13
lem that plagues refractor-type telescopes, since light passes
through both air and glass to form an image. Most astronomical
objects emit a spectrum comprised of many different wave-
lengths of light, so each wavelength will be bent by a slightly
different amount when passing through a lens. This results in
each color of light reaching precise focus at a slightly different
point, which will provide unacceptable images.
Achromatic refractors, like the AstroView 120mm, are
designed to minimize chromatic abberation to acceptable
levels. The objective lens is actually comprised of two individ-
ual lenses, called elements, made of different materials,
which bend light in slightly different ways. By precisely spac-
ing and shaping the elements, the chromatic abberation
incurred when light passes through air and the first glass ele-
ment is reduced by the way the second element bends the
light. The result is an image that is much better color correct-
ed than a non-achromatic (one element) objective lens.
Even with the achromatic lens design, however, the AstroView
120 will suffer a bit from chromatic abberation due to its large
aperture. This will be noticeable, to some degree, on extreme-
ly bright objects, such as the brightest stars in the sky. What
you will notice is that the object, when focused, has a slight
“purple-halo” around it. This will not present a problem for most
observers, as the eye readily adapts to the view and is still
able to distinguish fine details. Chromatic abberation will never
inhibit deep sky observing, as deep sky objects are too faint to
cause any noticeable color distortion.
How to Find Interesting Celestial Objects
To locate celestial objects with your telescope, you first need
to become reasonably familiar with the night sky. Unless you
know how to recognize the constellation Orion, for instance,
you won’t have much luck locating the Orion Nebula, unless,
or course, you look up its celestial coordinates and use the
telescope’s setting circles. Even then, it would be good to
know in advance whether that constellation will be above the
horizon at the time you plan to observe. A simple planisphere,
or star wheel, can be a valuable tool both for learning the con-
stellations and for determining which ones are visible on a
given night at a given time.
A good star chart or atlas will come in very handy for helping
find objects among the dizzying multitude of stars overhead.
Except for the Moon and the brighter planets, it’s pretty time-
consuming and frustrating to hunt for objects randomly,
without knowing where to look. You should have specific tar-
gets in mind before you begin observing.
Start with a basic star atlas, one that shows stars no fainter
than 5th or 6th magnitude. In addition to stars, the atlas will
show the positions of a number of interesting deep-sky
objects, with different symbols representing the different types
of objects, such as galaxies, open star clusters, globular clus-
ters, diffuse nebulas, and planetary nebulas. So, for example,
your atlas might show a globular cluster sitting just above the
lid of the “Teapot’ pattern of stars in Sagittarius. You then know
to point your telescope in that direction to home in on the clus-
ter, which happens to be 6.9-magnitude Messier 28 (M28).
You can see a great number and variety of astronomical
objects with your AstroView 120mm, including:
A. The Moon
With its rocky, cratered surface, the Moon is one of the easi-
est and most interesting targets to view with your telescope.
The best time to observe our only natural satellite is during a
partial phase, that is, when the Moon is NOT full. During par-
tial phases, shadows on the surface reveal more detail,
especially right along the border between the dark and light
portions of the disk (called the “terminator”). A full Moon is too
bright and devoid of surface shadows to yield a pleasing view.
Try using a Moon Filter (Orion part #5662) to dim the Moon
when it is very bright. It simply threads onto the bottom of the
eyepieces (you must first remove the eyepiece from the star
diagonal to attach the Moon filter).
B. The Planets
The planets don’t stay put like the stars (they don’t have fixed
R.A. and Dec. coordinates), so you’ll have to refer to charts
published monthly at our website, www.telescope.com, or in
monthly magazines like Astronomy, Sky & Telescope, to
locate them. Venus, Mars, Jupiter, and Saturn are the bright-
est objects in the sky after the Sun and the Moon. Not all four
of these planets are normally visible at any one time.
JUPITER The largest planet, Jupiter, is a great subject to
observe. You can see the disk of the giant planet and watch
the ever-changing positions of its four largest moons, Io,
Callisto, Europa, and Ganymede. If atmospheric conditions
are good, you may be able to resolve thin cloud bands on the
planet’s disk.
SATURN The ringed planet is a breathtaking sight when it is
well positioned. The tilt angle of the rings varies over a period
of many years; sometimes they are seen edge-on, while at
other times they are broadside and look like giant “ears” on
each side of Saturn’s disk. A steady atmosphere (good see-
ing) is necessary for a good view. You might see a tiny, bright
“star” close by; that’s Saturn’s brightest moon, Titan.
VENUS At its brightest, Venus is the most luminous object in
the sky, excluding the Sun and the Moon. It is so bright that
sometimes it is visible to the naked eye during full daylight!
Ironically, Venus appears as a thin crescent, not a full disk,
when at its peak brightness. Because it is so close to the Sun,
it never wanders too far from the morning or evening horizon.
No surface markings can be seen on Venus, which is always
shrouded in dense clouds.
MARS If atmospheric conditions are good, you may be able
to see some subtle surface detail on the Red Planet, possibly
even the polar ice cap. Mars makes a close approach to Earth
every two years; during those approaches its disk is larger
and thus more favorable for viewing.
C. Stars
Stars will appear like twinkling points of light in the telescope.
Even powerful telescopes cannot magnify stars to appear as
more than points of light! You can, however, enjoy the different col-
ors of the stars and locate many pretty double and multiple stars.