14
Star-Testing the Telescope
When it is dark, point the telescope at a bright star and accu-
rately center it in the eyepiece’s field-of-view. Slowly defocus
the image with the focusing knob. If the telescope is correctly
collimated, the expanding disk should be a perfect circle
(Figure 19). If the image is unsymmetrical, the scope is out of
collimation. The dark shadow cast by the secondary mirror
should appear in the very center of the out-of-focus circle, like
the hole in a doughnut. If the “hole” appears off-center, the tel-
escope is out of collimation.
If you try the star test and the bright star you have selected is
not accurately centered in the eyepiece, then the optics will
always appear out of collimation, even though they may be
perfectly aligned. It is critical to keep the star centered, so over
time you will need to make slight corrections to the telescope’s
position in order to account for the sky’s apparent motion.
8. Astronomical Observing
For many users, the Atlas 8 EQ telescope will be a major leap
into the world of amateur astronomy. This section is intended
to get you ready for your voyages through the night sky.
Observing Tips
A. Site Selection
Pick a location away from street lights and bright yard lighting.
Avoid viewing over rooftops and chimneys, as they often have
warm air currents rising from them, which distort the image
seen in the eyepiece. Similarly, you should not observe
through an open window from indoors. Better yet, choose a
site out-of-town, away from any “light pollution”. You’ll be
stunned at how many more stars you’ll see! Most importantly,
make sure that any chosen site has a clear view of a large
portion of the sky.
B. Seeing and Transparency
Atmospheric conditions play a huge part in quality of viewing.
In conditions of good “seeing”, star twinkling is minimal and
objects 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. Typically, seeing
conditions will be better at sites that have an altitude over
about 3000 feet. Altitude helps because it decreases the
amount of distortion causing atmosphere you are looking
through.
A good way to judge if the seeing is good or not is to look at
bright stars about 40° above the horizon. If the stars appear to
“twinkle”, the atmosphere is significantly distorting the incom-
ing light, and views at high magnifications will not appear
sharp. If the stars appear steady and do not twinkle, seeing
conditions are probably good and higher magnifications will
be possible. Also, seeing conditions are typically poor during
the day. This is because the heat from the Sun warms the air
and causes turbulence.
Good “transparency” is especially important for observing
faint objects. It simply means the air is free of moisture,
smoke, and dust. All tend to scatter light, which reduces an
object’s brightness.
One good way to tell if conditions are good is by how many
stars you can see with your naked eye. If you cannot see stars
of magnitude 3.5 or dimmer then conditions are poor. Magni-
tude 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 “dipper”. If you cannot see
Megrez, then you have fog, haze, clouds, smog, light pollution
or other conditions that are hindering your viewing (See
Figure 20).
C. Cooling the Telescope
All optical instruments need time to reach “thermal equilibri-
um” to achieve maximum stability of the lenses and mirrors,
which is essential for peak performance. When moved from a
warm indoor location outside to cooler air (or vice-versa), a
telescope needs time to cool to the outdoor temperature. The
bigger the instrument and the larger the temperature change,
the more time will be needed.
Allow at least 30 minutes for your Atlas 8 EQ to equilibrate. If
the scope has more than a 40° temperature adjustment, allow
Figure 20.
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.
Figure 19.
A star test will determine if a telescope’s optics are
properly collimated. An unfocused view of a bright star through the
eyepiece should appear as illustrated on right if optics are perfectly
collimated. If circle is unsymmetrical, as in illustration on left, scope
needs collimation.
Out of collimation
Collimated
2.4
1.7
3.4
2.4
4.9
1.9
1.9
2.5
