There isn’t a simple fraction like “1/3 moon is OK.” For astrophotography, the effect depends on what you’re shooting and conditions such as the Moon’s altitude, haze/water vapor, particulates, and how close the Moon is to your framing.

General rule:

• For faint deep-sky subjects and Milky Way shots: ideally no Moon above the horizon.
• For brighter targets like planets: moonlight matters less, so “it depends.”

Moonlight acts like light pollution: it brightens the sky background, lowers contrast, and adds noise, which means you need more total exposure time to get a clean result. A full moon is especially bad for wide-field star and Milky Way images.

Best practice is to shoot under the darkest skies possible, away from city lights, and aim for nights when the Moon is below the horizon or near new moon if you want the best star-field detail.