Orion STARBLAST 6/6I - Manual (Page 30)

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in the secondary mirror holder about a quarter of a turn only
(counterclockwise). That should be enough to free up the sec-
ondary mirror to rotate slightly in either direction. Look into the
collimation cap and rotate the mirror slightly in each direction
to get an idea of how it affects the view of the secondary mir-
ror. Now rotate the mirror as needed so that it precisely faces
the focuser. Hold the mirror holder stationary in that position
while turning the center screw clockwise until it is just tight
(do not over-tighten). Sometimes the mirror may rotate slightly
when tightening the screw, so keep at it until the mirror faces
the focuser squarely and is secured in place.
adjusting the Secondary Mirror’s Tilt
Finally, the tilt of the secondary mirror may occasionally
require adjustment. If the entire primary mirror reflection is not
visible in the secondary mirror when using the collimation cap,
as in
Figure 27c, you will need to adjust the tilt of the second-
ary mirror. Using a 2mm hex key, first loosen one of the three
alignment set screws by, say, one full turn, and then tighten the
other two to take up the slack. Do not loosen the center screw
during this process. The goal is to center the primary mirror
reflection in the secondary mirror, as in
Figure 27d. When it
is centered, you’re done adjusting the secondary mirror. Don’t
worry that the reflection of the secondary mirror (the dark cir-
cle with the four spider vanes adjoining it) is off-center, since
that adjustment is made when aligning the primary mirror in
the next step.
aligning the Primary Mirror
The final collimation adjustment is made to the primary mir-
ror. It will need adjustment if, as in
Figure 27d, the second-
ary mirror is centered under the focuser and the reflection of
the primary mirror is centered in the secondary mirror, but the
reflection of the secondary mirror (dark circle containing the
light reflective surface and center black “dot” of the collimation
cap) is off-center.
The tilt of the primary mirror is adjusted with three spring-
loaded collimation thumbscrews on the back end of the optical
tube (bottom of the primary mirror cell); these are the wide
thumbscrews (
Figure 31). The other three thin thumbscrews
lock the mirror’s position in place; these thin thumbscrews
must be loosened before any collimation adjustments can be
made to the primary mirror.
To start, turn the thin thumbscrews that lock the primary mirror
in place counterclockwise a few turns each.
Now, while looking into the focuser through the collimation
cap, turn one of the wide collimation thumbscrews a half turn
or so in either direction and see if the secondary mirror reflec-
tion moves closer to the center of the primary. That is, does the
“dot” of the collimation cap appear to move closer to the ring on
the center of the primary mirror? If it does, great, keep going
until you get it as close as you can. If it doesn’t, try turning
the collimation thumbscrew in the opposite direction. If turning
the one collimation thumbscrew does not seem to bring the
dot closer to the ring, try using one of the other collimation
thumbscrews. It will take some trial-and-error using all three
collimation thumbscrews 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 possible in the
ring, your primary mirror is collimated. Now lightly tighten the
three thin locking thumbscrews to secure the primary mirror
in place.
The view through the collimation cap should now resemble
Figure 27e. A simple star test will indicate how well the tele-
scope optics are 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 correct-
ly collimated, the expanding disk should be a perfect circle
(
Figure 32). 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 tele-
scope is out of collimation.
Figure 31. The collimation and mirror locking thumbscrews
Figure 32. 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.