Celestron G-9.25 - Calculating Magnification; Determining Field of View

24 • Telescope Basics
You can change the power of your Celestron G-9
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telescope just by changing the
eyepiece (ocular). To determine the magnification for your telescope, you would
simply divide the focal length of the telescope (2350mm) by the focal length of the
eyepiece that you are using. In equation format, the formula looks like this:
Focal Length of Telescope (mm)
Magnification = ————————————————
Focal Length of Eyepiece (mm)
Let’s say, for example, that you are using a 26mm eyepiece. To determine the
magnification, simply divide the focal length of your G-9
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(2350mm) by the focal
length of the eyepiece (26mm). Dividing 2350 by 26 yields a magnification of 90
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, the G-9
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is 9.25" in diameter. Multiplying 9.25 by 60 gives a
maximum useful magnification of 555 power. Although this is the maximum useful
magnification, due to limiting atmospheric conditions most observing is done in the
range of 20 to 35 power for every inch of aperture which is 185 to 324 power for the G-
9
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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:
Apparent Field of Eyepiece
True Field = ———————————————
Magnification
Using the example we started with above, we can determine the field of view using the
same 26mm eyepiece. The 26mm eyepiece has an apparent field of view of 50°.
Divide the 50° by the magnification, which is 90 power. This yields an actual field of
.55°, or a little more than a half of a degree.
For terrestrial viewing, field size is often referred to as feet at a thousand yards. To
convert this to feet at one thousand yards, multiply the actual field of .55° by 52.5.
This produces a field width of 29 feet at one thousand yards.
The apparent field of each eyepiece that Celestron manufacturers is found in the
Celestron Accessory Catalog (#93685).
Calculating
Magnification
Determining Field of
View