IC4756: An oft-overlooked open cluster worth ogling

Aug 18

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8/18/2014 7:26 AM  RssIcon

IC 4756: A huge, star-rich object

It's always interesting researching objects that are not Messiers or well-known NGC objects. Particularly with IC objects, the data available is, sadly, even harder to come by. But we need not know the nitty gritty details (IC 4756 has 6 blue stragglers!) to find and appreciate these clusters. No, what we need most often is simply knowing they exist, that they are easily seen, and worth finding/observing.

Star hop to IC4756 in OphiuchusI often find NGC objects that appear "bright" on Stellarium or on other lists that indicate a bright overall combined magnitude, but then when I see the size of the object, I have to roll my eyes sometimes. "A cluster just 10 arc seconds across? That's like looking at MARS!" (which, if you don't already know, looks incredibly tiny at the eyepiece). So when I saw an object that lists an angular diameter of 52 arc minutes (almost twice is large as the Moon) AND an integrated magnitude of 5.00, I knew I had to investigate further. 

The skies for me this summer have been horrendously and atypically cloudy for very long periods of time at night, so when I got the opportunity to view that was not only clear, but also Moonlight-free, I perused this area of sky. I began with binoculars, because a nearly one-degree-across object in the sky is not necessarily best-viewed with a telescope. Many telescopes are limited to a 1 or 1 and a quarter degree field of view. That' doesn't adequately frame a large cluster. 

Finding the wayStar hop jump 1 to IC 4756

I could trace my star-hop in the sky without difficulty, though the cluster itself was not visible in binoculars or a finderscope under my "red-orange zone" light polluted skies. In planning to share how to get to this area with others, I noticed the small grouping of stars near Delta Aquilae that were good for beginning the star hop. The next easily-visible object along the way required a rather long hop, but because the starter stars offered a nice pointer on where to go, hopping then landing at Theta Serpentis was not difficult at all. The harder part was determining where to go FROM there. 

Theta itself is the brightest marker along the way. At a "bright" 5-th magnitude, it stand out in most any finderscope of 6x30 or larger under most skies. The challenge was in determining where to go from there - or rather not so much where (the destination is IC 4756, so the "where" was not in question) - but HOW to know the way there. In other words, what were the landmarks? What could be used to point the way?

Useful pointers

Often when developing a star hop, there are several different ways to do so. One is use bright stars that guide you right to the object - Beta and Mu Andromedae for example, point right to Messier 31 in Andromeda. Another way is to use a right angle; if no bright stars exist to point right to an object, use a right angle from two bright stars, and move that way. In this case, there weren't any really bright stars pointing exactly the way, but there was a right triangle that helped serve as a bit of a pointer in the right direction.

Star hop 2 to IC 4756 (Theta Serpentis)To the south of Theta Serpentis is a wide pairing of stars. The eastern and marginally brighter one is 5.5 magnitude 64 Serpentis. The western one at 6.1 magnitude makes a nice pairing, as well as a useful reference point. These star pointed the way NOT to go, but from a line drawn up to Theta, I could now use this as a way to help me know which way I SHOULD go. The hop from Delta Aquila to IC 4756 is about 12 degrees - that's easily two full finderscope fields of view. That isn't a short star hop by any means. So using new reference points at Theta Serpentis helps. 

From that "64 Ser to Theta Ser" line, I found a right angle: A similar distance away, in the direction I wanted to go, was a 6-th magnitude star. Now, there's a brighter star to the south, that can also make a right triangle, but then 64 Serpentis would be the apex. In this case, 64 formed one part of the base, Theta the apex, and the 6.2 star the other side of the base. And I would follow that Theta to 6.2 magnitude star line. And I was well on my way at this point; Theta is about 7.5 degrees from Delta Aquilae. I knew I had less than 5 degrees to go to get to IC 4756. Now it was a matter of finding what was close to there along my line of direction to reach the open cluster.

