Calculating Magnification; Determining Field Of View - Celestron 31055 AstroMaster 130AZ Instruction Manual

CALCULATING MAGNIFICATION

You can change the power of your telescope just by changing the eyepiece (ocular) To determine the magnification of
your telescope, simply divide the focal length of the telescope by the focal length of the eyepiece used In equation format,
the formula looks like this:
Focal Length of Telescope (mm)
Magnification = ______________________________
Focal Length of Eyepiece (mm)
Let's say, for example, you are using the 20 mm eyepiece that came with your telescope To determine the magnification
you simply divide the focal length of your telescope (the AstroMaster 130AZ for this example has a focal length of 660
mm) by the focal length of the eyepiece, 20 mm Dividing 660 by 20 yields a magnification of 33 power
Although the power is variable, each instrument under average skies 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 AstroMaster 130AZ is 5 inches in
diameter Multiplying 5 by 60 gives a maximum useful magnification of 300 power 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 is 100 to 175 times
for the AstroMaster 130AZ telescope You can determine the magnification for your telescope the same way

DETERMINING FIELD OF VIEW

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 eyepiece manufacturer)
by the magnification In equation format, the formula looks like this:
Apparent Field of Eyepiece
_________________________
True Field =
Magnification
As you can see, before determining the field of view, you must calculate the magnification Using the example in the
previous section, we can determine the field of view using the same 20 mm eyepiece that is supplied standard with
the AstroMaster 130AZ telescope The 20 mm 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°
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 5° by 52 5 This produces a linear field width of 78 75 feet at a
distance of one thousand yards
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