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 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.a).
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.b).
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.c. The screws on the back of your
telescope may be different.)
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.d), 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 or loosen one and then compensate for the slack by tightening
the other two. Stop when you see all three mirror clips (Fig.e). Make sure that all three small alignment screws
are tightened to secure the secondary mirror in place.
Fig.b
Primary mirror
Support for
secondary mirror
Secondary mirror
Focuser
Fig.a
Corretly aligned
Needs collimation
Fig.d
Primary mirror clip
Ignore the reflected
image for now
Fig.e
Primary mirror clip
Primary mirror clip
Primary mirror clip
COLLIMATING A NEWTONIAN
Fig.c
Adjusting screw
Primary mirror
Mirror cell
Locking screw
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