Telescope Reviews: LATE SEPTEMBER AT ROCKFORD LAKE

DS092209
..........RECENT OBSERVATIONS..........
by David Knisely

DATE: September 22nd, 2009, 0330 to 0619 hrs UTC.

LOCATION: Rockford Lake, Nebraska: 40.227N, 96.579W, 1403ft (427.7m) elev.

INSTRUMENTS: 9.25 inch f/10 SCT: 59x, 98x, 168x, 196x, 235x, 277x, 298x.

CONDITIONS: Clear, Temp. 55 deg. F. Wind N. less than 5mph.

UNAIDED-EYE ZENITH LIMITING MAGNITUDE: 6.6

SEEING (above 45 deg. altitude): 1"-1.5" arc (Antoniadi II).

OBJECTS OBSERVED: NGC 6712, IC 1295, PK25-4.1, Pegasus-1 Galaxy Cluster (IC 5309 target object), NGC 7634, M30, M2, M15, NGC 6940, NGC 7293, NGC 7380/Sh2-142, NGC 253, M33, NGC 891.

OBSERVATIONS: I had a clear Monday night off for once, so after some time wasted watching "Big Bang Theory", I got my stuff together and headed out to Rockford Lake for some much needing time under the stars. Once set up, I hit some objects I had been meaning to revisit. My first target was the nice globular cluster NGC 6712 in Scutum. Even at only 59x, it was starting to show some faint stars in the group. In fact, it appeared to posses a faint outer halo of very faint stars that almost went out to the plotted boundaries shown on Megastar (9.8' arc diam.), although with this object, the main core region tended to dominate the view. The main bright region is only around 2.5 arc minutes in diameter and is the only part I tend to see well from my driveway at home. Although roughly spherical, it also gives the impression of a broad fan of stars with the fan's base pointing roughly west. At 277x, it was rich in stars, with the inner regions looking very irregular and patchy. However, the cluster was not quite fully resolved, as there still was a little weak haziness to it. I estimated between 50 and 100 stars were seen with certainty and others hinted at.

At the Nebraska Star Party some years ago, Brian Skiff had shown me a "two for the price of one" pair of planetary nebulae near a globular cluster, but I had lost track of which ones they were. However, near NGC 6712, I finally found them again. The larger and more prominent one is IC 1295, a very dim and somewhat diffuse object located not far south-southeast of the globular. It was visible without a filter as a faint diffuse roughly circular patch in a very rich starfield, but that impression changed once I went to the OIII filter. Then, the object really stood out well, showing up as a somewhat annular patch with a glowing center and slightly brighter north and south sides. It was pretty good sized (about 90 arc seconds across), so at 98x which I had been using to study the cluster, it really made an impression. The other object, PK25-4.1 (Sanduleak 2-374), was only about 4.7 arc minutes to the west-northwest of IC 1295. It was considerably more difficult (14th magnitude), as it was very small and in a very rich starfield, requiring me to "blink" it using the OIII. Even then, it was difficult to discern from several nearby faint stars, so it took a while to identify it for certain. The object was pretty much stellar even at 298x, so, given the seeing, I must conclude that its size is probably around an arc second or so.

I had wanted to go back to M30 in Capricornus for a more detailed examination of this globular after seeing some recent images of it. 168x confirmed the impression that the cluster seems to have several strings of faint stars going north from it. I could identify at least three broad strings of anywhere from three to five stars that give the impression of a broad but rather spotty fan extending northward from the core region. The cluster has a small tight core rich in stars which became even prettier at 298x. The star density rises rather dramatically in an almost linear fashion as you look closer to the center (sometimes known as an "even concentration"). While the group isn't exactly my very favorite globular, it is well worth looking at.

With both M2 and M15 high in the sky, I decided to do a quick comparison between the two big globular clusters. M2 was very nice at 198x, with a somewhat concentrated core region but not as much as M30 was. Its star density seems to rise towards the center in a somewhat more "broad" and gradual fashion. At 235x, it was nicely resolved with many many hundreds of faint stars easily seen even in the core. M15 at 198x was stunning, being much more concentrated towards the center than M2 was. Its core region was perhaps a bit smaller with a "sharper" star density increase towards the center. 298x showed a very tiny brighter slightly elongated core spot that was distinct and maybe made up of two tiny short strings of a few stars that intersect in sort of a "V" shape. M15, like NGC 6712, showed an extended outer halo of faint stars that is somewhat more spread-out.

