34
Figure 4-3 -
The emblem on
the end of the focus knob
shows the correct rotational
direction for focusing your
telescope
.
Focusing
The Schmidt-Cassegrain 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 (approximately 60 feet) 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.
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-S 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.
Although the power is variable, each instrument under average skies has a limit to the highest useful magnification. The
general rule is that 60 power can be used for every inch of aperture. For example, the C8-S is 8 inches in diameter.
Multiplying 8 by 60 gives a maximum useful magnification of 480 power. Although this is the maximum useful
magnification, most observing is done in the range of 20 to 35 power for every inch of aperture which is 160 to 280 times for
the C8-S telescope.
Determining Field of View
Determining the field of view is important if you want to get an idea of the angular size of the object you are observing. To
calculate the actual field of view, divide the apparent field of the eyepiece (supplied by the eyepiece manufacturer) by the
magnification. In equation format, the formula looks like this:
Apparent Field of Eyepiece
True Field =
Magnification
As you can see, before determining the field of view, you must calculate the magnification. Using the example in the
previous section, we can determine the field of view using the same 40mm eyepiece. The 40mm Plossl eyepiece has an
apparent field of view of 46°. Divide the 46° by the magnification, which is 51 power. This yields an actual field of .9°, or
nearly a full degree.
To convert degrees to feet at 1,000 yards, which is more useful for terrestrial observing, simply multiply by 52.5. Continuing
with our example, multiply the angular field .9° by 52.5. This produces a linear field width of 47 feet at a distance of one
