Deep Sky This Month – July 2018 (Ophiuchus: The Globular Bearer - Part 2 of 3)

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by , 07-02-2018 at 10:29 PM (885 Views)
(continued from Part 1)

With our basic background data in place, let’s begin our journey through the brightest globulars of Ophiuchus. Using Chart 2 below, we will start at 2.8 magnitude Delta Ophiuchi (Yed Prior) and 3.2 Epsilon (Yed Posterior). From Delta we will sweep northeast for 5.5° to pick up 5.9 magnitude 12 Oph. From here we slip just north of due east for another 2.75° to reach Messier 12 (NGC 6218, mag=6.1, size=15.0’, class=9). This is a large and showy cluster, and you may notice some resolution of stars across its face. I find it slightly out of round, which may be an illusion caused by some uneven brightness across its dimension.

Our next target, Messier 10 (NGC 6254, mag=6.6, size=15.0’, class=7), is found only 3.25° southeast of M12. This is another bright and large globular cluster, which should also show some resolution across its face and into its outliers. Though M10 is listed with the same angular diameter as M12, I always find that it seems just slightly smaller to the eye. You should notice the mag 4.8 star 30 Oph just 1° east of M10, which will be our launching point to the next stellar globe.

From 30 Oph we will sweep nearly 6.5° ESE to find the dim globular, NGC 6366 (mag=9.5, size=8.3, class=11) lying just east of 4.5 magnitude HD 157950. This dim glob presents a subtle round glow in the eyepiece, and is rather loose in structure. Even from a darker site, if one were not looking for it, they might pass over it unknowingly. Finding its field is certainly helped by its proximity to the bright star.

We now head northeast from HD 157950 for just over 3° to pin down another brighter cluster. Messier 14 (NGC 6402, mag=7.6, size=12.0’, class=8) appears bright and fairly large in the eyepiece. It presents as round and, for the most part, fairly even in illumination. One may also be able to pick out some resolution of members as well.

We now begin a three star hop in a generally northward direction to locate the dimmest and smallest globular on our hit list. Starting at M14, nudge just over 1° NNE to mag 6.1 HD 160471. Turning due north for 1.5° we pick up HD 160438 at mag 6.5. Turning slightly NNE again for almost 3.5° brings us to the mag 6.5 double star 61 Oph. NGC 6426 (mag=10.9, size=3.2’, class=9) will be found just a hair over half a degree north of the double star, and slightly east of due north. It presents a very small homogenous glow in the eyepiece, and for some may appear as nothing more than a slightly bloated star in the field. The take away from this object is simply to identify it in the star field.

Wrapping up our first leg of this long journey, we backtrack to M14. Turning south from this showpiece globular, we slide almost 5° to land on mag 4.6 Mu Oph. Next sweep 5.5° to the ESE to pick up Nu Oph, shining at mag 3.3. Nearly 2° NNE of Nu you should spot mag 4.8 Tau Oph. Here is where you have to employ your trained eye. Slightly over halfway from Nu to Tau and slightly east of a direct line between the two, look for a small round glow that will be NGC 6517 (mag=10.1, size=4.3’, class=4). It is not an eye catcher at first blush, but does reveal some central brightness within an understated outer halo of cotton. Try pushing magnification higher to reveal a brighter and at times stellar core within the small unresolved outer halo.

Chart 2

Referring back to Chart 1 momentarily to reorient ourselves, we now sight in on 2.6 magnitude Zeta Oph (Han). Now we switch over to Chart 3 below, as we will be moving just over 2.5° to the SSW from Han where we will find the magnitude 7.8 glow of Messier 107 (NGC 6171, mag=7.8, size=10.0’, class=10). It presents as a smallish round and mostly homogenous globe of star light sitting south of a triangle of 7th magnitude stars. Try pushing up the magnification to see if you can resolve any of its members.

We now make a jump of 9.5° ESE to east-southeast to mag 2.4 Eta Oph (Sabik). From Sabik, slide 3.5° SSE to bring the soft glow of mag 7.8 Messier 9 (NGC 6333, mag=7.8, size=9.3’, class=8) into view. This cluster reveals a smallish round ball, and is easily seen. Despite being a little smaller, it does display a noticeably bright core area. Our next target is slightly over 1° northeast of M9. NGC 6356.(mag=8.2, size=7.2’, class=2) is a slightly dimmer and smaller than M9, yet it still shows up well with readily seen central brightness within its small halo.

Our final cluster in this section lays just over 1° SSE of M9. So we move back to M9 and nudge to the SSE, where we can pick up NGC 6342 (mag=9.5, size=4.4’, class=4). It is small in diameter and dim, yet it does display some core brightness within its diminutive round dimension. You will also notice a bright field star, mag 6.5 HD 156846 about 17’ NNW of the dim cluster which you will pass over as you nudge down from M9.

That completes the second section of our journey through the brighter globular clusters of Ophiuchus. If you’ve managed to bag them all so far, then you have a total of 10 out the 21 we are chasing down. Well done!

Chart 3

We now begin the final leg of this odyssey, and it will be a wild one. We are headed into the deep southern portion of Ophiuchus on a winding, twisting star hopping experience! Looking at Chart 4 below, we start at Sabik (Eta) and move our scope SSE for just almost 6° to the 4.4 mag double star Xi Oph. Then slide south just shy of 4° to Theta Oph (mag 3.3). We are now staged at our jumping off point for the final leg.

Chart 4

(completed in Part 3)
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