5
For example, the GoScope has a focal length of 350mm,
which when used with the supplied 20mm eyepiece yields:
350mm
= 17.5x
20mm
The magnification provided by the 10mm eyepiece is:
350mm
= 35x
10mm
The maximum attainable magnification for a telescope is
directly related to how much light it can gather. The larger the
aperture, the more magnification is possible. In general a fig-
ure of 2x per millimeter of aperture is the maximum attainable
for most telescopes. Your GoScope has an aperture of 70 mil-
limeters, so the maximum magnification would be about 140x.
This level of magnification assumes you have ideal conditions
for viewing.
Keep in mind that as you increase magnification, the bright-
ness of the object viewed will decrease; this is an inherent
principle of the laws of physics and cannot be avoided. If mag-
nification is doubled, an image appears four times dimmer.
If magnification is tripled, image brightness is reduced by a
factor of nine!
Start by centering the object you wish to see in the 20mm
eyepiece. Then you may want to increase the magnification to
get a closer view. If the object is off-center (i.e., it is near the
edge of the field of view) you will lose it when you increase
magnification, since the field of view will be narrower with the
higher-powered eyepiece.
To change eyepieces, first loosen the securing thumbscrew
on the diagonal. Then carefully lift the eyepiece out of the
holder. Do not tug or pull the eyepiece to the side, as this will
knock the telescope off its target. Replace the eyepiece with
the new one by sliding it gently into the holder. Re-tighten the
thumbscrew, and refocus for your new magnification.
terrestrial Viewing
Most users will use their GoScope extensively for viewing ter-
restrial (Earth-based) objects during daylight hours. It excels
for viewing birds, wildlife, sporting events, vistas, or anything
else you want to see close-up.
Remember to aim well clear of the Sun, unless the front of
the telescope is fitted with a professionally made solar filter
(available from Orion). Otherwise, permanent eye damage
could result. Children should use the GoScope only with adult
supervision.
astronomical observation
The GoScope also is a good telescope for casual stargazing.
It can show much more than what is visible on Earth during
the day. Once the Sun sets, there are literally thousands of
objects in the night sky that can be inspected more closely.
For many, this will be the first foray into the exciting world of
amateur astronomy. The following information and observing
tips will help you get 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
heat which 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 ris-
ing from them. Similarly, avoid observing from indoors through
an open (or closed) window, because the temperature differ-
ence 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
turbulence causes objects viewed through the telescope to
“boil”. If you look up at the sky with your eyes, and the stars
are twinkling noticeably, then the seeing is bad and you will
be limited to viewing with low magnifications. Planetary and
Moon 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 (brightness) of the
faintest stars you can see with the unaided eye (6th magni-
tude or fainter 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. For optimal performance,
allow at least 30 minutes for your telescope to cool to the tem-
perature outdoors.
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 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.