NGC 6633 is a bright and loosely scattered open cluster that presents an excellent target for wide-field astrophotography in the constellation Ophiuchus. Spanning an impressive 43.2 arcminutes across the sky—larger than the full Moon—this cluster is best captured with short to medium focal length telescopes or even telephoto camera lenses. At 1,260 light-years distant, NGC 6633 offers a rich field of moderately bright stars spread across a physical diameter of 16 light-years, creating a striking composition against the background star fields of the Milky Way. The cluster’s relatively low surface brightness and large angular extent make it ideal for imaging with fast optical systems and modest exposure times.
Names and Catalog numbers
- NGC 6633
- Mel 201 (Melotte 201)
Position and the cosmic neighborhood
NGC 6633 lies in the northern reaches of Ophiuchus, positioned along the galactic plane where the constellation borders Serpens and Aquila. The cluster’s coordinates place it in a region rich with deep-sky objects, immersed in the dense star fields characteristic of our view toward the Milky Way’s central regions. With an angular diameter of 43.2 arcminutes, NGC 6633 occupies a substantial area of sky, making it an easy target to frame even with relatively short focal lengths. The cluster exhibits a slight approaching motion with a radial velocity of -28.7 km/s, moving toward our solar system through the spiral arm structure of our galaxy.
Nice to Know
- NGC 6633’s angular size of 43.2 arcminutes makes it larger than the full Moon in apparent diameter, allowing astrophotographers to capture the entire cluster with wide-field setups and even standard camera lenses at 200mm focal length or less.
- Also cataloged as Melotte 201, this open cluster is bright enough to be visible through binoculars from reasonably dark sites, making it an accessible target for both visual observers and photographers.
- The cluster’s 16 light-year physical diameter and relatively modest distance of 1,260 light-years result in well-resolved individual member stars that photograph beautifully with amateur equipment, showing distinct color variations among the stellar population.
- NGC 6633’s location in Ophiuchus places it within rich Milky Way star fields, providing a spectacular backdrop of fainter stars that add depth and context to images, especially when captured with sufficient exposure time to reveal the galactic background.
Brightness, distance and size
It lies about 1,260 light-years from Earth and spans roughly 16 light-years across. On the sky it appears about 0.7° (43′) in size.
