The first thing
that a salesman will hawk will be "power." This will be
represented by a large number, such as "200," and followed by an
"X." This indicates that when you marry an eyepiece to its
holder (at the other end of the tube from the "main
objective"--the big lens at the front or the light-gathering mirror in
the back), you will get that much magnification...everything
will be 200 times larger in the "field of view." That sounds
wonderful, and very powerful, but unless you have the resources of Mount
Palomar or the Kit Peak Observatory, you may be quite disappointed in the
In order to get a
highly magnified image, the lens has to focus in, much like a magnifying
glass does. For example, if you look at your hand, you will see not
only your hand, but your fingers, your arm, and everything around your
hand. Then, if you take a strong magnifying glass and use it to look at
your hand, you will lose all imaging except for a very small area that shows
either ridges and whorls, or a cracked landscape with branchless trees.
magnification works much the same way, except that the objects that you are
observing are much more distant, and often suffer from atmospheric
distortion. When you use high magnification, the distortion is also
magnified. That is why "high powered lenses" are not the
best selling point for a telescope, although that is
the only thing a salesman is likely to know about the merchandise.
A smart shopper will
check to find out which good "low power" eyepieces come with the
telescope. It is much easier to find something if you have a larger
"field of view." You can always zoom in later, after you have
the image centered. Low powered eyepieces give you enough sky to work
with to find your way from star... to star... to desired object. They
also give you a crisper, clearer image, because distortions are not magnified
as much as they will be with a high powered lens.
analogies....if someone told you that they thought they felt something sting
their back and wanted you to find the site, you could easily "eyeball
scan" their back for a tiny red dot. However, if they gave you a
strong microscope lens, and you had to scan using only the
lens--one tiny portion of skin at a time, you can see how frustrated
you might get looking over an entire back for a tiny red speck. That is
how you might feel if you depend only on all that "power" hawked by
the salesman. First, you'd want to "eyeball" the area until
you found the spot, and then you would get out your
magnifier. That is why you will check for a good set of low powered
eyepieces...they will allow you to "get there." If you want
to magnify further, you may then do so.
A series of good
low power eyepieces to choose from includes: 25mm, 30 mm, 32mm, 35mm
or 40 mm focal length with a 1 1/4" barrel. Kellner
is the least expensive, and is acceptable in performance. Erfles and Plossls cost
considerably more, but offer even wider fields of view. Do not get
Huygens or Ramsden...they have a very narrow field
of view, and irritating color distortions.
A series of good
mid-power eyepieces to choose from includes: 18 mm, 16 mm, 15 mm, 13
mm, and 12 mm. The lower the number, the higher the magnification you
will get. Orthoscopic is the best mid-range eyepiece obtainable at a
reasonable price. Plossl and TeleVue designs are more expensive, but their performance
matches their price.
High power eyepieces
to choose from include: 10.5 mm, 10 mm, 9 mm, 8 mm, 7 mm, 6 mm, 5 mm, 4
mm, or 3.5 mm. Rarely will atmospheric conditions be stable enough to
push for the highest magnification (3.5 - 5). Orthoscopic, again
has the best high power eyepiece obtainable at a reasonable price. Plossl and TeleVue designs are
more expensive, but again, their performance matches their price.
Field of View
of view" is the amount of area that you can see through any given
eyepiece. The field of view varies with different eyepieces...low to
medium power eyepieces give you a broader field of view than high power
The Huygens and Ramsden eyepieces have the narrowest field of view, and
are not recommended for use. They are quite inexpensive, but you will
be disappointed in the performance and your 'scope will quickly end up in a
yard sale where experienced amateurs will pick it up for a song and add some
Kellner and Orthoscopic eyepieces have moderate fields of
view, and very good color correction--they yield true colors.
The Erfle has a very wide field of view, but has some
distortion at the outside edges of the field of view.
Plossl and TeleVue offer wide
fields of view without the edge distortion.
Resolution is the
ability to detect fine detail. The larger the objective (large
lens or light-gathering mirror), the better its ability to
resolve detail. This is very important with high magnification viewing
of the moon, the planets, open or globular clusters, and double stars.
