Telescope Basics • 23
Each of the Celestron 8 telescopes uses the same focusing mechanism. The
primary mirror is mounted on a ring which slides back and forth on the primary
baffle tube (see figure 4-5). The focusing knob, which moves the primary
mirror, is on the rear cell of the telescope. To focus, turn the focusing knob
until the image is sharp. If the knob will not turn, it has reached the end of its
travel on the focusing mechanism. Turn the knob in the opposite direction until
the image is sharp. Once an image is in focus, turn the knob clockwise to
focus on a closer object and counterclockwise for a more distant object (see
figure 4-4). A single turn of the focusing knob moves the primary mirror only
slightly. Therefore, it will take many turns (about 40) to go from close focus
(approximately 25 feet) to infinity.
For astronomical viewing, out of focus star images are very diffuse making
them difficult, if not impossible, to see. If you turn the focus knob too quickly,
you can go right through focus without seeing the image. To avoid this prob-
lem, your first astronomical target should be a bright object (like the Moon or a
planet) so that the image is visible even when out of focus.
Critical focusing is best accomplished when the focusing knob is turned in
such a manner that the mirror moves against the pull of gravity. In doing so,
any mirror shift is minimized. For astronomical observing, both visually and
photographically, this is done by turning the focus knob counterclockwise.
Focusing
Figure 4-4
The decal on the end of the
focus knob shows the
correct rotational direction
for focusing the C8.
Figure 4-5
This diagram shows the focusing mechanism of the C8 telescopes.
