Orion SKYQUEST #27189 XT12I - Manual (Page 17)

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of the secondary mirror (with the “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 opti-
cal tube (bottom of the primary mirror cell); these are the
larger thumbscrews. 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
(Figure 23). Use a screwdriver in the slots, if
necessary.
Now, try tightening or loosening one of the larger collimation
thumbscrews with your fingers. Look into the focuser and see
if the secondary mirror reflection has moved closer to the cen-
ter 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 further away
from the “ring” on the center of the primary mirror. If turning
that one thumbscrew does not seem to be bringing you closer
to collimation, try using one of the other collimation thumb-
screws. It will take some trial and error using all three thumb-
screws to adjust the telescope properly. 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 19e. Now be sure to
re-tighten the locking thumbscrews to lock the primary mirror
in that position.
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 24). If the image is unsymmetrical, the scope
is out of collimation. 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.
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 per-
fectly 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. astronomical observing
SkyQuest IntelliScope Dobsonians provide prodigious capa-
bility for observing the many wonders of the heavens, from
the major planets to deep-space nebulas and galaxies. In this
section we give you some observing tips and briefly summa-
rize what you can expect to see.
a. selecting an observing site
Since most astronomical objects are faint, observing them
from dark skies will give you the best views. While some
objects, such as the planets and Moon, are bright enough
to see clearly even from light-polluted city skies, for nebulas,
galaxies, and most star clusters, the less ambient light there
is to reduce contrast, the better.
When it isn’t possible or convenient to get out of town to a
pitch-dark observing location, try to set up in a spot that is
removed from street and building lights and that has a clear
view of a large portion of the sky. For observing faint deep-
sky objects, choose a Moonless night. Use of a light-pollution
filter can mitigate the effects of background sky brightness,
enhancing the view of faint objects.
Figure 23. The three small thumbscrews that lock the primary
mirror in place must first be loosened before any adjustments can
be made. Then the tilt of the primary mirror can be adjusted by
turning one or more of the three larger thumbscrews.
Figure 24. 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