tions for the time being. The secondary mirror should be centered
in the field of view. If it isn’t, as in Figure 8b, it must be adjusted.
(It helps to adjust the secondary mirror in a brightly lit room with
the telescope pointed toward a bright surface, such as white
paper or a wall.)
If the secondary mirror is not centered in the focuser draw-
tube (in the direction parallel to the length of the telescope),
loosen the three small alignment screws in the center hub of
the secondary mirror holder several turns. Now hold the sec-
ondary mirror stationary (be careful not to touch the surface
of the secondary mirror!), while turning the central Phillips-
head bolt (as in Figure 9). Turning the bolt clockwise will move
the secondary mirror toward the front opening of the optical
tube, while turning the bolt counterclockwise will move the
secondary mirror toward the primary mirror. When the sec-
ondary mirror is centered in the focuser drawtube (as in
Figure 8c), rotate the secondary mirror holder slightly side-
to-side until the reflection of the primary mirror is as centered
in the secondary mirror as it will get. It still may not be per-
fectly centered yet, but that is OK. Now tighten the three small
alignment screws to secure the secondary mirror in that posi-
tion. This adjustment will rarely need to be done, if ever.
If the entire primary mirror reflection is not visible in the sec-
ondary mirror (as it is not in Figure 8c), adjust the tilt of the
secondary mirror by alternately loosening one of the three
alignment screws a turn or two and tightening the other two
(Figure 10). The goal is to center the primary mirror reflection
in the secondary mirror, as depicted in Figure 8d. Don’t worry
that the reflection of the secondary mirror (the smallest cir-
cle, with your eye reflected in it) is off-center (as also is the
case in Figure 8d); you will fix that in the next step.
Adjusting the Primary Mirror
The final adjustment is made to the primary mirror. It will need
adjustment if, as in Figure 8d, 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 your eye inside) is off-center.
The tilt of the primary mirror is adjusted with the three spring-
loaded collimation screws on the back end of the optical tube
(bottom of the primary mirror cell); these are the knurled thumb-
screws with the larger diameter heads. The three smaller-headed
slotted thumbscrews lock the primary mirror’s position in place;
these screws must be loosened before any collimation adjust-
ments can be made to the primary mirror (Figure 11).
To start, unthread the slotted thumbscrews a few turns each
with your fingers (Figure 12). If the screws are too tight to turn
with your fingers, use a flat-head screwdriver.
11
Figure 10. Adjust the tilt of the secondary mirror by loosening or
tightening the three alignment screws with a Phillips screwdriver.
Figure 9. To center the secondary mirror under the focuser, hold the
secondary mirror holder in place with one hand while adjusting the
center bolt with a Phillips screwdriver. Do not touch the mirror’s surface!
Figure 11. The back end of the optical tube (bottom of the primary
mirror cell). The three spring-loaded collimation thumbscrews adjust
the tilt of the primary mirror. The three slotted thumbscrews lock the
mirror’s tilt in place.
Figure 12. Before collimation
adjustments can be made to
the primary mirror, the three
slotted thumbscrews must first
be loosened.
Spring-loaded
collimation
thumbscrews
Slotted locking
thumbscrews
