more heat, which disturbs the surrounding air and degrades
the images seen through the telescope. Avoid viewing over
rooftops 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 tempera-
ture 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, when you look up at the sky with your naked eyes, the
stars are twinkling noticeably, 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.
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 (6th magnitude or fainter is
desirable).
If you cannot see stars of magnitude 3.5 or dimmer then con-
ditions are poor. Magnitude is a measure of how bright a star
is—the brighter a star is, the lower its magnitude will be. A
good star to remember for this is Megrez (mag. 3.4), which is
the star in the “Big Dipper” connecting the handle to the “dip-
per”. If you cannot see Megrez, then you have fog, haze,
clouds, smog, or other conditions that are hindering your
viewing (See Figure 11).
Eyepiece Selection
By using eyepieces of different focal lengths, it is possible to
attain many magnifications or powers with the Observer
60mm. Your telescope comes with two Kellner eyepieces
(Figure 12): a 25mm, which gives a magnification of 36x, and
a 10mm, which gives a magnification of 90x. Other eyepieces
can be used to achieve higher or lower powers. It is quite
common for an observer to own five or more eyepieces to
access a wide range of magnifications.
To calculate the magnification, or power, of a telescope-eye-
piece combination, simply divide the focal length of the tele-
scope by the focal length of the eyepiece.
Telescope Focal Length (mm)
= Magnification
Eyepiece Focal Length (mm)
For example, the Observer 60mmEQ , which has a focal
length of 900mm, used in combination with the 25mm eye-
piece, yields a magnification of
900mm
= 36x
25mm
Whatever you choose to view, always start by inserting your
lowest-power (longest focal length) eyepiece to locate and
center the object. Low magnification yields a wide field of
view, which shows a larger area of sky in the eyepiece. This
makes finding and centering an object much easier. Trying to
find and center objects with a high power (narrow field of
view) eyepiece is like trying to find a needle in a haystack!
Once you’ve centered the object in the eyepiece, you can
switch to a higher magnification (shorter focal length) eye-
piece, if you wish. This is recommended for small and bright
objects, like planets and double stars. The Moon also takes
higher magnifications well.
The best rule of thumb with eyepiece selection is to start with
a low power, wide-field eyepiece, and then work your way up
in magnification. If the object looks better, try an even higher
magnification eyepiece. If the object looks worse, then back
off the magnification a little by using a lower-power eyepiece.
Figure 11.
Megrez connects the Big Dipper’s handle to it's “pan”.
It is a good guide to how conditions are. If you can not see Megrez
(a 3.4 mag star) then conditions are poor.
16
2.4
1.7
3.4
2.4
4.9
1.9
1.9
2.5
Figure 12.
The 10mm and 25mm
Explorer II eyepieces.
