12
Now, try tightening or loosening one of the spring-loaded col-
limation thumb screws one turn (Figure 13). Look into the
focuser and see if the secondary mirror reflection has moved
closer to the center of the primary mirror reflection. Repeat
this process on the other two collimation thumb screws, if
necessary. It will take a little trial and error to get a feel for how
to tilt the primary mirror in this way to center the secondary
mirror reflection. Do not loosen (rotate counter-clockwise)
each collimation thumb screw too much, or the screw will
completely unthread from the mirror cell casting. Rather, try
tightening the other two collimation thumb screws. Once the
secondary mirror reflection is centered in the primary mirror
reflection, retighten the smaller thumb screws with your fin-
gers to lock the primary mirror in place.
The view down the focuser should now resemble Figure 8a.
The secondary mirror is centered under the focuser; the
reflection of the primary mirror is centered in the secondary
mirror, and the reflection of the secondary mirror is centered
in the reflection of the primary mirror.
A simple star test will tell you whether the optics are accu-
rately collimated.
Star-Testing Your Telescope
When it is dark, point the telescope at a bright star and center it
in the eyepiece’s field of view with the R.A. and Dec. slow-
motion controls. Slowly rack the image out of focus with the
focusing knob. If the telescope is correctly collimated, the
expanding disk should be a circle (Figure 14). If it is unsymmet-
rical, 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 telescope 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.
7. Using Your Telescope—
Astronomical Observing
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
more heat. Heat 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 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.
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).
Cooling the Telescope
All optical instruments need time to reach “thermal equilib-
rium.” The bigger the instrument and the larger the
temperature change, the more time is needed. Allow at least
a 30 minutes for your telescope to cool to the temperature
outdoors. In very cold climates (below freezing), it is essen-
tial to store the telescope as cold as possible. If it has to
Figure 14. 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
Figure 13. Use the three
spring-loaded primary mirror
collimation screws to adjust
the tilt of the primary mirror.