Star hop three to IC 4756Four gently curving stars

I knew I had found my last landmark when I noticed a nice little line of stars. They were not straight, but rather curved gently. But what was helpful was their magnitude: All similar in brightness, even if not evenly separated from each other. A 7.1, two 6.7's and a 6.5 magnitude star all showed up in the same field of view as Theta Serpentis, and just slightly above my "right angle apex to base" line. It made the hop easy and do-able; there was no guessing of, "Gosh I hope I'm going the right way!" All landmarks were visible for this second, shorter hop.

Now was the moment of truth: Was the cluster there? What was visible? What would I see? 

I wasn't prepared for what greeted me at the eyepiece.

Stars, stars everywhere 

Have you ever ventured out on a late summer evening and witnessed the swarms of "no-see-um" insects? They seem to swarm everywhere, yet trying to slap them them or wave them away is useless. You just have to sort of deal with them while there there for that half hour of dusk. But while the sky bright enough to see them, that's seemingly ALL you see - swarms of black against a light sky.

Well, without the annoyance, IC 4756 is much like that. Swarms of stars. Too many, it seemed! I initially had begun with a moderately low power eyepiece, a 20mm Orion Expanse with a 66 degrees apparent field of view in my 10" Dobsonian. But that wasn't wide enough to fully take in this area. While the cluster itself is only 52 arc minutes across - the additional stars along this line of sight reveals even MORE than just the cluster. Just like the swarms of gnats that are even beyond what you can see in the light of the sky, the stars around IC 4756 seemed to go for some distance beyond.

IC 4756 at 1.5 degrees field (graphic representation)Observing IC 4756

Certainly, this open cluster lies right in the line of sight of our Milky Way galaxy, so some additional star fields are expected to be seen. But it was a wonderful problem to have - SO many stars seen at the eyepiece that I first brought out a 32mm Plossl, then had to go back for a 26mm 2" eyepiece with a 65 degrees field, then back AGAIN for a 42mm 2" eyepiece with a similar FOV. FINALLY I could see all of the cluster, but still nearly filling the view I could see. 

THIS, I realized, was a special object, and others surely needed to know it was there, and how to find it. A rich field telescope is undoubtedly best; I do have a 5" f/4.4 scope, which I will have to pull out and use to observe this cluster again. I can get 2.7 degrees field with that scope, and that's certainly what is needed to see this one to one and a half degrees swarm of stars. It reminded me of Messer 44, the Beehive, but with some important differences. 

  1. It cannot be seen well with 50mm aperture binoculars. 
  2. It is larger than it seems to be listed, though I don't doubt that the actual star cluster members are only 52 arc minutes across.
  3. The Beehive seems to have some slightly brighter members, while IC 4756 has these very "even" quality to it. While there are dimmer stars, there are MANY stars of the brighter members that all seem to be within one magnitude of each other. 

There is no real 'core' visible either; just a rich, dense field of stars. It's why framing it with a wide view is so necessary; one could mistake the area as just a dense section of the Milky Way, yet when framed adequately, it's true appearance as a cluster really stands out. But even with a narrower field of view, an observer can "sweep" back and forth across this cluster to find the edges, slowly tilting up and down to see the full extent. 

It's a useful exercise for a lovely cluster that is often overlooked, and seems to have few who have really found it and taken the time to view it adequately. Perhaps that's due to it's sheer large angular diameter; slower focal ratios and narrower AFOV's in eyepieces of yesteryear would have made this cluster hard to view. Not so today with fast, rich field reflectors and refractors coupled with affordable 65 and 70 plus degrees AFOV eyepieces. 

Check out this cluster during the summer months. It's a worthy Beehive alternative. 

There's a lot more here on Eyes on the Sky. For example, every week on the homepage there is a new astronomy video about observing objects in the night sky. They're only 5 minutes long - why not check out the latest one right now?

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The Nightlight

This blog includes what to see in the night and daytime skies, thoughts on telescopes, binoculars, and other astronomy observing accessories and equipment, plus my own occasional notes on objects I've seen and observed. Oh, and the random theater or other "my take on life" post. In other words, there is always something interesting. Check it out.