6
to-right. (Correct-image diagonals are available, and may be
purchased separately, though the image quality is slightly reduced.)
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.
Eyepiece selection
Always start viewing with your lowest-power, widest-field eye-
piece. After you’ve located and looked at the object with a
low-power eyepiece, switch to a higher-power eyepiece and see
if the object looks better or worse. Keep in mind that at higher
power, an image will always be fainter and less sharp (this is a
fundamental law of optics). Many viewers use the lowest-power
eyepiece practically all the time! Naturally, higher magnifications
are desirable for viewing some celestial objects, but stay with low
powers when searching for an object and for extended viewing.
To calculate the power, or magnification of a telescope, divide the
focal length of the telescope by the focal length of the eyepiece:
Telescope focal length ÷ Eyepiece focal length = Magnification
For example, if you’re using this telescope, which has a focal
length of 800mm, and a 25mm eyepiece, the power would be
800 ÷ 25 = 32x.
We recommend having a selection of several eyepieces of
different focal lengths, so that you can choose the optimal
magnification, brightness level, and contrast for each object
and for different observing conditions.
useful limit of Magnification
Every telescope has a useful limit of power of about 45x-60x
per inch of aperture (diameter of objective lens). Claims of
higher power by some telescope manufacturers are a mislead-
ing advertising gimmick and should be dismissed. The Observer
60mm Ultra Altazimuth Refractor, with its 60mm-diameter
(2.4") objective lens, is capable of a maximum useful magnifi-
cation of about 140x. Any higher and the images will virtually
always appear very faint and fuzzy. As a general rule, if the
image you see is not crisp and steady, reduce the magnifica-
tion by switching to a longer-focal-length (lower-power)
eyepiece. A small but well-resolved image will show more detail
and be more rewarding than a dim, fuzzy, over-magnified one.
6. astronomical Viewing
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. Many observers
notice improvements after several hours of total darkness. 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 least 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
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, or you can cover the front of a
regular incandescent flashlight with red cellophane or paper.
Beware, too, that nearby porch and street lights and car head-
lights 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 at high
powers more severely). Planetary observing may also be poor.
Make sure you are not looking over buildings or any other
source of heat: that will also cause image degradation.
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 “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).
tracking celestial objects
Celestial objects appear to move slowly across the sky
because of the rotation of the Earth on its polar axis. When
you observe an object through the telescope, you’ll see it drift
gradually across the field of view. To keep the object centered
in the field, give the tube a light tug or push in altitude and/or
azimuth as needed (make sure the altitude and azimuth lock
knobs are loosened). Objects will appear to move faster at
higher magnifications, when the field of view is narrower.
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. A simple planisphere,
or star wheel, can be a valuable tool for learning the constella-
tions and seeing which ones are visible in the sky on a given
night.
A good star chart or atlas can 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-