Celestron CGEM II 1100 EdgeHD Mode D'emploi page 4

can greatly enlarge the image, but remember that the
contrast and brightness will be low due to the high
magnification. High powers can be used occasionally
when conditions allow – you will achieve the power
d f
eTeRmiNiNg ield of
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 manufacturer) by the
Magnification =
magnification. In equation format, the formula looks like this:
True (Actual) Field of View =
g o
eNeRal bSeRviNg
When using any optical instrument, there are a few things to
remember to ensure you get the best possible image.
• Never look through window glass. Glass found in household
windows is optically imperfect, and as a result, may vary
in thickness from one part of a window to the next. This
inconsistency can and will affect the ability to focus your
telescope. In most cases you will not be able to achieve a
truly sharp image, while in some cases, you may actually see
a double image.
• Never look across or over objects that are producing heat
waves. This includes asphalt parking lots on hot summer days
or building rooftops.
©2011 Celestron • All rights reserved.
v
iew
Focal Length of Telescope (mm)
Focal Length of Eyepiece (mm)
Apparent Field of Eyepiece
Magnification
h
iNTS
but the image will be dark with low contrast because
you have magnified the object to considerably. For the
brightest images with the highest contrast levels, use
lower powers.
As you can see, before determining the field of view, you must
calculate the magnification. Using the example above, we can
determine the field of view using the same 25 mm eyepiece.
The 25 mm eyepiece has an apparent field of view of 50°.
Divide the 50° by the magnification, which is 40 power. This
yields an actual field of 1.25°.
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 of 1.25° by 52.5
and this produces a linear field width of 65.6 feet at a distance
of one thousand yards. The apparent field of each eyepiece
that Celestron supplies with its telescopes can be found under
specifications for the particular telescope model on
Celestron's website.
• Hazy skies, fog, and mist can also make it difficult to focus
when viewing terrestrially. The amount of detail seen under
these conditions is greatly reduced.
• If you wear corrective lenses (specifically glasses), you may
want to remove them when observing with an eyepiece
attached to the telescope. When using a camera, however,
you should always wear corrective lenses to ensure the
sharpest possible focus. If you have astigmatism, corrective
lenses must be worn at all times.
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