Globular clusters are the senior citizens of our galaxy – they contain suns at least 12 billion years old. Their tightly packed stars make them extremely spectacular when seen through the telescope, and easy to observe even from light-polluted areas.
Summer nights are the best time to start hunting for globular clusters, almost one third of them are to be found in the summer constellation of Sagittarius.
When observing try to use high magnifications, for most clusters this gives the best views, making stars visible even close to the center. Most globular clusters can’t be completely resolved into stars using amateur telescopes, in most cases the center remains a diffuse glow.
For your first observation of a globular cluster choose M13, the brightest cluster visible from the northern sky, teetering on the edge of naked-eye visibility and appearing as a small, hazy glow in binoculars. A 200-mm telescope resolves the globular’s outer fringes into myriad of stars, and you might even see stars close to the cluster’s core.
Globular clusters observed with a 150-mm telescope
Palomar 8 is a difficult globular in Sagittarius. Its feeble five arcminutes glow is visible only by using averted vision, on a very clear night. It is completely diffuse, without condensation towards the core.
NGC 6934 is large and bright. With averted vision it is partially resolved into stars towards the edges.
NGC 7006 is a 10.6 magnitude cluster in Delphinus. It is very small and faint, condensation is visible towards the core.
NGC 6440 is a 9.7 magnitude cluster in Sagittarius. It has a bright core, but it is unresolved into stars even with averted vision.
NGC 6642 is very small and faint. With averted vision the globular cluster is partially resolved towards the edges, but the core remains diffuse even at high power.
Globular clusters observed with a 114-mm telescope
M30 is a small globular cluster in Capricornus. It has a bright core and towards the edge, at 50x, is resolved into stars.
M75 is extremely small and bright. It has an almost stellar aspect, with a very bright nucleus.
M55 is a huge cluster in Sagittarius. It is partially resolved into stars.
M72 is unimpressive, diffuse, faint and unresolved into stars even at 120x.
NGC 4147 is small, bright, strongly condensed towards the core.
NGC 5466 in Bootes is large and faint. Towards the edges it is partially resolved into stars. Using averted vision I’ve managed to see a star near the core.
M54 has a diffuse aspect, with a very bright core.
M13 a.k.a. “The king of the northern sky” is one of the most beautiful deep sky objects. The cluster is resolved into myriads of stars, only the center remains diffuse.
M22 is a beautiful globular cluster, its shape is a little oval. It is well resolved into stars.
M5 is very interesting, at 36x is partially resolved towards the edges.
M14 is extremely diffuse, without condensation towards the core. At 53x it is completely unresolved.
NGC 6712 is easily seen without averted vision. It is small and diffuse. Only with averted vision I’ve managed to see condensation towards the core.
M4 is very scattered, even at very low power numerous stars are visible.
M79 in Lepus is unresolved even at 120x.
Other clusters observed by me, that are partially resolved into stars even at low power are M3, M10, M12 and M53. Some remain diffuse, unresolved, with a bright core. These include M107, M9, M28 and M80.
NGC 6544 is faint, without condensation towards the core.
NGC 6553 is somewhat brighter then NGC 6544. With averted vision I’ve managed to see some condensation towards the core.
NGC 6638 it is extremely small and faint. It is slightly condensed towards the stellar nucleus.
NGC 6528 and NGC 6522 share the same field at 53x. NGC 6528 is extremely difficult. NGC 6528 is more easily seen, but only with averted vision.
M69 is unresolved into stars at 53x. I’ve seen a bright star near the northern edge.
M70 is pretty difficult to see, at 53x only the core is visible. It is small and faint.
M71 in Sagitta is very beautiful, resolved into stars toward the edges. Some stars are visible even near the core.
Globular clusters observed with a 60-mm telescope
Seen through a 60-mm telescope all globular clusters are small, nebulous objects. The only cluster I’ve managed to resolve into stars, only towards the edges, is M13.
The other globular clusters I’ve seen with my 60-mm refractor are: M4, M28,M22, M80, M62, M19, M9, NGC 6356, M15, M2, M92, M5, M71 and M56.