16
4. collimating the
optical system
To get the sharpest images, your telescope’s optical sys-
tem must be in precise alignment. The process of aligning
the primary and secondary mirrors with each other and with
the mechanical axis of the telescope is called collimating.
Collimating is relatively easy to do and can be done in day-
light or at night.
Because the primary mirror is shipped separately from the
optical tube, the telescope’s optics must be collimated before
it can be used. Most of the adjustments will be to the tilt of the
primary mirror, as the secondary mirror has been pre-aligned
at the factory. It is also good idea to check the collimation
(optical alignment) of your telescope before each observing
session and make any necessary adjustments.
To check collimation, remove the eyepiece and look down the
focuser drawtube. You should see the secondary mirror cen-
tered in the drawtube, as well as the reflection of the primary
mirror centered in the secondary mirror, and the reflection of
the secondary mirror (and your eye) centered in the reflection
of the primary mirror, as depicted in
Figure 28a. If anything
is off-center, as in
Figure 28b, proceed with the following col-
limation procedure.
the collimation cap and Mirror center Mark
Your XXg comes with a collimation cap. This is a simple cap
that fits on the focuser drawtube like a dust cap, but has a
hole in the center and a reflective inner surface. The cap
helps center your eye so that collimating is easier to perform.
Figures 28b-e assume you have the collimation cap in place.
As an additional aid in collimating, the primary mirror of
the XXg has a tiny adhesive ring marking its exact center
(
Figure 3a). This center ring will not affect the images you
see when observing with the telescope in any way (since
it lies directly in the shadow of the secondary mirror), but it
will greatly facilitate collimating when using the supplied col-
limation cap or other, more sophisticated collimation devices,
such as the Orion LaserMate Deluxe II laser collimator.
Preparing the telescope for collimating
Once you get the hang of collimating, you will be able to do it
quickly even in the dark. For now, it is best to collimate in day-
light, preferably in a brightly lit room and aimed at a white wall.
It is recommended that the telescope tube be oriented hori-
zontally. This will prevent any parts from the secondary mirror
from falling down onto the primary mirror and causing dam-
age if something comes loose when you are making adjust-
ments. Place a sheet of white paper inside the optical tube
directly opposite the focuser. This will provide a bright “back-
ground” when viewing into the focuser. When properly set up
for collimation, your telescope should resemble
Figure 29.
Figure 28.
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)
.
a.
Primary mirror
center mark
e.
d.
c.
b.
(XX12g
only)
