10
Adjusting the Primary Mirror
The final adjustment is made to the primary mirror.
It will need adjustment if, as in Figure 12d, 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 your eye inside) is off-center.
The tilt of the primary mirror is adjusted with three
spring-loaded collimation thumb screws on the back
end of the optical tube (bottom of the primary mirror
cell); these are the larger thumb screws. The other
three smaller thumb screws lock the primary mirror’s
position in place; these thumb screws must be
loosened before any collimation adjustments can be
made to the primary mirror.
To start, unthread the thumb screws that lock the
primary mirror in place a few turns each (Figure 14).
The thumb screws are slotted, so if they are too
difficult to loosen with your fingers, use a flat-head
screwdriver.
Now, try tightening or loosening one of the spring-
loaded collimation thumb screws one turn. Look into
the focuser and see if the secondary mirror reflection
has moved closer to the center of the primary mirror
reflection. Repeat this process on the other two
collimation thumb 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.) Do not loosen (i.e., rotate
counter-clockwise) each collimation thumb screw too
much, or the thumb screw will completely unthread
from the mirror cell. Rather, try tightening the other
two collimation thumb screws. Once the secondary
mirror reflection is centered in the primary mirror
reflection, retighten the thumb screws that lock the
primary mirror’s position in place.
The view through the focuser should now show 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 wheth-
er the optics are accurately collimated.
Shows the Primary Mirror locking screws
Shows the collimation adjustment screws
Star-Testing the Telescope
When it is dark, point the telescope at a bright star
and accurately center it in the eyepiece’s field-of-
view. Slowly defocus the image with the focusing
knob. If the telescope is correctly collimated, the
expanding disk should be a perfect circle (Figure
16). 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 doughnut.
If the “hole” appears off-center, the telescope is out
of collimation.
If you try the star test and the bright star you have
selected is not accurately centered in the eyepiece,
then the optics will always appear out of collimation,
even though they may be perfectly aligned. It is crit-
ical to keep the star centered, so over time you will
need to make slight corrections to the tele-
scope’s position in order to account for the sky’s ap-
parent motion.
Shows Star images when testing collimation