This is the
proper alignment of the optics in any optical device. With refractors,
this is often fixed, but in the various forms of reflector, the mirrors may
at times need adjustment in order to line up in order to properly focus the
light. This is critical to good telescope performance. The more
often you move a reflector telescope, the more frequently you will have to
The amount of
light that an objective collects is proportional to its surface area, or the square
of its radius. This is why larger telescopes allow you to view much
Right Ascension & Declination
If you consider
it as terrestrial latitude and longitude extended into space, declination
corresponds to latitude (north/south axis),
while right ascension corresponds to longitude (east/west
axis). Many telescopes have "setting
circles" with the degrees measured off in order for you to find an
object by its celestial coordinates. Like latitude, declination is
measured in degrees north or south of the equator. Right ascension,
however, is a 24-hour clock face with 0 (zero hours) automatically set for
the vernal equinox (the point that the sun crosses the
equator as it heads north to begin spring). In that
sense, right ascension is similar to the 24-hour time zones that cross the
Back to Section
Eyepieces and Accessory Lenses--Types & Functions
ACHROMATIC or ACHROMATIC
This is a double lens used as a refractor objective to give better
This is a negative lens that you place in the eyepiece holder.
It holds the eyepiece. Barlow lenses extend the focal length (the
distance between the lens or mirror, and where it focuses light to a
point--the greater the focal length, the higher the magnification)
of the main objective, which, in effect, causes the eyepiece to yield a
higher magnification...generally 2-3 times greater than the original power of
ERFLE LENS (ur' full)
This is an expensive wide-angle lens that is very complex....it is a
five-element lens (it uses five different pieces of glass).
It gives you a very wide field of view, but it does have some distortion at
the edges of the field of view. This is your best low power lens.
The wide field of view makes it very easy for you to locate objects.
HUYGENS LENS (hi' gens)
This is a two-element lens that has a very narrow field of view,
making it difficult to find any object in any magnification range. It
also suffers from color distortion. This cheap eyepiece comes as
standard equipment on most Japanese telescopes (Tasco/Jason/Swift),
and should be discarded as soon as possible. Get a Kellner
lens instead, if you want to actually use the telescope for viewing
anything. Telescopes used as decorative items, however, may keep the
If you can't afford an Erfle, this is the
best value for your money...in low and medium power lenses. It is a
three-element lens (three pieces of glass) that
gives a flatter field of view than the Erfle (very
little distortion at the edges), but not as wide.
A four-element lens, this lens gives very good images at high power (9mm;
6mm; 4mm--smaller focal length=higher power/magnification in eyepieces),
although you can get low power Orthoscopic lenses (30mm to 20mm), and medium
power (18mm-12mm). The low and medium powered Orthos
are not superior to the lower priced Kellners, but
the high power Ortho lenses are the cream of the crop in high power lenses.
PLOSSL LENS (just like it looks...ploss' ul)
This is a four-element eyepiece that has a wide, flat field of
view. It is a good general purpose eyepiece in all magnifications...more
expensive than a Kellner but less expensive than Orthoscopics and Erfles.
It doesn't have as wide a field of view as the Erfle,
but they do not have edge or color distortions, either.
This is the other lens that comes with your Japanese low cost
telescopes (Jason/Tasco/Swift). It is the
high power lens that has a very narrow field of view and produces a very dim
image. This lens is probably the main reason you can find soooo many telescopes at yard sales. Also known as
the SR-4, this lens will frustrate practiced observers and drive novices into
easier pastimes such as calculus. Get a good high powered Kellner or, ideally, an Orthoscopic lens instead.
The main objective of the Japanese 'scopes are fine. The problem is
with the eyepieces.
STAR DIAGONAL (link to
construction of a star-diagonal)
This accessory for refractors (in any form)
and Cassegrain reflectors allows the observer to
look through the eyepiece without straining his neck or getting down on his
knees. The image will be erect instead of being inverted, as usual, but
the image is a mirror image with left and right
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Types of Telescopes
The refractor takes in light through the main objective lens (the
large lens at the front), and delivers it straight to the
eyepiece. The old ship's spyglass is a good example of this type of
Light enters the tube and is collected by a mirror which reflects it
back to a focuser via a secondary mirror....the eyepiece is set on the side
instead of directly at the end of the tube.
SCHMIDT-CASSEGRAIN (shmit' - cass' uh grin)
This is a folded version of telescope that combines the best of a
refractor with a reflector....the short, folded optical design focuses light
through a hole in the main mirror. The eyepiece is placed at the end
just like the refractor.
