10
mary mirror centered in the sec-
ondary mirror, and the reflection
of the secondary mirror (and your
eye) centered in the reflection of
the primary mirror, as in Figure
8a. If anything is off-center, pro-
ceed with the following collimat-
ing procedure.
The Collimation Cap and
Mirror Center Mark
Your ShortTube 4.5 EQ comes
with a collimating 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 collimating is
easy to perfor m. Figures 8b
through 8e assume you have the
collimation cap in place.
In addition to providing the colli-
mation 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 position (described below)
until the reflection of the hole in
the collimation cap is centered
inside the ring.
NOTE: The center ring sticker
need not ever be removed
from the primar y mirror.
Because it lies directly in the
shadow of the secondary mir-
ror, its presence in no way
adversely affects the optical
performance of the telescope
or the imag e 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 cen-
tered in the focuser drawtube, in the direction parallel to the
length of the telescope. If it isn’t, as in Figure 8b, it must be
adjusted. Typically, this adjustment will rarely, if ever, need to
be done. It helps to adjust the secondary mirror in a brightly
lit room with the telescope pointed toward a bright surface,
such as white paper or wall. Placing a piece of white paper in
the telescope tube opposite the
focuser (i.e., on the other side of
the secondary mirror) will also be
helpful in collimating the second-
ary mirror. Using a 2.5mm Allen
wrench, loosen the three small
alignment setscrews in the cen-
ter hub of the 3-vaned spider
several turns. Now hold the mir-
ror holder stationary (be careful
not to touch the surface of the
mirrors), while turning the center
screw with a Phillips head screw-
driver (Figure 9). Turning the
screw clockwise will move the
secondar y mirror toward the
front opening of the optical tube,
while turning the screw counter-
clockwise will move the second-
ary mirror toward the primary
mirror.
When the secondary mirror is
centered in the focuser draw-
tube, 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 centered,
but that is OK. Now tighten the
three small alignment screws
equally to secure the secondary
mirror in that position.
If the entire primary mirror reflec-
tion is not visible in the second-
ary mirror, as in Figure 8c, you
will need to adjust the tilt of the
secondary mirror. This is done by
alternately loosening one of the
three alignment setscrews while
tightening the other two, as
depicted in Figure 10. The goal is
to center the pr imar y mirror
reflection in the secondary mir-
ror, as in Figure 8d. Don’t worry
that the reflection of the second-
ary mirror (the smallest circle,
with the collimation cap “dot” in
the center) is off-center. You will
fix that in the next step.
Aligning 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 colli-
mation cap) is off-center.
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).
a.
b.
c.
d.
e.
Summary of Contents for ShortTube 4.5 EQ 9849
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