I have often noted the large open star cluster NGC 6940 in Vulpecula when viewing the summer Milky Way in binoculars or my 100mm f/6 refractor, but I don't know if I had ever put it in my 9.25 inch SCT. Once in the field at 59x, I got a pretty good look at it. It is a large oval group of perhaps 50 stars with magnitudes in the 8-13 range that is not well detached from the rich star background. It was probably 30' arc by 20' arc in size, so it filled up a good portion of my field.

I went down into Aquarius to check on the Helix (NGC 7293), comparing the performance of the DGM Optics NPB filter to that of the OIII. The object was fairly easily visible without filtration at 59x, but both filters brought up the contrast quite a bit and made the object much easier to see. The NPB shows perhaps a hair greater brightness to this big "fuzzy donut", but the OIII had a definite edge in contrast. The OIII seems to show the southeast and northwest sides of the nebula as kind of "tufted out" a bit as if someone had grabbed the nebula on those sides and pulled outward to form hints of filamentary helical formations or the beginning of faint "wings".

At the Nebraska Star Party in July, I had gone after the Pegasus-1 galaxy cluster, located on the Pegasus-Pisces border and anchored by the 11th magnitude elliptical galaxies NGC 7619 and NGC 7626. However, after looking through my notes, I saw that I had missed one I should not have: IC 5309. A quick slew, and I was back on the group, quickly picking up IC 5309 at 98x as a tiny faint elongated spot of light. After looking around, I started going back over the galaxies and, in an area about 40 arc minutes across, I counted 14 galaxies in total. Only a handful are all that easy, but it was pleasing to track all these tougher ones down even from my "regular" dark sky site. This bettered the record of 12 I had logged at NSP, so I felt I had probably just missed the other two in my haste to view as much at the star party as I could. I moved around a little and found NGC 7634, a spiral galaxy a little distance north of the main group which might be an outer member of the cluster. 98x revealed it as a small oval fuzzy spot with a brighter middle, while 168x revealed a small concentrated core and a faint star on the south side of the galaxy.

I went back up north to try for a nebula or two before going back to galaxies again. The nebulous open star cluster NGC 7380/Sh2-142 in Cepheus was an interesting target. The cluster is a sort of wedge or "U" shaped group of maybe 40 to 50 stars that kind of reminds me of a "hat" with string-like groups of stars on either end of the wedge to form the brim. The cluster was not terribly detached from the rich Milky Way background. The nebula was only hinted at without filters, appearing as an indistinct barely visible haze over and around the cluster. The OIII, H-beta, and NPB filters clearly showed the nebula as a distinct irregular glow mostly around the cluster with a notable inclusion or darker band on the south and eastern sides of the group. There were a few vague extensions of the nebula beyond the central part of the cluster, but none were very prominent. The NPB seemed to provide the best overall view, with the OIII coming in a close 2nd and the H-beta providing nearly as good a view as the OIII.

I finished with quick looks at NGC 253, M33, and NGC 891. I used my Orion Skyglow on NGC 253 and quickly noted that the contrast and dark detail showed up better at low power (59x) with the filter than without it. Increasing the power helped even more, as even at 98x, the galaxy shows a wealth of faint patchy detail. Indeed, one of the better views came at 168x, where the galaxy almost filled the field of view. M33 also responded to the Skyglow filter, making the arm and star-cloud structure come out a bit better than without a filter, although the difference was small. NGC 891 showed its dark lane, but this time, the filter did not provide much of a gain in contrast, as again, increasing the power slightly helped about as much. The dust lane was mainly visible across the broad nuclear bulge and only hinted at as irregularities in the elongated extensions of the galaxy away from the core. I found 98x seemed to work best, as 168x still showed the galaxy but made it quite faint and difficult to see. Part of this may have been due to dew, as I noted that with my flashlight, the corrector plate was just starting to show a little fogging. I was tired as well, so I decided to "pull up stakes and head for the barn". Still, it had been a pretty darn good night out under the first decent skies I had had since NSP.

Clear skies to you.

David Knisely
--

--------------------
David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org