As the Milky Way becomes prominently placed in June, we turn our attention away from the "Realm of the Galaxies" to the wondrous nebulae and star clusters of the summer skies. Among my favorites objects this time of year are the globular clusters-ancient spherical swarms of stars which Hubble showed form a halo around our galaxy. As we gaze towards the center of our galaxy in Sagittarius and Scorpius we naturally encounter more of these objects. The constellation of Ophiuchus is another great place to start as it contains a total of 20 bright NGC globulars. Though a 4"-5" scope will reveal almost all of these, an 8"-16" is really necessary to fully appreciate the splendor of these dense swarms of stars.
The principal challenge for the visual observer is the degree of resolvability into stars for each system. This depends largely on two factors: the degree of concentration of the stellar members in the cluster and the distance to the globular. A third factor which can come into play is the extent of dust in the plane of the milky way along our line of sight to the object. Tightly packed clusters will tend to resist resolution but will have a high surface brightness. This aids some extremely distant clusters such as NGC 2419 in Lynx to be easily visible in an 8" scope although its distance is comparable to the Magellanic Clouds-our 2 neighboring satellite systems. Medium or loosely concentrated clusters may allow a high degree of resolution in an 8" if they are relatively nearby. Some globulars discovered photographically in the past 35 years are visually unobservable due to a combination of a sparse population, extreme distance, and high obscuration of dust in the galactic plane. Nevertheless, about half of the so-called "Palomar Globulars" discovered in the Schmidt survey of the early 1950's are detectable and a description of Palomar 6 in Ophiuchus will be found below.
To compare the degree of resolution in different apertures I've included my notes taken with a C-8, 13.1" Odyssey 1 and 17.5" open-tube Dobsonian. The single most crucial visual factor affecting resolution is the "seeing". On mediocre or poor evenings a globular may be reduced to a large fuzzy blob as the individual images smear each other out. So, wait until those nights or even moments when the atmosphere is calm and then use the maximum power that the seeing will allow. With my C8, I regularly used 200X (25 times the aperture in inches) when the seeing allowed. Finally, since many globulars lie well south of the celestial equator, wait until the object is near culmination to minimize the effects of haze and seeing at low elevations.
M107 (NGC 6171): Even the dimmest of the Messier globulars is fairly bright in my C8. The outer halo appears grainy and a few magnitude 14 stars appear on the NW edge at 220x. The small, bright unresolved core is very prominent in my 13" and the outer halo is resolved into a number of faint stars.
M10 (NGC 6254): The C8 will resolve this symmetrical beauty down to the core. Faint stars cover the bright central region and scores of stragglers stream away from the nucleus in lanes. The core is intensely bright in the 13" and densely packed with resolved stars. In my 17.5" this globular appears fully resolved as hundreds of stars are layered over the entire disc.
M12 (NGC 6218): Although the bright core admits only a partial resolution in the C8, the irregular outer halo highly resolves into layers of mag 12-13 stars with a few brighter stars embedded. The core itself yields to the 13" at high powers and faint stars are scattered over the center. Although M10 and M12 are similar in total brightness their appearance is different and I do not find M10 quite as rich as M10 in faint stars.
M62 (NGC 6266): The C8 reveals a non-symmetrical appearance; flattened in the SE direction due to intervening dust and fanning out to the west where a few faint stars can be discerned. The mottled outer shell just resolves into many faint stars at 288X in my 13" Odyssey 1.
M19 (NGC 6273): Though lively in a dark sky, the C8 only resolves a few faint stars at the north edge at 200x. This globular is clearly elongated north-south in the Odyssey 1, which resolves 1-2 dozen faint stars at high powers, particularly south of the core.
NGC 6293: With the 13" Odyssey 1 a small, bright, compact core is evident at 220x. The mottled outer halo is much fainter than the core but with averted vision about 10 extremely faint stars are resolved. Using the 17.5", 15 to 20 stars can be resolved at 286X and the globular appears on the verge of more extensive resolution.
M9 (NGC 6333): Again, the C8 shows a very grainy disk with a few faint stars resolved around the periphery at 220x, especially on the east side. The unresolved core appears mottled in the 13", but many faint stars are bunched at the edges of the core at 288x, and the outer halo breaks up into numerous stars. Look for the large dark nebula B64 just SW of M9. In my 17.5" at 220x the globular is 4' diameter and the bright core is elongated N-S somewhat like M4. The outliers or field stars appear to extend the halo E-W. The halo is fairly well resolved into two dozen mag 13.5-14.5 stars. The core is very mottled and lively and just breaks up into numerous densely packed mag 14-15 stars. At 420x, the core is easily well-resolved and two very close double stars are at the W and E edges of the halo.
NGC 6366: This is a strange object, easily located just 17' east of a mag 4.5 star. Visually, it appears large and diffuse with a low surface brightness due to its loose concentration, although some brighter field stars appear at the edges. I could just make out about a dozen faint stars over the unconcentrated haze in my 13".
M14 (NGC 6402): This cluster lies in a heavily obscured region of the milky way which dims its members and makes resolution difficult with a C8. In my 13", I recorded M14 as fairly large with a moderate central condensation. The outer halo is fairly smooth and a dozen very faint stars are superimposed at 288x. In the 17.5" over two dozen mag 14 and 15 stars are resolved, mostly at the edges of the outer halo and the large disc appears more ragged or irregular.
Palomar 6: This highly obscured globular is barely visible on the blue sensitive POSS print at UC Berkeley but is quite prominent on the red print. Using my 17.5" a fairly large but very low surface brightness haze is just visible. Six to eight faint stars are superimposed and there are 3 faint collinear stars just off the south end. The field is easily located as the globular lies just 6' northwest of a mag 8 star. In addition, the huge naked-eye dark rift known as the Pipe Nebula is located immediately to the west.