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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 rooftops and chim-
neys, 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 distor-
tion.
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!
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 a half-hour
for your telescope to cool to the temperature outdoors. In very
cold climates (below freezing), it is essential to store the tele-
scope as cold as possible. If it has to adjust to more than a
40° temperature change, allow at least one hour.
aiming the telescope
To view an object in the main telescope, first loosen both the
R.A. and Dec. lock bolts. Aim the telescope at the object you
wish to observe by "eyeballing" along the length of the tele-
scope tube (or use the setting circles to "dial in" the object's
coordinates). Then look through the (aligned) finder scope and
move the telescope tube until the object is centered on the
crosshairs. Retighten the R.A. and Dec. lock bolts. Then center
the object on the finder's crosshairs using the R.A. and Dec.
slow-motion controls. The object should now be visible in the
main telescope with a low-power (long focal length) eyepiece.
Focusing the telescope
Practice focusing the telescope in the daytime before using it
for the first time at night. Start by positioning the focuser near
the center of its adjustment range. Insert an eyepiece into the
focuser and secure with the thumbscrew. Point the telescope
at a distant subject and center it in the field of view. Now,
slowly rotate one of the focusing knobs until the object comes
into sharp focus. Go a little bit beyond sharp focus until the
image just starts to blur again, then reverse the rotation of the
knob, just to make sure you hit the exact focus point. The
telescope can only focus on objects at least 50 to 100 feet
away.
Do You Wear Eyeglasses?
If you wear eyeglasses, you may be able to keep them on
while you observe, if your eyepieces have enough "eye relief"
to allow you to see the whole field of view. You can try this by
looking through the eyepiece first with your glasses on and
then with them off, and see if the glasses restrict the view to
only a portion of the full field. If they do, you can easily
observe with your glasses off by just refocusing the telescope
the needed amount.
4
calculating the Magnification
It is desirable to have a range of eyepieces of different focal
lengths, to allow viewing over a range of magnifications. To calcu-
late the magnification, or power, of a telescope, simply divide the
focal length of the telescope by the focal length of the eyepiece:
Telescope focal length ÷ Eyepiece focal length =
Magnification
For example, the ShortTube 4.5" EQ, which has a focal length
of 1000mm, used in combination with a 25mm eyepiece,
yields a power of
1000 ÷ 25 = 40x.
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 tolerate.
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
conditions 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, gal-
axies, 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. Exposing your eyes to very bright daylight for
extended periods of time can adversely affect your night
vision for days. So give yourself at a little while to get used to
the dark before you begin observing.
To see what you're doing in the darkness, use a red-filtered flash-
light 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, like the RedBeam LED Flashlight (Orion #5744), is ideal ,
or you can cover the front of a regular incandescent 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, when you look up at the sky with just your eyes, the stars
are twinkling noticeably, the seeing is bad and you will be
limited to viewing with low powers (bad seeing affects images
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