M 21 (NGC 6531, Lund 806) Open Cluster in Sagittarius

Located at: RA 18 hours 04 minutes 13 seconds, Dec -22 degrees 39 minutes 51 seconds

Size: 13' (16') Magnitude: 5.9 Class: I 3 r

North is up

West to the right

Telescope:

8" f5 Newtonian reflector

Camera:

 ST-8XME, self-guided, binned 1x1, temp -10c, camera control MaxIm DL 4.56

Image:

Lumicon Deep Sky filter, 150 minutes (15 x 10 minute subs), 09/1/2007

Processing:

CCDStack 1.3, Photoshop 7.0

Location:

 Rolling Roof Observatory, Thousand Oaks, CA 91360 (+34d 13m 29s -118h 52m 20s)

Notes: According to "Star Clusters", by Brent Archinal and Steven Hynes, the size of this open cluster is 16 arc minutes.

From the NGC / IC Project:

Contemporary Visual Observation(s) for NGC 6531

NGC 6531 = M21 = E521-SC19 = Cr 363
18 04 13 -22 29.4
V = 5.9;  Size 13

17.5": M21 consists of roughly 50 stars in a 5' diameter.  Very bright, fairly 
small although outliers greatly increase the diameter.  Includes s wide double 
star S 698 = 7.9/8.8 at 30".  Just N of these stars is a remarkably symmetric 
ring consisting of a mag 9.5 star and ten mag 12-13 stars.  A close mag 14/14 
double star is on the W side of the bright double star.  Just W is a 10' string 
of mag 8/9 stars oriented NW-SE including two double stars.  M20 lies 45' SW.

- by Steve Gottlieb