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In addition to providing the collimation cap, you'll notice a tiny
ring (sticker) in the exact center of the primary mirror. This
“center mark” allows you to achieve a very precise collimation
of the primary mirror; you don't have to guess where the cen-
ter of the mirror is. You simply adjust the mirror position
(described below) until the reflection of the hole in the collima-
tion cap is centered inside the ring. This center mark is also
required for best results with other collimating devices, such
as Orion's LaserMate Laser Collimator, obviating the need to
remove the primary mirror and mark it yourself.
NOTE: The center ring sticker need not ever be removed from
the primary mirror. Because it lies directly in the shadow of
the secondary mirror, its presence in no way adversely affects
the optical performance of the telescope or the image quality.
That might seem counterintuitive, but it’s true!
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 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 wall. Placing a piece of white paper in
the telescope tube opposite the focuser (i.e., on the other side
of the secondary mirror) will also be helpful in collimating the
secondary mirror. Use a small Phillips head screwdriver to
loosen the three small alignment screws in the center hub of
the 3-vaned spider several turns. Now hold the mirror holder
stationary (be careful not to touch the surface of the mirrors),
while turning the center screw with a larger Phillips head
screwdriver (see Figure 9). 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.
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. This adjustment will rarely, if
ever need to be done.
If the entire primary mirror reflection is not visible in the sec-
ondary mirror, as in Figure 8c, you will need to adjust the tilt of
the secondary mirror. This is done by alternately loosening
one of the three alignment screws while tightening the other
two, as depicted in Figure 10. The goal is to center the primary
mirror reflection in the secondary mirror, as in Figure 8d. 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 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 the “dot” of the collimation cap) is
off-center.
The tilt of the primary mirror is adjusted using the three sets of
two collimation screws on the back end of the optical tube.
Adjusting the tilt of the mirror requires a “push-pull” technique
involving adjustment of each set of collimation screws. Loosen
one of the screws one full turn, and then tighten the adjacent
screw until it is tight as in Figure 11 (do not overtighten). Look
into the focuser and see if the secondary mirror reflection has
moved closer to the center of the primary. You can tell this eas-
ily with the collimation cap and mirror center mark by simply
watching to see if the “dot” of the collimation cap is moving
closer or farther away from the ring on the center of the pri-
Figure 9. To center the secondary mirror under
the focuser, hold the secondary in place with
your fingers while adjusting the primary screw
with a Phillips head screwdriver. Do not touch
the mirror’s surface.
Figure 10. Adjust the tilt of the secondary mirror
by loosening or tightening the three alignment
screws with a small Phillips head screwdriver.