Appendix A: Collimation (Aligning The Mirrors) - Orion XT4.5 Instruction Manual

Appendix A:
Collimation
(Aligning the
Mirrors)
Collimation is the process of adjust-
ing the mirrors so they are perfectly
aligned with one another. Your tele-
scope's optics were aligned at the
factory, and should not need much
adjustment unless the telescope is
handled roughly. Accurate mirror
alignment is important to ensure the
peak performance of your telescope,
so it should be checked regularly.
Collimation is relatively easy to do
and can be done in daylight.
To check collimation, remove the eye-
piece and look down the focuser
drawtube. You should see the sec-
ondary mirror centered 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 pri-
mary mirror, as in Figure 16a. If
anything is off-center, as in Figure
16b, proceed with the following colli-
mation procedure.
The Collimation Cap and
Mirror Center Mark
Your SkyQuest XT4.5 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 silver bottom. This
helps center your eye so that colli-
mation is easy to perform. Figures
16b-e assume you have the collima-
tion cap in place.
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 center of the mirror
is. You simply adjust the mirror posi-
tion (described below) until the reflection of the hole in the collimation
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 pri-
mary 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
14
a.
b.
d.
Figure 16. 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).
c.
e.
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 secondary
mirror, as in Figure 16c, you will need to adjust the tilt of the second-
ary mirror. This is done by alternately loosening one of the three
alignment set screws while tightening the other two, as depicted in
Figure 18. The goal is to center the primary mirror reflection in the
secondary mirror, as in Figure 16d. Don't worry that the reflection of
optical performance of the tele-
scope 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 centered in the focuser
drawtube, in the direction parallel to
the length of the telescope. If it isn't,
as in Figure 16b, it must be adjusted.
It helps to adjust the secondary mir-
ror in a brightly lit room with the
telescope pointed towards a bright
surface, such as white paper or wall.
Placing a piece of white paper in the
telescope tube opposite the focuser
(in other words, on the other side of
the secondary mirror) will also be
helpful in collimating the secondary
mirror. Using a 2mm Allen wrench,
loosen the three small alignment set
screws in the center hub of the 4-
vaned spider several turns. Now
hold the secondary mirror holder
stationary (be careful not to touch
the mirror's surface), while turning
the center screw with a Phillips head
screwdriver (See Figure 17). 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
making these adjustments, be care-
ful not to stress the spider vanes or
they may bend.
When the secondary mirror is cen-
tered in the focuser drawtube, 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 cen-
tered, but that is OK. Now tighten the
three small alignment screws equally