophotos taken today employ an equatorial mount
several times more massive and stable than would be required for
simple visual observing. This approach relates to the need for stability,
breeze-resistance, tracking accuracy, and minimized vibrations.
Typically, good astro-imaging also requires some kind of guiding
mechanism, often meaning the use of a second guide scope on the
same mount. Even if your mount has a clock drive, it is not perfect.
Continual corrections are required during a long exposure to make
sure the object stays in the center of the field, to an accuracy
that is near the resolution limit of the telescope being used. There
are both manual guiding approaches and CCD “auto-guiders” that come
into play in this scenario. For film approaches, “long exposure”
can mean 10 minutes to more than an hour. Excellent guiding is needed
during the entire exposure. This is not for the faint-hearted.
Piggy-back photography is substantially easier, and
can give excellent results. The idea is to mount a normal camera
with a medium or wide-field lens on the back of a telescope. You
use the telescope (with a special illuminated reticle guiding eyepiece)
to track a “guide star” in the field. Meanwhile, the camera takes
a 5 to 15 minute exposure of a large patch of sky at a fast setting,
F/4 or better. This approach is ideal for vista shots of the Milky
Way or other star fields.
Below are a few images taken with a 35mm Olympus OM-1
(still a preferred camera among astrophotographers) with exposures
ranging from 25 minutes to 80 minutes on fairly standard Fuji ASA
400 film.
More advanced imaging techniques include hyper-sensitizing
film to increase its sensitivity to light, using sophisticated astro-CCD
cameras and auto-guiders, and performing a wide variety of post-processing
techniques (such as “stacking” and “mosaic alignment”) on digital
images.
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Upper Left:M42, The Great Nebula in
Orion; Upper Right, Sagittarius Star Field (piggy back); Lower
Left:the Pleiades and reflection nebula; Lower Right, M8,
the Lagoon Nebula in Sagittarius.
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If you like imaging, are a technophile, and have patience,
the field of astro-imaging may be for you. Many amateur imagers
today produce results that rival the achievements of professional
observatories only a few decades ago.
Manufacturers
With the recent rise of popularity of astronomy, there
are now more telescope manufacturers and retailers than ever before.
The best way to find out who they are is by going down to your local,
high-quality magazine rack and picking up a copy of Sky and Telescope
or Astronomy magazines. From there, the Web will help you get more
detail on their offerings.
There are two major manufacturers that have dominated
the marketplace for the past two decades: Meade Instruments and
Celestron. Each has a several lines of telescope offerings in the
refractor, Dobsonian, and Schmidt-Cassegrain design categories,
along with other specialty designs. Each also has comprehensive
eyepiece sets, electronics options, photo and CCD accessories, and
much more. See www.celestron.com,
and www.meade.com. Both operate
through dealer networks, and pricing is set by the manufacturer.
Don’t expect to bargain or get a special deal other than close-outs
and seconds.
Close on the heels of the big two is Orion Telescopes
and Binoculars. They import and re-brand several lines of telescopes,
along with reselling selected other brands. The Orion web site (www.telescope.com) is full of information
about how telescopes work, and which type of telescope is right
for your needs and budget. Orion is probably the best source for
a broad selection of quality, entry-level telescopes. It is also
a great source of accessories, such as eyepieces, filters, cases,
star atlases, mounting accessories, and more. Sign up for the catalog
on their web site – it too is full of useful, general-purpose information.
Televue (www.televue.com)
is a purveyor of very high quality refractors (APOs) and premium
eyepieces (Naglers and Panoptics). Takahashi (www.takahashiamerica.com)
produces world-renown fluorite APO refractors. In America, Astro-Physics
(www.astro-physics.com)
has produced perhaps the highest quality, most sought-after APO
refractors; they typically have a 2 year waiting list, and their
telescopes have actually appreciated in value on the used market
over the past decade.
Obsession Telescopes was the first, and still most
highly rated, producer of premium large Dobsonians. Sizes
range from 15” to 25”. Be prepared to get a trailer to move one
of these telescopes to dark skies. See www.obsessiontelescopes.com.
Resources
The Web is full of astronomical resources, from manufacturer’s
web sites to publishers, classifieds, and message forums. Many individual
astronomers maintain sites showing their astrophotography, observing
reports, equipment tips and techniques, etc. A comprehensive listing
would be many pages. The best bet is to start with Google, and search
on a variety of terms, such as “telescope observing techniques”,
“telescope reviews”, “amateur telescope making”, etc. Also search
on “astronomy clubs” to find one in your area.
Two sites are worth mentioning explicitly. The first
is the Sky & Telescope web site (www.skyandtelescope.com)
which is full of great information about observing generally, what’s
up in the sky right now, and past equipment reviews. The second
is Astromart (www.astromart.com),
a classifieds site dedicated to astronomy equipment. High quality
telescopes don’t really wear out or have many problems due to use,
and they are usually meticulously cared for. You might want to consider
obtaining a used instrument, especially if the seller is in your
area and you can check it out in person. This approach also works
well for obtaining accessories like eyepieces, filters, cases, etc.
Astromart also has discussion forums where the latest chatter on
equipment and techniques is abundant.
Books
There are thousands of books on astronomy. Below are
some recommendations suitable for folks new to the field. Click
on the links to read more about them at Amazon. By the way, Amazon
is also an authorized dealer of both Meade and Celestron telescope
lines.
Telescopes
Here are some good telescopes to start with:
A note on light pollution
If you are interested in the fight against light pollution,
or want to know what you can do in your local community to improve
the situation, visit the International Dark Sky Association (IDA)
at www.darksky.org.
Next?
If you haven’t already done so, get out there and
do some observing with friends or a local astronomy club. Amateur
astronomers are a gregarious bunch, and given the chance, will generally
tell you more about any given topic than you can possibly absorb
in one sitting. Next, inform yourself with magazine sources, web
searches and sites, and a visit to the book store. If you find you
really have the bug, then decide your parameters and constraints
to narrow down your telescope choices in terms of size, design,
and budget. If that is all too much work, and you just want to get
a telescope yesterday, then go to Orion and buy the venerable 6”
F/8 Dobsonian.
Happy Star Trails!
Copyright (c) 2004 by Chuck Fuller
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