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telescope pointed toward a bright surface, such as white
paper or a wall.)
If the secondary mirror is off-center in the focuser drawtube,
loosen the three small Phillips-head alignment screws in the
center hub of the spider a few turns. Now position the cylin-
drical secondary mirror holder forward or back by turning the
larger central screw head with a Phillips screwdriver while
holding the secondary mirror holder stationary. When the mir-
ror is centered in the focuser drawtube (as in Figure 3B),
rotate it slightly side to side until the reflection of the primary
mirror is as centered in the secondary mirror as it will get. It
still may not be perfectly centered yet, but that is OK. Now
tighten the three small Phillips-head alignment screws to
secure the secondary mirror in that position.
If the entire primary mirror reflection still is not visible in the
secondary mirror (as it is not in Figure 3B), adjust the tilt of
the secondary mirror further by alternately loosening one of
the three alignment screws a turn or two and tightening
another one. The goal is to center the primary mirror reflec-
tion in the secondary mirror, as depicted in Figure 3C. Don't
worry that the reflection of the secondary mirror (the smallest
circle with your eye in it) and spider are off-center (as also is
the case in Figure 3C); you will fix that in the next step.
Adjusting the Primary Mirror
The final adjustment is made to the primary mirror. It will need
adjustment if, as in Figure 3C, the secondary mirror is cen-
tered under the focuser and the reflection of the primary mirror
is centered in the secondary mirror, but the small reflection of
the secondary mirror (with your eye inside) is off-center.
The tilt of the primary is adjusted with the three sets of two
collimation screws on the back end of the optical tube.
Adjusting the tilt requires a "push-pull" technique involving
adjustment of each set of collimation screws. Loosen one of
the screws one full turn, and then tighten the adjacent screw
until it is just tight. Then 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 sets of collimation screws, if necessary. It will take a little
trial and error to get a feel for how to tilt the mirror in this way
to center the reflection. (It helps to have two people for primary
mirror collimation, one to look in the focuser while the other
adjusts the collimation bolts.) When the adjustment is com-
plete, make sure all the Phillips-head collimation screws are
tight (but do not overtighten), to secure the mirror tilt.
The view through the Collimating Eyepiece should now
resemble Figure 3D. The secondary mirror is centered in 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. Look into
the eyepiece and slowly rack the image out of focus with the
focusing knob. If the telescope is correctly collimated, the
expanding disk should be a perfect circle. If it is unsymmetri-
cal, the scope is out of collimation. In reflectors and
Schmidt-Cassegrains, the dark shadow cast by the second-
ary 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.
7. Using the 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 streetlights, 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, 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!
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. Allow at least a half-hour for
your telescope to cool to the temperature outdoors. In very
cold climates (below freezing), it is essential to store the tele-
scope as cold as possible. If it has to adjust to more than a
40° temperature change, allow at least one hour.
Aiming the Telescope
To view an object in the main telescope, first loosen both the
R.A. and Dec. lock knobs. Aim the telescope at the object you
wish to observe by "eyeballing" along the length of the tele-
scope tube (or use the setting circles to "dial in" the object's
coordinates). Then look through the (aligned) finder scope and
move the telescope tube until the object is centered on the
crosshairs. Retighten the R.A. and Dec. lock bolts. Then center
the object on the finder's crosshairs using the R.A. and Dec.
slow-motion controls. The object should now be visible in the
main telescope with a low-power (long-focal-length) eyepiece.
Focusing the Telescope
Practice focusing the telescope in the daytime before using it
for the first time at night. Start by positioning the focuser near
the center of its adjustment range. Insert an eyepiece into the
focuser and secure with the thumbscrew. Point the telescope
at a distant subject and center it in the field of view. Now,
slowly rotate one of the focusing knobs until the object comes
into sharp focus. Go a little bit beyond sharp focus until the
image just starts to blur again, then reverse the rotation of the
knob, just to make sure you hit the exact focus point. The tel-
7
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