Celestial Observing Tips; Field Of View - Celestron POPULAR SCIENCE AstroMaster 80AZS Instruction Manual

CELESTIAL OBSERVING TIPS

This section covers visual observing hints for both solar system and deep-sky objects as well as general observing
conditions which will affect your ability to observe.
MAGNIFICATION
You can change the magnification of your telescope by simply changing eyepieces. Both telescopes and eyepieces
have focal lengths that are measured in millimeters. To calculate the magnification of a given telescope and eyepiece
combination, take the focal length of the telescope and divide it by the focal length of the eyepiece. The resulting number
is the magnification factor.
For example, the AstroMaster 80AZS has a focal length of 1000mm. Using the 20mm eyepiece:
_________________________
Magnification =

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 it gives you in the scope you are using.
Using the example in the previous section, we can determine the field of view using the same telescope and 20mm
eyepiece. The 20mm eyepiece has an apparent field of view of 40°.
Actual Field Of View =
The 20mm eyepiece has an apparent field of view of 40°. Divide the 40° by the magnification, which is 50 power.
This yields an actual field of view of 0.9 degrees or 54 minutes of arc.
10
Focal length of Telescope
Focal length of Eyepiece
Apparent Field of View of Eyepiece
________________________________
Magnification
1000mm
____________ = 50x
=
20mm
40°
______ = 0.9 degrees of arc
=
50