Reducing Vibration - Celestron C90 Instruction Manual

Apparent Field of Eyepiece
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True Field (in degrees) =
Magnification
Using the example we started with in the previous section, we can determine the field of view using the
same 30mm eyepiece. The 30mm eyepiece has an apparent field of view of 50°. Divide the 50° by the
magnification, which is 33 power. This yields an actual field of 1.5°, or a degree and a half. To convert this
to feet at one thousand yards, multiply the actual field of 1.5° by 52.5. This gives us a field width of 78.8
feet at one thousand yards.
PHOTOGRAPHY HINTS
Metering
Celestron spotting scopes have fixed apertures and, as a result, fixed f/ratios. To properly expose your
subjects photographically, you need to set your shutter speed accordingly. Most 35mm SLR cameras offer
through-the-lens metering which lets you know if your picture is under or overexposed. Adjustments for
proper exposures are made by changing the shutter speed. Consult your camera manual for specific
information on metering and changing shutter speeds.

Reducing Vibration

Releasing the shutter manually can cause vibrations, producing blurred photos. To reduce vibration when
tripping the shutter, use a cable release. A cable release keeps your hands clear of the camera and lens, thus
eliminating the possibility of introducing vibration. Mechanical shutter releases can be used, though air-
type releases are best.
Blurry pictures can also result from shutter speeds that are too slow. To prevent this, use films that produce
shutter speeds greater than 1/250 of a second when hand-holding the lens. If the lens is mounted on a
tripod, the exposure length is virtually unlimited.
Another way to reduce vibration is with the Vibration Suppression Pads (#93503). These pads
rest between the ground and tripod feet. They reduce the vibration amplitude and vibration time.