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6. collimating the optics
(aligning the Mirrors)
Collimation is the process of adjusting the mirrors so they are
perfectly aligned with each other. Your telescope’s optics were
aligned at the factory, and should not need much adjustment
unless the telescope was roughly handled during shipment.
Accurate alignment is important to insuring the peak perfor-
mance of your telescope, so it should be checked occasionally.
Collimation is easy to do and should be done in daylight.
To check the collimation, remove the eyepiece and look down
the focuser drawtube. You should see the secondary mirror
centered in the drawtube, 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 in Figure 4D. 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 (Orion
#3640). We highly recommend you purchase one.
aligning the secondary Mirror
With eyepiece removed, look straight down the open focuser
drawtube at the secondary (diagonal) mirror. Ignore the reflections
for the time being. The secondary mirror 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 pointed
toward a bright surface, such as white paper or a wall.)
If the secondary mirror is not centered in the focuser draw-
tube (in the direction parallel to the length of the telescope),
loosen the three small alignment screws in the center hub of
the secondary mirror holder several turns with the provided
Allen wrench. Now hold the secondary mirror stationary (be
careful not to touch the surface of the secondary mirror!),
while turning the central Phillips-headed bolt. Turning the bolt
clockwise will move the secondary mirror towards the front
opening of the optical tube, while turning the bolt counter-
clockwise will move the secondary mirror towards the primary
mirror. When the secondary mirror is centered in the focuser
drawtube (as in Figure 4B), rotate the secondary mirror holder
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 alignment screws to secure the secondary mirror in that
position. This adjustment will rarely need to be done, if ever.
If the entire primary mirror reflection is not visible in the sec-
ondary mirror (as it is not in Figure 4B), adjust the tilt of the
secondary mirror by alternately loosening one of the three
alignment screws of the secondary mirror holder a turn or two
and tightening another one. Use the provided Allen wrench to
do this. The goal is to center the primary mirror reflection in
the secondary mirror, as depicted in Figure 4C. Don’t worry
that the reflection of the secondary mirror (the smallest circle,
with your eye reflected in it) is off-center (as also is the case
in Figure 4C); 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 4C, 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 knurled
thumbscrews on the back end of the optical tube (bottom of
the mirror cell). The Phillips-head screws next to each of the
three knurled thumbscrews serve to lock the mirror in place
once the tilt has been adjusted. Start by loosening each of
these Phillips-head screws a few turns. Now, adjust the tilt of
the mirror by turning one of the knurled thumbscrews either
clockwise or counterclockwise. Look into the focuser and see
if the secondary mirror reflection has moved closer to the
center of the primary mirror reflection. If not, try turning the
knurled thumbscrew in the opposite direction. 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 center the reflection. (It helps to have two
people for primary mirror collimation, one to look in the
focuser while the other adjusts the collimation thumbscrews.)
When the reflection of the secondary mirror (and your eye)
are centered, retighten the three Phillips-head screws.
The view through the Collimating Eyepiece should now
resemble Figure 4D. 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 and accu-
rately center it in the eyepiece’s field-of-view with the R.A. and
Dec. slow-motion controls. Slowly rack the image out of focus
with the focusing knob. If the telescope is correctly collimated,
the expanding disk should be a perfect circle. If it is unsym-
metrical, 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 doughnut. If the
“hole” appears off-center, the telescope is out of collimation.
7. using Your 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,
