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turns of these set screws or force them past their normal trav-
el. A simple 1/2 turn of the screw can dramatically change the
tilt of the secondary mirror. The goal is to center the primary
mirror reflection in the secondary mirror, as in Figure 31d.
Don't worry that the reflection of the secondary mirror (the
smallest circle, with the collimation cap "dot" in the center) is
off-center. You will fix that in the next step.
Aligning the Primary Mirror
The final adjustment is made to the tilt of the primary mirror.
It will need adjustment if, as in Figure 31d, the secondary
mirror is centered 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 the "dot" of the
collimation cap) is off-center.
The tilt of the primary mirror is adjusted with the three large
spring-loaded collimation knobs on the rear end of the opti-
cal tube (Figure 35). The three smaller thumb screws lock
the mirror's position in place. These thumb screws must be
loosened before any collimation adjustments can be made to
the primary mirror.
To start, turn the smaller thumb screws counterclockwise a
few turns each. Use a screwdriver in the slots, if necessary.
Now, try tightening or loosening one of the collimation knobs
Look into the focuser and see if the secondary mirror reflec-
tion has moved closer to the center of the primary mirror. You
can easily determine this with the collimation cap and mirror
center mark by simply watching to see if the "dot" of the col-
limation cap is moving closer or further away from the "ring"
on the center of the primary mirror. If turning the one knob
does not seem to bring the dot closer to the ring, try using one
of the other collimation knobs. It will take some trial-and-error
using all three knobs to properly align the primary mirror. Over
time you will get the feel for which collimation screws to turn to
move the image in a given direction.
When you have the dot centered as much as is possible in the
ring, your primary mirror is collimated. The view through the
collimation cap should resemble Figure 31e. Re-tighten the
locking thumb screws in the bottom of the mirror cell.
A simple star test will tell you whether the optics are accu-
rately collimated.
Star-Testing the Telescope
When it is dark, point the telescope at a bright star high in
the sky and 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 36). If the image is unsymmetrical, the tele-
scope is out of collimation. The dark shadow cast by the sec-
ondary 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
26
Collimation
knob
Locking
thumbscrew
Figure 35.
The tilt of the primary mirror is adjusted by turning
one or more of the three collimation (large) knobs.
time you will need to make slight corrections to the telescope's
position in order to account for the sky's apparent motion.
10. Using Your Telescope
Focusing the Telescope
The SkyQuest XXg Dobsonians come standard with a 2"
dual-speed (11:1) Crayford focuser (Figure 28). The large 2"
format focuser allows use of 2" or 1.25" eyepieces and the
Crayford design prevents imaging shifting while focusing. The
focuser has coarse focus knobs and a fine focus knob for very
precise focusing.
With the 35mm DeepView eyepiece in the focuser and secured
with the thumb screws, move the telescope so the front end is
pointing in the general direction of an object at least 1/4-mile
away. Now, with your fingers, slowly rotate one of the coarse
focus 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're
close to the focus point.
Now, use the fine focus knob to achieve precise focus. Eleven
turns of the fine focus knob is equivalent to one turn of the
coarse focus knobs, so much finer adjustment is possible
than with just the coarse focus knobs alone. You'll find this is
a great convenience, especially when attempting to focus at
high magnifications.
If you have trouble focusing, rotate the coarse focusing knob
so the drawtube is inward as far as it will go. Now look through
the eyepiece while slowly rotating the focusing knob in the
opposite direction. You should soon see the point at which
focus is reached.
The thumb screw on the bottom of the body of the focuser
(Figure 28) will lock the focuser drawtube in place once the
telescope is properly focused. Before focusing, remember to
first loosen this thumb screw.
If you find the drawtube tension when focusing is either too
tight (i.e., focus knob is difficult to turn) or too loose (i.e., draw-
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