14
Adjusting the Primary Mirror
The final adjustment is made to the primary mirror.It will need
adjustment if, as in Figure 18d, the secondary mirror is cen-
tered 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 colli-
mation 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
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 22).Use a screwdriver in the slots, if
necessary.
Now, try tightening or loosening one of the larger collimation
thumbscrews with your fingers (Figure 23). Look into the
focuser and see if the secondary mirror reflection has moved
closer to the center of the primary.You can tell this easily with
the collimation cap and mirror center mark by simply watch-
ing 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 thumbscrews. It will take some trial and error
using all three thumbscrews 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 18e. Re-tighten the
locking thumbscrews.
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 defo-
cus 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 unsymmetr ical, 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 cir-
cle, 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 tele-
scope’s position in order to account for the sky’s apparent
motion.
Figure 22. The three small thumbscrews that lock the primary
mirror in place must first be loosened before any adjustments can
be made.
Figure 23. The tilt of the primary mirror is 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