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Posted by Robert May on November 9, 2003 00:54:27 UTC

The first thing is the aperture of the scopes. Larger apertures will mean being able to see fainter objects and at higher magnification.
The general rule of thumb is that you can use 2X diameter (mm) of the objective as the max usable power of the system. This means that the 600x magnification of the 60mm is not really valid but rather 120 power is more like the max.
Next is the focal length of the telescope and that determines the image scale of the views. The radius of the aperture stop of the EP over the focal length is the tan of the angle that will be seen of the sky times two (we're interested in the diameter, not the radius but you have to do the radius for the correct angle) and that is another physical limit. Shorter focal lengths have a smaller image scale and you can see more of the sky with the same EP as the magnification will be less. Magnification is the focal length of the objective over the focal length of the EP.

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