11
The tilt of the primary mirror is adjusted with the three
knurled thumbscrews on the back end of the optical tube
(bottom of the mirror cell) (Figure 11). The Phillips head
screws next to each of the three knurled thumbscrews serve
to lock the mirror in place once the tilt has been adjusted.
Start by loosening each of these Phillips-head screws a few
turns. Now adjust the tilt of the mirror by turning one of the
knurled thumbscrews either clockwise or counterclockwise.
Look into the focuser and see if the secondary mirror reflec-
tion has moved closer to the center of the primary. You can
tell this easily with the collimation cap and mirror center
mark by simply watching to see if the “dot” of the collimation
cap is moving closer or farther away from the ring on the
center of the primary mirror. If it is not getting closer, try turn-
ing the knurled thumbscrew in the opposite direction. Repeat
this process for the other two knurled thumbscrews, if neces-
sary. It will take a little trial and error to get the feel for how to
adjust the mirror to center the “dot” of the collimation cap in
the “ring” of the mirror mark.
When you have the dot centered as much as possible in the
ring, your primary mirror is collimated. The view through the
collimation cap should resemble Figure 8e. Make sure all the
Phillips-head screws are tight (but do not overtighten), to
secure the mirror tilt.
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 and accu-
rately center it in the eyepiece’s field of view. Slowly de-focus
the image with the focusing knob. If the telescope is correctly
collimated, the expanding disk should be a perfect circle
(Figure 12). 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 donut. 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, 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 tele-
scope’s position in order to account for the sky’s apparent
motion.
7. 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.
Figure 9.
To center the
secondary mirror under the
focuser, hold the secondary in
place with your fingers while
adjusting the center screw with
the Phillips head screwdriver. Do
not touch the mirror’s surface.
Figure 10.
Adjust the tilt of
the secondary mirror by
loosening or tightening the
three alignment screws with a
2.5mm Allen wrench.
Figure 11.
The tilt of the primary mirror is adjusted by turning
one or more of the knurled collimation thumbscrews.
Knurled thumbscrew
Phillips-head screw
Figure 12.
A star test will determine if the telescope's optics are
properly collimated. An unfocused view of a bright star through the
eyepiece should appear as illustrated on the right if optics are
perfectly collimated. If the circle is unsymmetrical, as illustrated on
the left, the scope needs collimation.
Out of collimation
Collimated
Summary of Contents for ShortTube 4.5 EQ 9849
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