18
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 32). The three smaller thumbscrews lock the
mirror’s position in place. These thumbscrews must be loos-
ened before any collimation adjustments can be made to the
primary mirror.
To start, turn the smaller thumbscrews 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 28e. Re-tighten the
locking thumbscrews 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 de focus
the image with the focusing knob. If the telescope is correct-
ly collimated, the expanding disk should be a perfect circle
(Figure 33). If the image is unsymmetrical, the telescope is
out of collimation. The dark shadow cast by the secondary mir-
ror 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.
5. using Your telescope
using the clutch tensioning Knobs
The XXg Dobs all now feature large clutch tensioning knobs
on both the altitude and azimuth axes. Located on the alti-
tude and azimuth motor/encoder housings (
Figure 34), these
knobs allow the user to adjust the amount of tension (fric-
tion) in both the altitude and azimuth motion for moving the
telescope by hand. Rotating the knob clockwise increases
the tension, while turning it counterclockwise decreases the
tension. The closed-loop encoder system of the XXg Dobs
allows the scope to be moved manually without losing the ini-
tial GoTo star alignment. The clutch knobs allow you to set the
amount of motion tension on each axis independently to the
level you desire for smooth manual slewing of the telescope.
Figure 32.
The tilt of the primary mirror is adjusted by turning one
or more of the three larger thumbscrews. (XX12g shown)
Figure 33.
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 the right if the optics
are perfectly collimated. If the circle is unsymmetrical, as in the
illustration on the left, the scope needs collimation.
Out of collimation
Collimated
Collimation
knob
Locking
thumbscrew
