# 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](https://astropilot.space/glossary/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.

**Related:** [Magnification](https://astropilot.space/glossary/magnification.md), [Seeing](https://astropilot.space/glossary/seeing.md), [Resolution](https://astropilot.space/glossary/resolution.md), [Exit pupil](https://astropilot.space/glossary/exit-pupil.md)

From the AstroPilot astronomy glossary: https://astropilot.space/glossary/max-magnification