M13 and the propeller
Tuesday 3 September 2024 · Hercules
| Date | 2024-09-03 |
|---|---|
| Time (UTC) | 21:10 – 22:25 |
| Instrument | 200 mm f/6 Dobsonian |
| Eyepieces | 24 mm (50×), 13 mm (92×), 9 mm (133×), 6 mm (200×) |
| Seeing | Antoniadi II |
| Transparency | 4/5, slight haze low in the south |
| Sky | SQM 21.2 at zenith; NELM about 6.1 in Ursa Minor |
| Moon | New (none) |
First clear, moonless night since proper darkness came back in August. New Moon was early this morning, so no excuses. Scope out at 20:15 to cool; it was ready by the time astronomical twilight ended, a little before nine. M13 was already down at about 50° in the west by the time I got to it, which is lower than I like, but the seeing was steady enough that it didn't matter much.
At 50× it sits neatly between its two seventh-magnitude guide stars, a bright round glow with a grainy edge. The grain is the giveaway that it wants more power. At 92× the outer halo breaks into a scatter of stars with curving chains running out from it — the “legs” I always see going off to the north-west and south-east. The core stays a bright mottled blur.
133× was the best view of the night. Resolved almost to the centre with averted vision, and after about ten minutes of looking, the propeller: three thin dark lanes meeting a little south-east of the core, making a lopsided Y. I have read about it for years and had only half-convinced myself of it once before. Tonight it held steady as soon as I stopped staring straight at it. I sketched it in the notebook before I could talk myself out of it.
Tried 200× for a few minutes. Darker background, individual stars fatter, nothing gained. Back to 133×.
Then NGC 6207, the little galaxy about half a degree to the north-east. At 92× it is a faint elongated smudge with a slightly brighter middle, lying roughly north–south, clearly not a star once you know where to put your eye. The tiny galaxy IC 4617, which sits between the two, I did not see and did not expect to.
Packed up at 22:30 with dew forming on the finder. The tube was dry.
Next timeDo M92 first, while it is higher, instead of leaving it as the afterthought it always ends up being.