Expert: Paul Wagner - 3/22/2004

Question
How does one determine the maximum useful magnification of a telescope?

In compound scopes do you have to correct for the effect of the central obstruction in calculations using the objective diameter as an input?

Answer
The useful magnification does depend on seeing conditions, as well as the quality of the scope.  That said, I would use an estimate of something between 30X and 50x the aperture in inches.  This doesn't really depend on the type of scope...and you don't need to subtract anything from compound scopes. It is just a nice general rule that helps beginners from getting too upset about the fact that their telescope won't show them the same images the Hubble scope does.

Paul Wagner