Maximum Useful Magnification:
The maximum useful magnification of a telescope is the highest magnification that a particular telescope can handle. This is a fairly theoretical limit, and in reality is usually much less. The highest useful magnification is more likely to be around 20-30 times the diameter, instead of 60. This limit of useful magnification is due to turbulence in the atmosphere, often referred to as “seeing“.
One thing to keep in mind is that the higher the magnification, the darker the image will be. Planets are most often viewed under high magnification, while DSO’s (deep space objects) are viewed using less magnification.
Max Useful Magnification = ( 60 ) × ( D_scope[inches] )
My telescope has a 130 mm diameter which is roughly 5 inches. Therefore…
Max Useful Magnification = ( 60 ) × ( 5 ) = 300x
Or more likely,
Max Useful Magnification = ( 20 ) × ( 5 ) = 100x
Lowest Useful Magnification:
The lowest useful magnification is the lowest magnification a telescope can utilize while having an image that is still resolvable to the human eye. Low magnification is great for observing large areas of rich star fields and for pulling details out of DSO’s. The general rule is 3 – 4 times the diameter of the telescope. Any less than this will cause the exit pupil of the eye piece to be larger than the pupil of your eye. The result is an image that your eye can’t adapt to.
Using the same telescope specifications as above ( 130 mm diameter ≈ 5 in ).
Lowest Useful Magnification = ( 4 ) × ( 5 ) = 20x
What this means is that it would not be wise to use an eyepiece with this telescope that provides less than 20x magnification.