M55
Globular Cluster in Sagittarius
About
A loose globular cluster in southern Sagittarius, about 17,600 light-years away. Nicolas Louis de Lacaille found it in 1752 from South Africa, while Messier, hunting for it from Paris, found it barely cleared the horizon. Only about 55 variable stars are known near its centre.
Imaging tips
- At 19′ it is large: 1,280–2,130 mm on APS-C or 1,950–3,260 mm on full frame.
- It stays under 10° altitude from 50°N and 24° from 35°N, so pick a site with a clear southern horizon; best in July.
- Faint for its size at 21.3 mag/arcsec². One Chile image gathered 7.5 hours of LRGB in 300 s and 600 s subs on a 24-inch scope over many nights.
- Dew is a practical risk: on one hazy night the corrector of an 8-inch SCT fogged, and the red channel was spoiled even with a hair dryer between filters.
Facts
- Size
- 19′
- Magnitude
- 6.3
- Coordinates
- RA 19h 40.0m, Dec −30° 57′
- Framing, APS-C
- 1280–2130 mm
- Framing, full frame
- 1950–3260 mm
- Best months
- Jul
- Peak altitude
- 9° from 50°N, 24° from 35°N