M46

About

A rich open cluster in Puppis of about 500 stars, roughly 4,900 light-years away and 250 million years old. The planetary nebula NGC 2438 appears inside its northern edge, though it is likely unrelated because its velocity differs.

Imaging tips

• About 27′ across: 900–1,500 mm on APS-C frames it with room around it.
• NGC 2438 lies 4.9′ from the centre in the same field. It is small and bright, so one imager advises watching it throughout processing to avoid overexposing it.
• M47 is about a degree away; one team fitted both clusters in one field up to about 800 mm, placing each in an opposite corner. They used 3 hours of 5-minute subs on a one-shot colour camera.
• Low for northern observers: it stays below 25° from 50°N, so choose a clear southern horizon. Best in January.