M21
Messier 21, NGC 6531
48’ x 32’ | 0.3”/px | 9576 × 6388 px
Sagittarius
RA 18h 04m 08s Dec -22° 29’ 43” | 0°
Messier 21, also known as NGC 6531, is an open star cluster located in the constellation Sagittarius, embedded in one of the richest regions of the summer Milky Way. It was discovered by Charles Messier on June 5, 1764, during the same observing session in which he also catalogued M20, the Trifid Nebula, which lies only around 40 arcminutes to the northwest. The two objects lie at different distances and are therefore an optical rather than a physical pairing. M21 lies at a distance of approximately 4,250 light-years from Earth and spans a true diameter of roughly 13 light-years. It is a young cluster, estimated at around 4 to 5 million years old, which places it among the younger open clusters in the Messier catalogue. It contains approximately 57 confirmed members, dominated by hot blue-white stars of spectral type B, a composition consistent with its youth. The cluster reaches a combined apparent magnitude of around 6.5, placing it just at the limit of naked eye visibility under good dark sky conditions.
Source: Claude.ai
Data Acquisition
Data was collected during 4 nights in April and May 2026, using a 14” reflector telescope with full-frame camera at the remote observatory in Spain. Data was gathered using standard RGB filters. A total of approximately 6 hours of data was finally combined to create the final image.
Location Remote hosting facility Roboscopes in Fregenal de la Sierra, Spain (38°N 6°W)
Sessions
Frames
Equipment
Telescope
Mount
Camera
Filters
Guiding
Accessoires
Software
Planewave CDK14 (2563mm @ f/7.2), Optec Gemini Rotating focuser
10Micron GM2000HPS, custom pier
Moravian C3-61000 Pro (full frame), cooled to -10 ºC
Chroma 2” RGB unmounted, Moravian filterwheel L, 7-position
Unguided
Compulab Tensor I-22, Dragonfly, Pegasus Ultimate Powerbox v2
Voyager Advanced, Viking, Mountwizzard4, Astroplanner, PixInsight 1.9.4
Processing
All processing was done in Pixsinsight unless stated otherwise. Default features were enhanced using scripts and tools from RC-Astro, SetiAstro, GraXpert, CosmicPhotons and others. Images were calibrated using 50 Darks, 50 Flats, and 50 Flat-Darks, registered and integrated using WeightedBatchPreProcessing (WBPP). The processing workflow diagram below outlines the steps taken to create the final image.
The processing followed a very standard approach. See the outline below for a detailed breakdown of the processing applied to the image.
Processing workflow (click to enlarge)
This image has been published on Astrobin.
