Aligning the Secondary Mirror
With the collimation cap in place, look through the hole in the
cap at the secondary (diagonal) mirror. Ignore the reflections
for the time being. The secondary mirror itself should be cen-
tered in the focuser drawtube, in the direction parallel to the
length of the telescope. If it isn’t, as in Figure 12b, it must be
adjusted. This adjustment will rarely, if ever, need to be done.
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 wall. Placing a piece of white paper in the tele-
scope tube opposite the focuser (i.e., on the other side of the
secondary mirror) will also be helpful in collimating the sec-
ondary mirror. Using a 2mm hex key, loosen the three small
alignment set screws in the center hub of the 4-vaned spider
several turns. Now hold the mirror holder stationary (be care-
ful not to touch the surface of the mirrors), while turning the
center screw with a Phillips head screwdriver (see Figure 13).
Turning the screw clockwise will move the secondary mirror
toward the front opening of the optical tube, while turning the
screw counter-clockwise will move the secondary mirror
toward the primary mirror.
Note: When making these adjustments, be careful not to
stress the spider vanes or they may bend.
When the secondary mirror is centered in the focuser draw-
tube, rotate the secondary mirror holder until the reflection of
the primary mirror is as centered in the secondary mirror as
possible. It may not be perfectly centered, but that is OK. Now
tighten the three small alignment screws equally to secure the
secondary mirror in that position. If the entire primary mirror
reflection is not visible in the secondary mirror, as in Figure
12c, you will need to adjust the tilt of the secondary mirror.
This is done by alternately loosening one of the three align-
ment setscrews while tightening the other two, as depicted in
Figure 14. The goal is to center the primary mirror reflection in
the secondary mirror, as in Figure 12d. Don’t worry that the
reflection of the secondary mirror (the smallest circle, with the
collimation cap “dot” in the center) is off-center. 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 12d, 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 reflec-
tion 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 optical
tube (bottom of the primary mirror cell); these are the larger
thumbscrews. The other 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 a few turns each
(Figure 15). Use a screwdriver in the slots, if necessary.
Now, try tightening or loosening one of the larger collimation
thumbscrews with your fingers (Figure 16). 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 watching
to see if the “dot” of the collimation cap is moving closer or far-
ther away from the ring on the center of the primary mirror.
Figure 13.
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 14.
Adjust the tilt of the secondary mirror by loosening or
tightening the three alignment set screws with a 2mm hex key.
Figure 15.
The three small thumbscrews that lock the primary
mirror in place must first be loosened before any adjustments can
be made.
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