Messier 4 (Scorpius)

Object: Constellation: Coordinates: Size: Brightness:
Messier 4
(NGC 6121)
Sco 16h24m / -26°32' 36' 5.4mag

The globular cluster Messier 4 (NGC 6121) in the constellation Scorpius on May 29, 2020. Messier 4 is approximately 7,000 light-years from Earth, making it the closest globular cluster. Its spatial diameter is approximately 75 light-years. Messier 4 is rather loosely concentrated and the first in which individual stars were resolved. The cluster features a characteristic "bar" structure across its core. The structure consists of 11th magnitude stars and is approximately 2.5' long and was first noted by William Herschel in 1783 (see single 3 minute exposure below). Messier 4 was discovered by Swiss astronomer Philippe Loys de Chéseaux in 1746 (source: Wikipedia).

Twenty-four 3 minute exposures at ISO 800 were added for this shot with the DeepSkyStacker software (resulting in a 1 hour 15 minute exposure) and the final image processing was done in Photoshop. Messier 4 never rises higher than about 16° above the mathematical horizon here in Switzerland. Atmospheric dispersion and turbulent air had a visible impact on the stars in the cluster. Low transparency, thin layers of clouds, light pollution and the high ISO value of the camera led to colorless stars in the image.
Equipment: Canon EOS 450D Baader modified camera, TeleVue Paracorr Type II coma corrector, 16" f/4.5 "Ninja" dobsonian telescope riding on a dual-axis Tom Osypowski equatorial platform, Lacerta MGEN autoguider, Lacerta off axis system (field of view comparison: image of the moon with the same equipment).

Search chart for Messier 4. Map © 2020 "The Mag-7 Star Atlas Project", Map is slightly modified. The map can be downloaded here .