Telescope Reviews: NGC 6540, NGC 6118 Smackdown

Having read a recent friendly debate as to which object was the tougher of the Herschel 400, NGC 6540 or NGC 6118 I was given pause to wonder. It was not so much as to which was tougher but how tough would they be to compare with my 6 inch dob? As a noted late night radio talk show host from San Francisco is known to say, “So, did you do the experiment?”. On the nights of July 21st and 22nd I did just such an experiment. I did it over the two nights for two reasons. The first is so the observations would be with essentially the same amount of darkness. I wanted to catch then as soon as possible after twilight but also before the early rising Moon became a factor. The second reason is that with the potential difficulty of these objects I knew I would probably only be able to get one before the fog that has been blowing in off the Pacific like regular clockwork around midnight happens. In the end, on both nights, the Moon became a factor before the fog did.

First up was NGC 6540. On July 21st I drove the few miles down the road to the Pearson College Observatory so I could get my best view to the south. This nice little observatory on southern Vancouver Island is perched on a hillside looking directly south towards the Olympic Mountains in Washington State. It’s a wonderful place to hang out and wait for darkness to fall. It wasn’t until almost 2300hrs that is was dark enough to consider my target. The only previous time I had observed this globular was actually just 9 days earlier and that was with my 12.5” dob. On that occasion I had been a bit preoccupied as I was observing with a friend who was doing a last observing session before going for some major liver surgery. I hadn’t taken very good notes. All I could remember was its general orientation SSE of the unique little open cluster NGC 6520.

It didn’t take me long at only 54X to pick up a very small faint glow in the appropriate area. As I increased the magnification in increments to 160X the haze became more defined. It was somewhat rectangular in shape in a mostly E-W orientation. The cluster itself is located to the NE of a small asterism of sorts, which reminded me somewhat of Capricorn. It for a moment made me think (warning GEEK alert) of the Star Trek episode where that Dooms Day machine was roaming around gobbling up worlds. I did a sketch. On the whole I didn’t find this an extremely tough object and have certainly gone deeper with this little scope. Don’t get me wrong though, as it wasn’t an easy find. SQM reading was pretty bad too for this site at only 20.87. By the time I finished the sketch around 2330hrs the Moon had been up for almost an hour but hadn’t risen over the treed hill to the east. At that point ease of seeing the Glob was dropping.

Round two, with NGC 6118. On the next night under nearly identical conditions (SQM 20.89) I started my search for NGC 6118 with my little 6 inch. I wasn’t using an overly detailed finder chat because like the night before, I wanted to see how well the object would “jump right out at me”.

Finding the correct location wasn’t difficult at all, because NGC 6118 is located just 17 arc minutes to the SW of the naked eye star HD 147550. This star is located right on the western border of Ophiucus where it meets the SE border of Serpens Caput. It took awhile playing with magnification to create the right balance between image scale and background brightness until I was sure that what I believed I was seeing, was actually what I was seeing. Eventually just to the NW of a very faint close pair of stars I could detect a roughly EW orientated extremely faint elongated glow. This was at 80X using my recently acquired 15mm Televue ploessl. Again I did a sketch.

Now I know that this only settles any debate for me. How sensitive people’s eyes are and how well they can detect individual objects can vary widely. For me though, NGC 6118 appeared to be a much more difficult object. If both of these objects are considered reasonably close in difficulty I would had thought NGC 6540 would have been more difficult considering how low in the sky it is located for my latitude. (48* 22’ N) but it wasn’t.

I’m including links to the sketches with inversions that I did. For the inversion I’ve tried as best as I could to dim the objects so they show as well as possible how they appeared against the background sky. Just a hint, NGC 6118 is in the left side of the FOV. When you click on the image it will enlarge.

NGC 6540,
http://rascvic.zenfolio.com/p566114947/?photo=h29E2A037#702718007

NGC 6118
http://rascvic.zenfolio.com/p566114947/?photo=h29E2A037#1068626272

You be the judge. Or, do the experiment for yourself.

I found it interesting to see that these two objects were observed with 70mm “Refractor Red”. I hope to get back to the observatory Saturday night with my 12.5” dob but just may have to drag my ED 80 along.

Bill

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6'' Orion SkyQuest
12.5'' f/5 Custom Truss Dob
William Optics 80mm ZenithStar II ED Doublet
f/5 25" newtonian on a giant GEM, any time I want

Observing sessions grand total for 2007, 171.
So far in 2008, 108