Telescope Reviews: After Messier

I had relatively little trouble with NGC 6540, which was the second most difficult H400 object according to some sources, but my first sighting of NGC 6118 absolutely required dark skies.

In June of 2002, my friend Tony Donnangelo, who is a truly first class observer, and I saw no hint of NGC 6118 through his 10" Meade LX5 SCT under slightly better than sixth magnitude skies at an ASH dark site in western Perry County, Pennsylvania. I've had no succcess with detecting NGC 6118 from Cherry Springs State Park, the IDA's second International Dark Sky Park, with my 101mm Tele Vue refractor. The Blinking Galaxy is visible through larger apertures from that location, of course.

Ed Ting relates his experience with observing NGC 6118 from a dark site in Arizona at http://www.scopereviews.com/az.html

"Halfway through the night, I laid some time aside to look for NGC 6118 in Serpens, which is roundly considered the most difficult Herschel object of them all. It's a relatively large galaxy with almost no surface brightness. Many experienced observers go their whole lives without seeing it. On this night, I found NGC 6118 in about thirty seconds. It looks a little bit like M33 does in my TeleVue Ranger under modest light pollution."

Here's what Jay Reynolds Freeman had to say about NGC 6118 and NGC 6540 during his quest to observe the Herschel 400 with a 55mm refractor:

"NGC 6118 lies in eastern end Serpens Caput, near a sixth-magnitude star (which is actually in Ophiuchus), about two degrees south of the celestial equator. That star was visible to the naked eye, and even if it hadn't been, nearby lambda, epsilon, and delta Ophiuchi made the field easy to locate. I observed with two eyepieces, alternating between a 12 mm Brandon (37x) and a 20 mm Meade Research Grade Erfle (22x). I used the _Millennium_Star_Atlas_, which shows plenty of nearby stars, so the precise location of the object was not in doubt.

With each of the eyepieces, I saw a faint, diffuse, and not very centrally concentrated glow, popping in and out at the limit of averted vision, at the charted position. Jiggling the telescope, or moving it slightly with the slow motions, helped a bit. The glow was detectable only ten or twenty percent of the time, but it kept reappearing at the same place, and I do not see similar fluctuations of intensity at random places in such fields, so I logged it.

Make no mistake -- this was a very tough object, certainly the toughest so far in my Herschel-400 survey with Refractor Red. When I say "detection", I mean no more than that. NGC 6118 would have gone unnoticed had I not known in advance exactly where to look, or had I not been patiently willing to pull every trick in my book to find it. I suspect that the root of the difficulty is that the object does not have nearly as large a central concentration to its brightness as do most galaxies; such a bright core to an image seems to draw the eye, and give the brain a reference point for locating the fainter, outer periphery of the object. Or so I would conjecture.

Since I had essentially an equally good view at 22x (2.5 mm exit pupil) and 37x (1.5 mm exit pupil), I suspect that an interim magnification -- perhaps with a 2 mm exit pupil -- might have been best for that object on that night. Unfortunately, I had only brought a handful of eyepieces, and did not have one available.

On the next evening I was at Fremont Peak again, this time with my Meade 5-inch refractor (model 127 ED). Sky conditions were similar, so I looked for NGC 6118 in the larger telescope, using 36x (Orion 32 mm Sirius Plossl) -- a magnification very similar to one of the ones I had used with Refractor Red. The object was much easier -- with five times the light grasp, that's no surprise -- and I was able to confirm the appearance that I had seen in the smaller instrument.

NGC 6540, located just off the spout of the Sagittarius "Teapot", is something of a puzzle. The visual description from the original catalog is a faint, sparse, open cluster which is relatively small in angular size. Yet what _Millennium_ plots is a ten-arc-minute globular. What I saw with the 12 mm Brandon in Refractor Red (37x) was a six or seven arc-minute unresolved circular glow, just noticeably brighter than the background (which was pretty bright -- this object is in the Sagittarius Milky Way, after all), with a smaller, brighter core superimposed. The core might have been one or two arc-minutes in diameter, it was unresolved, and it did not appear to have diffuse edges. The entire apparition was dead on the atlas position for NGC 6540, and was notably easier than NGC 6118."

Refractor Red: NGC 6118 and NGC 6540 by Jay Reynolds Freeman
[Edit: shortened the hyperlink]

Dave Mitsky

Edited by Olivier Biot (07/13/08 08:22 AM)