Telescope Magnification Calculator: Work Out Your Real Power

Magnification is not a property of the telescope, it is decided by the eyepiece you put in it. Enter your telescope's focal length and the eyepiece, and this tells you the real magnification, whether it is sensible for your aperture, and the exit pupil. The trap to avoid: pushing past roughly twice your aperture in millimetres just gives you a bigger, dimmer, blurrier image.

Two limits bracket the useful range. The maximum useful magnification is about twice the aperture in millimetres (a 130mm scope tops out near 260x); beyond that the image gets larger but no more detailed, just dimmer and softer. The minimum useful magnification is where the exit pupil grows past about 7mm, the widest a dark-adapted eye opens, so light spills around your pupil and is wasted. Most of your best views sit between those two, often around 25x to 150x.