Maximum useful magnification
Also called: highest useful power, empty magnification.
About twice the aperture in millimetres (50× per inch) — but the air rarely allows more than 250–350×.
Beyond about 2× the aperture in mm, a telescope shows no new detail: the image only gets bigger and dimmer (“empty magnification”). In practice the seeing sets the ceiling — most nights allow 150–250×, exceptional nights 300× or more, whatever the telescope.
At the eyepiece
Start at medium power and step up only while the view keeps improving.
In AstroPilot
AstroPilot avoids suggesting powers above what your telescope and typical seeing support.