Celestron Onyx80EDF Manuale d'uso - Pagina 3
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Using the example we started with in the previous section, we can determine the field of view using the
same 25mm eyepiece. The 20mm eyepiece has an apparent field of view of 50°. Divide the 50° by the
magnification, which is 25 power. This yields an actual field of 2°. To convert this to feet at one thousand
yards, multiply the actual field of 2° by 52.5. This gives us a field width of 105 feet at one thousand yards.
PHOTOGRAPHY HINTS
Metering
Celestron telescopes 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
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.
ASTRONOMY WITH YOUR TELESCOPE
Observing the Moon
In the night sky, the Moon is a prime target for your first look because it is extremely bright and easy to
find. Often, it is tempting to look at the Moon when it is full. At this time, the face we see is fully
illuminated and its light is overpowering. In addition, little or no contrast can be seen at this time.
One of the best times to observe the Moon is during its partial phases (around the time of first or third
quarter). Long shadows reveal a wealth of detail on the lunar surface. At low power you can see the entire
lunar disk at one time. Change to higher power (magnification) to focus in on a smaller area.
Keep in mind that since you are not using a clock drive, the Earth's rotation causes the Moon to drift out of
your field of view. This effect is more noticeable at high power. You must manually adjust your tripod to
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-