This extract from the Digital Sky Survey shows M21 in the upper left, the Trifid in the lower right. Additional Gamma correction provided. Both clusters shine through a rich, variegated Milky Way field. Dreyer describes NGC6522 as bright, fairly large, round, with considerable, gradual brightening toward the middle. Dreyer's estimate of magnitude 16 stars is a little extreme.
More recent measurements list stars from mag.
In this case, the magnitude estimate is relatively accurate, and the cluster harder to resolve than NGC6522. Image from the Digital Sky Survey. Map Printable Map More info. This is perhaps the most exquisite spot in the heavens. Cluster and nebula sit in the powdery Saggitarius Milky Way, in a region sliced by dark lanes.