8
For example, the Skywatcher 90 EQ, which has a focal length
of 910mm, used in combination with a 25mm eyepiece, yields
a power of
910 ÷ 25 = 36x.
Every telescope has a useful limit of power of about 45x-60x
per inch of aperture. Claims of higher power by some tele-
scope manufacturers are a misleading advertising gimmick
and should be dismissed. Keep in mind that at higher powers,
an image will always be dimmer and less sharp (this is a fun-
damental law of optics). The steadiness of the air (the
“seeing”) will limit how much magnification an image can tol-
erate.
Always start viewing with your lowest-power (longest focal
length) eyepiece in the telescope. After you have located and
looked at the object with it, you can try switching to a higher
power eyepiece to ferret out more detail, if atmospheric con-
ditions permit. If the image you see is not crisp and steady,
reduce the magnification by switching to a longer focal length
eyepiece. As a general rule, a small but well-resolved image
will show more detail and provide a more enjoyable view than
a dim and fuzzy, over-magnified image.
Let Your Eyes Dark-Adapt
Don’t expect to go from a lighted house into the darkness of
the outdoors at night and immediately see faint nebulas,
galaxies, and star clusters—or even very many stars, for that
matter. Your eyes take about 30 minutes to reach perhaps
80% of their full dark-adapted sensitivity. As your eyes
become dark-adapted, more stars will glimmer into view and
you’ll be able to see fainter details in objects you view in your
telescope.
To see what you’re doing in the darkness, use a red-filtered
flashlight rather than a white light. Red light does not spoil
your eyes’ dark adaptation like white light does. A flashlight
with a red LED light is ideal, such as the Orion RedBeam
(part #5744), or you can cover the front of a regular incan-
descent flashlight with red cellophane or paper. Beware, too,
that nearby porch lights, streetlights, and car headlights will
ruin your night vision.
“Seeing” and Transparency
Atmospheric conditions vary significantly from night to night.
“Seeing” refers to the steadiness of the Earth’s atmosphere at
a given time. In conditions of poor seeing, atmospheric turbu-
lence causes objects viewed through the telescope to “boil.” If
the stars are twinkling noticeably when you look up at the sky
with just your eyes, the seeing is bad and you will be limited
to viewing with low powers (bad seeing affects images at high
powers more severely). Planetary observing may also be
poor.
In conditions of good seeing, star twinkling is minimal and
images appear steady in the eyepiece. Seeing is best over-
head, worst at the horizon. Also, seeing generally gets better
after midnight, when much of the heat absorbed by the Earth
during the day has radiated off into space.
Avoid looking over buildings, pavement, or any other source
of heat, as they will cause “heat wave” disturbances that will
distort the image you see through the telescope.
Especially important for observing faint objects is good “trans-
parency”—air free of moisture, smoke, and dust. All tend to
scatter light, which reduces an object’s brightness.
Transparency is judged by the magnitude of the faintest stars
you can see with the unaided eye (6th magnitude or fainter is
desirable).
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 clusters,
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 cluster,
which happens to be 6.9-magnitude Messier 28 (M28).
You can see a great number and variety of astronomical
objects with your Skywatcher 90 EQ, including:
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 one and only natural satellite is
during a partial phase, that is, when the Moon is NOT full.
During partial 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 pleas-
ing 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).
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
