The Double Cluster, slowly
Monday 20 October 2025 · Perseus
| Date | 2025-10-20 |
|---|---|
| Time (UTC) | 20:35 – 22:05 |
| Instrument | 200 mm f/6 Dobsonian; 10×50 binoculars |
| Eyepieces | 32 mm Plossl (37×), 24 mm (50×), 13 mm (92×) |
| Seeing | Antoniadi II |
| Transparency | 5/5, after a day of north-westerly showers |
| Sky | SQM 21.3 at zenith; NELM about 6.2 |
| Moon | Thin crescent, not up until morning |
A showery day that cleared to the kind of sky you get after rain has washed everything out of the air. I’ve looked at the Double Cluster more times than I can count — it is the first thing I show anyone — but I realised I had never written down more than “lovely, as ever”. So tonight the rule was one object, an hour and a half, no hopping off to anything else.
Naked eye, a brighter knot in the Milky Way between the W of Cassiopeia and the top of Perseus, about 58° up in the north-east when I started. Obvious tonight, not just averted vision.
10×50: two separate patches side by side, the western one (NGC 869) a little smaller and brighter. Already starting to resolve at the edges. Chains of fainter stars trail away northwards.
37× in the 32 mm is the view. Both clusters sit in the field with room around them, and there is a curving arc of brightish stars running between them like a bridge. At this power they look like a pair; at higher power they look like two different things.
92×, one at a time. NGC 869 is the richer and more compact of the two, with a tight, bright middle where the stars crowd together and a near-empty gap just off the centre that I keep noticing and never see mentioned. NGC 884 to the east is looser and larger, and has the colour: I counted five orange or ruddy stars scattered across it, one quite deep, against a field that is otherwise blue-white. NGC 869 has a couple of warm stars too, but they don’t jump out the same way.
Finished with Stock 2, a couple of degrees north, which at 37× is a big sprawl of bright stars that fills most of the field and is easier in the finder than in the eyepiece. People call it the Muscleman Cluster. I can see why if I squint.
Next timeTry the 9 mm on the core of NGC 869, and make a proper sketch of NGC 884 so I can check the red stars against a chart.