Calculating Magnification (Power); Field Of View - Celestron C90 Instruction Manual

VISUAL OBSERVING INFORMATION
Focusing
When working with any optical instrument, there are several focusing hints that will ensure that you
get the sharpest possible image. The hints listed here are for visual observing and photography.
Never look through glass. Glass found in household windows is optically imperfect and may vary in
thickness from one location
your spotting scope. In most cases, you will not be able to achieve a truly sharp focus. In some cases,
you may actually see a double image. This effect is more pronounced with compound (catadioptric)
lenses, like the C90, than with refractive lenses.
Avoid looking across or over objects that are producing heat waves. This includes asphalt parking lots
on hot summer days or
impossible to get a truly sharp focus.
Avoid hazy skies, fog, and mist, all of which can make it difficult to focus, thus greatly reducing the
amount of detail seen. Remember that no matter how good the optics of your instrument are, they can
NOT perform to their fullest under adverse conditions.

Calculating Magnification (Power)

You can change the magnification of your spotting scope simply by changing the eyepiece (also
called an ocular). To determine the magnification with your spotting scope, simply divide the
focal length of your spotting scope by the focal length of the eyepiece. In equation format, the
formula looks like this:
Focal Length of Spotting Scope (mm)
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Magnification =
Focal Length of Eyepiece (mm)
For example, to determine the magnification of the C90 with a 30mm eyepiece, divide the focal length of
the C90 (l000mm) by the focal length of the eyepiece (30mm). 1000 divided by 30 yields 33 power.
Although the power is variable, each instrument 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, in a 3.5" diameter
spotting scope, such as the C90, the maximum useful magnification is 210 power. This is derived from
multiplying 60 times 3.5. 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 for the C90 is 70 to 105.

Field of View

Determining the field of view is important if you want to get an idea of the 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:
to the next. This inconsistency can and will affect the ability to focus
building rooftops. The turbulent air caused by these conditions makes it