Shift-and-add: finding faint moons

The technique behind Halimede’s 2002 discovery.

Published 2026-10-23

Halimede is magnitude 24 — far too faint for single images.^[1]

The technique

Holman’s team took many long exposures and aligned them to Neptune’s motion, so stars became streaks while moons became points of light.^[1][2]

Three at once

Halimede was found with Sao and Neso in Blanco Telescope images from 14 August 2002.^[1]

Follow-up

Follow-up observations used the VLT, Nordic Optical Telescope and Palomar’s Hale Telescope; precovery images went back to 1999.^[1]

The count

The announcement in January 2003 raised Neptune’s known moons to 11. Today there are 16.^[1][5]

Numbered citations link to the sources list on the Halimede overview page.