MAKSUTOV (mack' suitoff)
Like the Schmidt-Cassegrain, this is a
folded-optical design, but with even better images. It is the most
expensive telescope that you can buy, but worth it if you are a rabid amateur
or professional astronomer.
Back to Section
ALT-AZIMUTH MOUNT (alt- az' u muth)
This simple mount, such as a camera tripod, moves vertically and
horizontally with manual control. It comes on the least expensive
telescopes, but it requires constant adjustments of both axes to track
objects in the sky as the Earth rotates. For astronomy, therefore, the
equatorial mount is far superior.
A drive mechanism is an electrical driver that, once your equatorial
mount is aligned on Polaris, keeps your telescope aligned by physically
turning an equatorial mount's polar axis westward to match Earth's
rotation. There are controls for speed (+ & -), so you can
set the telescope for accurate tracking before you begin an observing or
photo session. You must have either an electrical outlet nearby, or an
auto adapter to use this equipment. Some amateurs carry small
generators with them to the field so that they can hook up.
This is a form of tripod for mounting the telescope. It has a
polar axis, which must be aligned to Polaris (the North Star), and
a SLOW MOTION CONTROL GEAR (a turnbuckle of some sort)
that allows you to gradually move the telescope westward in order to track an
object. As you have observed the sun, moon, planets and stars rising
and setting, they do so ever much faster when magnified in the lens of a
telescope. By tracking, you can keep an image in the eyepiece long
enough for observation. Often, the declination axis has a similar slow
motion control so that you can make minute north/south adjustments. (Slow
motion controls make telescopes much easier to use).
The finder scope is the miniature telescope often found attached to
the body of a larger telescope. The eyepiece often has
crosshairs. The purpose of this 'scope is to help you: 1) focus
on a particular image (it is much easier to eyeball the image as
well as getting it in the finder scope than trying to find the image through
a telescope eyepiece with the bulk of the telescope blocking your view of the
object), and 2) as a guide during astrophoto
sessions which allows you to position a "guide star" in a certain
relationship to the crosshairs, which position you then maintain--either by
clock drive or manual guidance.
SIDEREAL DRIVE (sid ear' ee ul)
The sidereal drive is a clock drive that is set to the actual motion
of the stars, rather than the rotation of the Earth. It is set for 360
degrees (normally, our 24-hour day) to be complete
every 23 hours and 56 minutes.
On a large equatorial mount tripod, this is the actual cradle that
holds the telescope body. It can be set to point the polar axis at
Polaris, and thus match the observer's latitude. As long as you stay in
the same place, you do not have to change it. If you move north or
south, it must be adjusted accordingly.
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These are simple color filters used for different effects in
observation or photography. Red allows you to view more clearly
through haze. Blue tends to show cloud formations on Mars and Venus
This cuts down on the brightness of the full moon in order for you to
observe contrasts better.
Tuned to particular atoms, this filter cuts down on sky pollution
while still allowing the bright emission lines (the spectrum of
just those atoms, such as hydrogen and oxygen) to show through.
SOLAR FILTERS--All should be placed in front of the telescope main
objective...NOT at the eyepiece!
These filters cut the sun's light down about 10,000 times to safe
MYLAR FILTERS give you a bluish solar image. This is
usually purchased in the form of a flexible film.
ICONEL FILTERS give you a nice, orangey solar image...beats
blue images, but is very expensive. This is a hard filter composed of a
thin metal film on glass.
THOUSAND OAKS POLYMER
PLUS gives you an
orange solar image. This is usually purchased in the form of a flexible
film similar to Baader Solar Filter Material, but
BAADER SOLAR FILTER
MATERIAL give you a white solar image
with a black, contrasting background. This flexible film is the best
solar filter material now in production. It comes in Neutral Density-3
for solar photography, or Neutral Density-5 for
visual observation. Draco
Productions only sells ND-5, which is not only safe for visual
observation, but which is also excellent for using as a filter for solar
photography when cells are constructed for your telephoto lens or telescope.
Baader material gives you
the best resolution and best contrast (according to the latest NASA tests) of
any of the currently available solar filters.
Good for daytime observation, when properly turned, it makes the
half-moon stand out against a much darker blue contrasting sky.
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