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C
ollimation
Fig.g
Fig.k
Corretly aligned
Fig.h
Primary mirror
Support for
secondary mirror
Secondary mirror
Focuser
Needs collimation
Fig.j
Primary mirror clip
Ignore the reflected
image for now
Primary mirror clip
Primary mirror clip
Primary mirror clip
Fig.i
Adjusting screw
Primary
mirror
Mirror cell
Locking screw
10
Collimation is the process of aligning the mirrors of your
telescope so that they work in concert with each other to
deliver properly focused light to your eyepiece. By observing
out-of-focus star images, you can test whether your
telescope's optics are aligned. Place a star in the centre of
the field of view and move the focuser so that the image is
slightly out of focus. If the seeing conditions are good, you
will see a central circle of light (the Airy disc) surrounded by a
number of diffraction rings. If the rings are symmetrical about
the Airy disc, the telescope's optics are correctly collimated
(Fig.g).
If you do not have a collimating tool, we suggest that you
make a "collimating cap" out of a plastic 35mm film canister
(black with gray lid). Drill or punch a small pinhole in the exact
center of the lid and cut off the bottom of the canister. This
device will keep your eye centered of the focuser tube. Insert
the collimating cap into the focuser in place of a regular
eyepiece.
Collimation is a painless process and works like this:
Pull off the lens cap which covers the front of the telescope
and look down the optical tube. At the bottom you will see the
primary mirror held in place by three clips 120º apart, and at
the top the small oval secondary mirror held in a support and
tilted 45º toward the focuser outside the tube wall (Fig.h).
The secondary mirror is aligned by adjusting the central bolt
behind it, (which moves the mirror up and down the tube),
and the three smaller screws surrounding the bolt, (which
adjust the angle of the mirror). The primary mirror is adjusted
by the three adjusting screws at the back of your scope. The
three locking screws beside them serve to hold the mirror in
place after collimation. (Fig.i)
Aligning the Secondary Mirror
Point the telescope at a lit wall and insert the
collimating cap into the focuser in place of a regular
eyepiece. Look into the focuser through your collimating
cap. You may have to twist the focus knob a few turns
until the reflected image of the focuser is out of your
view. Note: keep your eye against the back of the focus
tube if collimating without a collimating cap. Ignore the
reflected image of the collimating cap or your eye for
now, instead look for the three clips holding the primary
mirror in place. If you can't see them (Fig.j), it means
that you will have to adjust the three bolts on the top of
the secondary mirror holder, with possibly an Allen
wrench or Phillip's screwdriver. You will have to
alternately loosen one and then compensate for the
slack by tightening the other two. Stop when you see all
three mirror clips (Fig.k). Make sure that all three small
alignment screws are tightened to secure the
secondary mirror in place.
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