'((3 6.< TOUR
Some elusive clusters lurk in Ophiucus but they are worth searching out
Tick the box when you’ve seen each one 1 NGC 6426
This month’s objects can be found in a strip of sky passing across the middle of the large constellation of Ophiuchus, the Serpent Bearer. The first object of interest on our list is the most northerly in this month’s tour, located 1.5° to the south and slightly east of mag. +2.8 Cebalrai (Beta (`) Ophiuchi). NGC 6426 is a faint, mag. +11.1 globular cluster which best suits apertures over 200mm. Even then, this elusive cluster can be a struggle to spot, appearing merely as a gently glowing patch against the dark background sky. Matters don’t improve much through a 300mm scope, with the cluster appearing as a weak smudge of light with uneven brightness and no significant resolution to speak of. SEEN IT
2 COLLINDER 350
Extend the line from Cebalrai through NGC 6426 and keep going for the same distance again to arrive at another rather ill-defined object, the open cluster Collinder 350. This is another tricky target because it falls apart under magnification owing to its diffuse nature. There are about 15 viewable stars associated with the cluster, and – as suggested – the best way you’re going to get to see these is by using a very low power. A large number of background stars in this area also helps Collinder 350 camouflage itself into the background. The cluster has a large apparent diameter of around half-a-degree, roughly equivalent to the apparent size of the full Moon. SEEN IT
3 M14
Our next target this month is the far more promising mag. +7.6 globular cluster, M14. Located at a distance of 30,300 lightyears, this cluster appears off to the east of the giant box shape that forms the body of the serpent bearer. One way to find M14 is to place the mag. +3.2 star Eta ( ) Ophiuchi centrally in the field of view of adlow-power eyepiece and then swing the telescope west in RA by 00h 44m (11°). A 150mm telescope at high power reveals the cluster to have a distinctly mottled texture. Through larger apertures you can begin to discern its elongated shape with a few of the member stars starting to be resolved at powers over 200x. SEEN IT
4 NGC 6366
Our next stopping-off point is the globular cluster NGC 6366, which at mag. +8.9 lacks the brilliance of M14. It lies 16 arcminutes east of the mag. +4.5 star, HIP 85365. The faint cluster also has a 9th magnitude star located just
to the west. A 150mm telescope reveals an 11th magnitude pair of stars to the south-southwest of the cluster’s core. Small instruments aren’t likely to pick up much in the way of resolution when it comes to NGC 6366, instead revealing the cluster as a gentle glow. A 250mm scope at 250x power starts to resolve members against what looks like a more mottled background glow. Larger instruments continue this progression, creating the impression of a smattering of evenly spread faint stars against a mottled background almost 10 arcminutes across. SEEN IT
5 M10
Next we head westward for our penultimate target, another globular cluster known as M10, which lies at a distance of 14,300 lightyears from Earth. This particular cluster sits a degree west of 30 Ophiuchi, a mag. +4.8 star located southwest of the centre of the giant box shape representing the Serpent Bearer’s body. Unlike most of our previous targets on this tour, mag. +6.7 M10 resolves fairly well through even a 150mm scope, with many of the stars appearing superimposed over the glowing cluster background. Larger instruments only improve the view, revealing an elongation in the cluster’s core. Through a 200mm instrument, M10 appears with an apparent diameter of 10 arcminutes, with the well-defined core occupying approximately half this size. SEEN IT
6 M12
The end point for this month’s journey is yet another globular cluster and fortunately for us, it’s one that – at mag. +6.8 – finds itself at the brighter end of the spectrum. M12 lies 3.3° northwest of M10, the two objects often being cited as a pair. It’s an easy small telescope object which appears less condensed than M10. Around 70 stars are resolved through a 150mm scope at 200x magnification. Larger apertures vastly increase the number of individual stars that you can see, with the number increasing into the hundreds through a 250mm instrument. The outlying stars are quite unevenly spaced in M12 compared to those around M10. M12’s core also appears markedly smaller than M10’s. SEEN IT