7
Figure 2-3 -
The emblem on
the end of the focus knob
shows the correct rotational
direction for focusing your
telescope
.
Focusing
Your telescope's focusing mechanism controls the primary mirror which is mounted on a ring
that slides back and forth on the primary baffle tube. The focusing knob, which moves the
primary mirror, is on the rear cell of the telescope just below the star diagonal and eyepiece.
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. A single turn of the focusing knob moves the
primary mirror only slightly. Therefore, it will take many turns (about 30) to go from close
focus to infinity.
For astronomical viewing, out of focus star images are very diffuse, making them difficult to
see. If you turn the focus knob too quickly, you can go right through focus without seeing the
image. To avoid this problem, 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.
Mirror Support Clutches
The EdgeHD Optical tube is equipped with mirror tension
clutches to help support and minimize lateral movement of
the primary mirror during astrophotography.
To use the mirror clutches:
1.
Use the focus knob to adjust the primary mirror
to the desired focus.
2.
Once in focus, turn the two mirror lock knobs
clockwise until both are very tight and can be
turned no further.
Warning!
Once the mirror is locked down, do not turn
the focuser knob without loosening the mirror locks first.
Although turning the focus knob should not damage the
telescope, undue stress can be placed on the focus mechanisms causing excessive image shift while focusing.
Calculating Magnification
You can change the power of your telescope just by changing the eyepiece (ocular). To determine the magnification of your
telescope, simply divide the focal length of the telescope by the focal length of the eyepiece used. In equation format, the
formula looks like this:
Focal Length of Telescope (mm)
Magnification =
Focal Length of Eyepiece (mm)
Let’s say, for example, you are using the 40mm Plossl eyepiece. To determine the magnification you simply divide the focal
length of your telescope (the C8 for example has a focal length of 2032mm) by the focal length of the eyepiece, 40mm.
Dividing 2032 by 40 yields a magnification of 51 power.
Mirror
Lock
Knobs
Figure 2-4 – Mirror Support Clutches
