11
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 sec-
ondary mirror. Using an Allen wrench, loosen the three small
alignment setscrews in the center hub of the 3-vaned spider
several turns. Now hold the mirror holder stationary (be careful
not to touch the surface of the mirrors), while turning the center
larger Allen setscrew (
Figure 16
). 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 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 set screws equally to secure
the secondary mirror in that position.
If the entire primary mirror reflection is not visible in the sec-
ondary mirror, as in
Figure 15c
, 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 an equal amount. The goal is to center the primary mir-
ror reflection in the secondary mirror, as in
Figure 15d
. Don’t
worry that the reflection of the secondary mirror (the smallest
circle, with the collimation cap “dot” in the center) is off-center.
You will fix that in the next step. Be sure that the final adjust-
ment of the secondary tilt adjustment screws is to TIGHTEN
the screw. You cannot leave any of the screw loose, otherwise
the secondary mirror will not be secure and will wobble out of
alignment.
Aligning the Primary Mirror
The final adjustment is made to the primary mirror. It will need
adjustment if, as in
Figure 15d
, the secondary mirror is cen-
tered 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 collimation cap) is
off-center.
The tilt of the mirror is adjusted with three pairs of collimation
screws (
Figure 17
). The collimation screws can be turned by
hand without the need of a tool.
Each pair of collimation screws work together to adjust the tilt
of the primary mirror. The knob with the flat head screwdriver
slot cut into it is the locking screw, and the larger thumb knob
is the actual adjustment screw. First, back off all three locking
screws a turn or two (do not completely remove these screws).
Then try tightening or loosening one of the adjustment screws
one half to one turn. Look into the focuser and see if the sec-
ondary mirror reflection has moved closer to the center of the
primary. You can tell this easily with the collimation cap and
mirror center mark by simply watching to see if the “dot” of the
collimation cap is moving closer or farther away from the ring
on the center of the primary mirror. Repeat this process on
the other two collimation adjustment screws, if necessary. It
will take a little trial and error to get a feel for how to adjust the
mirror to center the “dot” of the collimation cap in the ring of the
mirror mark.
When you have the dot centered as much as possible in the
ring, your primary mirror is collimated. The view through the
collimation cap should resemble
Figure 15e
. Make sure to
then re-tighten all the primary collimation locking screws (but
do not overtighten), to secure the mirror tilt.
A simple star test will tell you whether the optics are accurately
collimated.
Star-Testing the Telescope
When it is dark, point the telescope at a bright star and accu-
rately center it in the eyepiece’s field of view. Slowly de-focus
the image with the focusing knob. If the telescope is correct-
ly collimated, the expanding disk should be a perfect circle
(
Figure 18
). If the image is unsymmetrical, the scope is out
of collimation. The dark shadow cast by the secondary mirror
should appear in the very center of the out-of-focus circle, like
the hole in a donut. If the “hole” appears off-center, the tele-
scope is out of collimation.
If you try the star test and the bright star you have selected is
not accurately centered in the eyepiece, the optics will always
appear out of collimation, even though they may be perfectly
aligned. It is critical to keep the star centered, so over time you
will need to make slight corrections to the telescope’s position
in order to account for the sky’s apparent motion.
X. Using Your Telescope—
Astronomical Observing
Choosing an Observing Site
When selecting a location for observing, get as far away as
possible from direct artificial light such as street lights, porch
lights, and automobile headlights. The glare from these lights
will greatly impair your dark-adapted night vision. Set up on
a grass or dirt surface, not asphalt, because asphalt radiates
more heat. Heat disturbs the surrounding air and degrades the
images seen through the telescope. Avoid viewing over roof-
tops and chimneys, as they often have warm air currents rising
from them. Similarly, avoid observing from indoors through an
open (or closed) window, because the temperature difference
between the indoor and outdoor air will cause image blurring
and distortion.
If at all possible, escape the light-polluted city sky and head
for darker country skies. You’ll be amazed at how many more
stars and deep-sky objects are visible in a dark sky!
“Seeing” and Transparency
Atmospheric conditions vary significantly from night to night.
“Seeing” refers to the steadiness of the Earth’s atmosphere at
a given time. In conditions of poor seeing, atmospheric turbu-
lence causes objects viewed through the telescope to “boil”. If,
when you look up at the sky with just your eyes, the stars are
twinkling noticeably, the seeing is bad and you will be limited
to viewing with low powers (bad seeing affects images at high
powers more severely). Planetary observing may also be poor.
In conditions of good seeing, star twinkling is minimal and
images appear steady in the eyepiece. Seeing is best over-
head, worst at the horizon. Also, seeing generally gets better
