the center screw with a larger
Phillips head screwdriver (see
Figure 9). Turning the screw
clockwise will move the sec-
ondary mirror toward the front
opening of the optical tube,
while turning the screw coun-
ter-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 reflec-
tion of the primary mirror is
as centered in the second-
ary mirror as possible. It may
not be perfectly centered, but
that is OK. Now tighten the
three small alignment screws
equally to secure the second-
ary mirror in that position.
If the entire primary mirror
reflection is not visible in the
secondary mirror, as in Figure
8c, you will need to adjust
the tilt of the secondary mir-
ror. This is done by alternately
loosening one of the three
alignment screws while tight-
ening the other two, as depict-
ed 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 small-
est 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 centered 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
the flush screw one full turn,
and then tighten the adjacent
raised screw until it is tight
as in Figure 11 (do not over-
tighten). Look into the focuser
and see if the secondary
mirror reflection has moved
closer to the center of the pri-
mary. 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 farther away from
the ring on the center of the
primary mirror. Repeat this
process on the other two
sets of collimation screws, if
necessary. It will take a little
trial and error to get a feel for
how to tilt the mirror in this
way. When you have the dot
centered as much as pos-
sible in the ring, your primary
mirror is collimated. The view
through the collimation cap
should resemble Figure 8e.
Make sure all the collimation
screws are tight (but do not
overtighten), to secure the
mirror tilt.
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 and
accurately center it in the eye-
piece’s field of view. Slowly
de-focus the image with the
focusing knob. If the tele-
scope is correctly collimated,
the expanding disk should be
a perfect circle (Figure 12). If
the image is unsymmetrical,
the scope is out of collima-
tion. The dark shadow cast
by the secondary mirror should appear in the very center of
the out-of-focus circle, like the hole in a donut. 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, 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
12
Figure 8.
Collimating the optics. (a) When the mirrors are
properly aligned, the view down the focuser drawtube should look
like this. (b) With the collimation cap in place, if the optics are out
of alignment, the view might look something like this. (c) Here, the
secondary mirror is centered under the focuser, but it needs to be
adjusted (tilted) so that the entire primary mirror is visible. (d) The
secondary mirror is correctly aligned, but the primary mirror still
needs adjustment. When the primary mirror is correctly aligned, the
“dot” will be centered, as in (e).
drawtube
Reflection
of primary
mirror clip
a.
b.
c.
d.
e.
