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4. Astronomical Observing
For many, this will be your first foray into the exciting world of
amateur astronomy. The following information and observing
tips will help get you started.
Choosing an Observing Site
When selecting a location for observing, get as far away as
possible from direct artificial light such as street lights, porch
lights, and automobile headlights. The glare from these lights
will greatly impair your dark-adapted night vision. Set up on
a grass or dirt surface, not asphalt, because asphalt radiates
more heat. Heat disturbs the surrounding air and degrades the
images seen through the telescope. Avoid viewing over roof-
tops and chimneys, as they often have warm air currents rising
from them. Similarly, avoid observing from indoors through an
open (or closed) window, because the temperature difference
between the indoor and outdoor air will cause image blurring
and distortion.
If at all possible, escape the light-polluted city sky and head for
darker country skies. You'll be amazed at how many more stars
and deep-sky objects are visible in a dark sky!
"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 you look up at the sky and stars are twinkling noticeably,
the seeing is poor and you will be limited to viewing at lower
magnifications. At higher magnifications, images will not focus
clearly. Fine details on the planets and Moon will likely not be
visible.
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.
Especially important for observing faint objects is good
"transparency"—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 (5th or 6th magnitude is
desirable).
Cooling the Telescope
All optical instruments need time to reach "thermal equilibri-
um." The bigger the instrument and the larger the temperature
change, the more time is needed. Allow at least 30 minutes for
your telescope to acclimate to the temperature outdoors before
you start observing with it.
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, galax-
ies, 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-adapt-
ed, 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. Beware, too, that nearby porch, street-
lights, and car headlights will ruin your night vision.
What to Expect
So what will you see with your telescope? You should be able
to see bands on Jupiter, the rings of Saturn, craters on the
Moon, the waxing and waning of Venus, and many bright
deep-sky objects. Do not expect to see color as you do in
NASA photos, since those are taken with long-exposure cam-
eras and have "false color" added. Our eyes are not sensitive
enough to see color in deep-sky objects except in a few of the
brightest ones. But as you become more experienced and your
observing skills get sharper, you will be able to ferret out more
and more subtle details and structure.
Objects to Observe
Now that you are all set up and ready to go, what is there to
look at in the night sky?
A. The Moon
With its rocky surface, the Moon is one of the easiest and most
interesting objects to view with your telescope. Lunar craters,
maria, and even mountain ranges can all be clearly seen from
a distance of 238,000 miles away! With its ever-changing
phases, you'll get a new view of the Moon every night. The
best time to observe our one and only natural satellite is dur-
ing a partial phase, that is, when the Moon is not full. During
partial phases, shadows are cast on the surface, which 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. Make sure to observe the Moon when it is well
above the horizon to get the sharpest images.
Use an optional Moon filter 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 focuser to attach a
filter). You'll find that the Moon filter improves viewing comfort,
and helps to bring out subtle features on the lunar surface.
B. The Planets
The planets don't stay put like the stars, so to find them you
should refer to Sky Calendar at our website OrionTelescopes.
com. Venus, Mars, Jupiter, and Saturn are the brightest objects
in the sky after the Sun and the Moon. Your telescope is capa-
ble of showing you these planets in some detail. Other planets
may be visible but will likely appear star-like. Because planets
are quite small in apparent size, optional higher power eye-
pieces and a Barlow lens are recommended and often required
for detailed observations. Not all the planets are usually visible
at any one time.
Jupiter: The largest planet, Jupiter, is a great subject for obser-
vation. You can see the disk of the giant planet and watch the
ever-changing positions of its four largest moons: Io, Callisto,
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