|Object name:||Constellation:||Coordinates:||Apparent size:||Visual brightness:|
|Messier 4 (NGC 6121)||Scorpius||16h24m / -26°32'||36'||5.4 mag|
The globular cluster Messier 4 (NGC 6121) in the constellation Scorpius. 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 (71 minutes total exposure) at ISO 800 taken on May 29, 2020 were added for this shot with the DeepSkyStacker software 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.
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).