Observer's guide
About NGC 6531
NGC 6531 (M21) is catalogued as an open cluster in Sagittarius. Its J2000 position is RA 18:04:13.45, Dec -22:29:24.2, corresponding to 271.0358 degrees RA and -22.505 degrees Dec. The local catalogue places it about 4,250 light-years away. Its referenced angular extent is approximately 110 by 110 arcminutes.
Observing tips
This is rated as a moderate target at magnitude 6.5. It is associated with summer observing, with midnight placement typically strongest around June. From 52 degrees north its theoretical meridian altitude is about 15.5 degrees; local horizons and the live altitude chart determine the practical window.
A small telescope under reasonably dark skies is a good starting point; use the live altitude chart to choose its highest point.
What to expect
The SIMBAD classification for the matched source is OpC. A published radial velocity value is -18.2 km/s. The SIMBAD record currently connects it to 158 bibliographic references.
At roughly 110 arcminutes across, it favors a wide-field imaging setup. Use the listed 110 by 110 arcminute extent to check framing, and treat catalogue magnitude as integrated brightness rather than a direct measure of visual contrast.