7
secondary mirror (and your eye) centered in the reflection of
the primary mirror, as is Figure 3D (see pg. 11 for figures 3A-D).
If anything is off-center, follow the collimation procedure below.
It helps to put a piece of white paper on the inside of the opti-
cal tube opposite the focuser. The white paper forms a bright
background behind the secondary mirror, making it easier to
distinguish the mirror holder from the background.
Use a Collimation Tool
To aid in centering your line of sight down the focuser draw-
tube, and in centering the mirror reflections during collimation,
it is very helpful to use a precision collimating tool containing
crosshairs, such as the Orion Collimating Eyepiece #3640.
We highly recommend you purchase one.
Aligning the Secondary Mirror
With eyepiece removed, look down the open focuser drawtube
at the secondary (diagonal) mirror. It should be centered in the
field of view. If it isn’t, it must be adjusted. (It helps to adjust the
secondary mirror in a brightly lit room with the telescope point-
ed toward a bright surface, such as white paper or a wall.)
If the secondary mirror is above or below center (in the direction
perpendicular to the length of the telescope, as in Figure 3A),
adjust the three-vane spider by turning the spider vane adjust-
ment thumbscrews on the outside of the optical tube. Loosen
one, then tighten another one the corresponding amount.
If the secondary mirror is to the left or right of center (in the
direction parallel to the length of the telescope) in the focuser
drawtube, loosen the three small Phillips-head alignment
screws in the center hub of the spider a few turns. Now rotate
the cylindrical secondary mirror holder forward or back on the
large center screw, while holding the screw head with a Phillips
screwdriver. When the mirror is centered in the focuser draw-
tube (as in Figure 3B), rotate it slightly side to side until the
reflection of the primary mirror is as centered in the secondary
mirror as it will get. It still may not be perfectly centered yet, but
that is OK. Now tighten the three small Phillips-head alignment
screws to secure the secondary mirror in that position.
If the entire primary mirror reflection still is not visible in the
secondary mirror (as it is not in Figure 3B), adjust the tilt of
the secondary mirror further by alternately loosening one of
the three alignment screws a turn or two and tightening
another one. The goal is to center the primary mirror reflec-
tion in the secondary mirror, as depicted in Figure 3C. Don’t
worry that the reflection of the secondary mirror (the smallest
circle with your eye in it) and spider are off-center (as also is
the case in Figure 3C); you will fix that in the next step.
Adjusting the Primary Mirror
The final adjustment is made to the primary mirror. It will need
adjustment if, as in Figure 3C, 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 your eye inside) is off-center.
The tilt of the primary is adjusted with the three collimation
thumbscrews on the back end of the optical tube. The
Phillips-head screws adjacent to the collimation screws lock
the mirror cell in place. Adjusting the tilt requires a “push-pull”
technique involving adjustment of one collimation thumb-
screw and its corresponding lock screw at a time. Loosen one
of the lock screws one full turn, and then tighten the adjacent
collimation thumbscrew until it is finger-tight. Then look into
the focuser and see if the secondary mirror reflection has
moved closer to the center of the primary mirror reflection. Or,
loosen the collimation thumbscrew one turn and tighten the
adjacent lock screw. Repeat this process on the other two
collimation screws, if necessary. It will take a little trial and
error to get a feel for how to tilt the mirror in this way to cen-
ter the reflection. (It helps to have two people for primary
mirror collimation, one to look in the focuser while the other
adjusts the collimation bolts.) When the adjustment is com-
plete, make sure the three Phillips-head lock screws are tight
(but do not overtighten), to secure the mirror tilt.
The view through the Collimating Eyepiece should now
resemble Figure 3D. The secondary mirror is centered in the
focuser; the reflection of the primary mirror is centered in the
secondary mirror, and the reflection of the secondary mirror is
centered in the reflection of the primary mirror.
A simple star test will tell you whether the optics are accu-
rately collimated.
Star-Testing Your Telescope
When it is dark, point the telescope at a bright star. Look into the
eyepiece and slowly rack the image out of focus with the focus-
ing knob. If the telescope is correctly collimated, the expanding
disk should be a perfect circle. If it is unsymmetrical, the scope is
out of collimation. In reflectors and Schmidt-Cassegrains, 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 doughnut.
If the “hole” appears off-center, the telescope is out of collimation.
7. Using the Telescope—
Astronomical Observing
Choosing an Observing Site
When selecting a location for observing, get as far away as pos-
sible from direct artificial light such as streetlights, porch lights,
and automobile headlights. The glare from these lights will great-
ly impair your dark-adapted night vision. Set up on a grass or dirt
surface, not asphalt, because asphalt radiates more heat, which
disturbs the surrounding air and degrades the images seen
through the telescope. Avoid viewing over rooftops and chim-
neys, 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!
Cooling the Telescope
All optical instruments need time to reach “thermal equilibri-
um.” The bigger the instrument and the larger the
temperature change, the more time is needed. Allow at least
