Calculating Magnification; Determining Field Of View; General Observing Hints - Celestron ASTROMASTER 90AZ 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 90AZ for this example has a focal length of
1000 mm) by the focal length of the eyepiece, 20 mm . Dividing 1000 by 20 yields a magnification of 50 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 90AZ is 3 .5 inches in
diameter . Multiplying 3 .5 by 60 gives a maximum useful magnification of 210 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 70 to 122 times for
the AstroMaster 90AZ 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 90AZ telescope . The 20 mm eyepiece has an apparent field of view of 50° . Divide the 50° by the
magnification, which is 50 power . This yields an actual field of 1 .0° .
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 .0° by 52 .5 . This produces a linear field width of 52 .5 feet at a distance of one
thousand yards .

GENERAL OBSERVING HINTS

When working with 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 .
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