I generally head to the Sierras to observe during the summer but last month decided to join a large group of San Francisco bay area observers at Willow Springs (south of Hollister) for Friday and Saturday nights. Observations were made with my 18-in f/4.3 Starmaster.
Willow Springs is a dark site (limiting mag between 6.5-7) with M33 a fairly easy naked-eye target and virtually no light extinction down to the southern horizon. It was a good weekend to go after some fall Abell galaxy clusters and galaxy groups. On both nights I worked together with an observer partner (18-inch Obsession) on the same targets hitting summer planetaries at high power (435x gave excellent images on both nights) in the early evening while the Milky Way was well placed and then switching over to some Hickson Compact Groups (HCG) and Abell Galaxy Clusters (AGC) as the fall constellations wheeled into view.
Usually I find working through more than one or two AGC's exhausting as you're pushing your scope and concentration to the limits on dim galaxies, but with good seeing and transparency it was easy to explore a half dozen rich clusters as well as a number of smaller Hickson groups that were nearby.
The first night I also took a look at a recently discovered globular, AL 3, located at 18 14 06.6 -28 38 06. This cluster was missed by the great visual observers of the 19th century and was first discovered by Andrews and Lindsay in 1967 on photographic plates taken at Armagh Observeratory in Ireland. Unfortunately, the cluster was left in obscurity for nearly 40 years and wasn't identified as a globular until earlier this year! Photometry of the individual stars and a Color-Magnitude Diagram done at ESO revealed a globular cluster that is located in the Galactic bulge at a distance of roughly 20,000 light years from the Sun. AL 3 is probably one of the least massive gc's in the Milky Way. Surprisingly this object is easier than the tougher Palomar globulars or another recent discovery, Djorgovski 2, and was immediately visible once I moved to the field in Sagittarius.
I was very surprised to find this globular was easily visible as a fairly faint, moderately large glow with a number of mag 13.5-14.5 stars superimposed, ~3' diameter. It appeared roughly round with no concentration, though with perhaps some mottling. A string of 6 stars oriented WSW to ENE is superimposed on the face of the glow along with a couple of additional stars.
Another interesting object we looked is a supermassive cluster located in the spiral arms of NGC 6946 in Cepheus. This cluster was discovered in 2000 (Astrophysical Journal, Vol 535: 748-758, "A Young Globular Cluster in the Galaxy NGC 6946") and appears to be a young (15 million year old) globular cluster embedded in a bubble-shaped star-forming region. At 323x it appeared as a 20" knot embedded within the halo of the galaxy just 3' from the core. The galaxy itself was stunning at low powers with long spiral arms that seemed to stretch on forever.
There are only a few visual supernova remnants and one of these, Sharpless 2-91, was discovered in 1997 in Cygnus, near Albireo. Previously, this object had been listed as an emission nebulae, but its actually the brightest thread of a huge supernova bubble, 4 degrees x 3.3 degrees in size. A quick peak at the field of 4.7-magnitude Phi Cygni (northeast of Albireo) revealed a huge, faint filament stretching across 13' to 15' of the rich star field using an OIII filter and a 20mm Nagler.
As far as the galaxy clusters and groups here's what we explored --
AGC 3744: This 570 million light year distant cluster is easy to locate as the central portion is less than 30' south of a 4.5-magnitude 24 Capricornia near the Chevron-shaped outline of Capricorn. The cluster contains three relatively easy members - NGC 7016, 7017 and 7018. A 4th fainter galaxy, MCG -4-49-16 is in the central core of the cluster while ESO 530-001 and -002 are just 15' away.
AGC 2572: I logged 7 members in this 600 million light distant cluster. The central core includes NGC's 7588, 7597, 7598 and 7602. The brightest of these is 15th magnitude NGC 7597 which I logged as "fairly faint, moderately large, irregularly round, broad concentration within the halo rising quickly to a small bright core". Several of the fainter members such as 2MASXi J2318294+183717 are in the 16th magnitude range and are just tiny knots less than 10" in size.
HCG 94: This Hickson group is located close west of AGC 2572! So, there was no need to starhop over to this Hickson group. The brightest member is the double galaxy NGC 7578A/B with a third much fainter member, HCG 94c close northeast.
HCG 93: This brighter Hickson group is just 30' NW of HCG 94 and contains 5 NGC galaxies -- 7547, 7549, 7550, 7553 and 7558. Only 7558 and 7553 required any effort and all 5 members fit in a 10' field!
Burbidge's Chain: With an excellent transparency low to the south, NGC 253 displayed a remarkable amount of dust structure and mottling along with HII knots. We then looked at NGC 247 which stretched across 20' of the field. Just 18' NNE is string of galaxies first described by a trio of well-known astronomers in 1963 -- Geoffrey and Margaret Burbidge and Fred Hoyle. For some reason Hoyle was subsequently left out of the picture as this group is dubbed "Burbidge's Chain". We were able to detect 4 members strung out along a north-south line including ESO 540-023, -024 and -025. PGC 842319 was a marginal object to the north of this trio. Though NGC 247 is a fairly popular target, Burbidge's Chain is still quite obscure.
AGC 4038: Another easy to locate Abell Galaxy Cluster found in the same field as 4.6-magnitude Delta Scultoris. I wrote about this cluster several years back in a Sky & Telescope article (October '99), and despite the southerly declination I was surprised the first time I took a look that a number of PGC galaxies were easily visible. The brightest members include IC 5349, 5350, 5353, 5354 and IC 5358. IC 5353 and IC 5358 are both double galaxies with very faint companions virtually superimposed! In addition we kept picking off numerous PGC galaxies -- I logged 14 total members and I believe my partner picked up a few more.
AGC 2593: This is another Pegasus rich cluster at a distance of 600 million light years. There's only one NGC galaxy here - 7649 - but a slew of tiny anonymous galaxies swarming around including 2MASXi J2324230+143930 and 2MASXi J2324122+143708. I logged 8 galaxies in the same field, nearly all 10" or smaller using 323x.
AGC 2666: This last Pegasus cluster is a bit closer at 400 million light years and contains one fairly bright galaxy, NGC 7768 as well as NGC 7765, 7766 and 7767. These are all crammed into 5' of sky! Several dim galaxies are in the same field including the unusual collisional ring galaxy MCG +04-56-014 (no structure seen visually), bringing the total to at least 11 members.
AGC 76: We moved south to Pisces to view this cluster which contains several IC galaxies -- 1565, 1566, 1568 and 1569 hiding among a field of relatively bright stars. 7 members were logged in total with the brightest members, IC 1565 and IC 1566 easy catches and with some tough objects such as 2MASX J00391571+0641188. I'm surprised this cluster was completely missed by both William and John Herschel.
HCG 5: Nearby is this small Hickson group including the double galaxy NGC 190 and two tiny companions to the north and south. (HCG 5C and 5D). I've looked at this group several previous times, but had never picked up 5D before and with a photographic magnitude of 17.3 I'm not surprised!
OK, now for the planetaries --
NGC 6742 in Draco is also catalogue as Abell 50. Although it was discovered by Herschel in 1788, Abell missed this when he compiled a list of "new" planetaries based on the recently completed POSS in the late '50's. The view was striking at 323x without a filter. The planetary was slightly elongated E-W, ~30"x25" with a slightly brighter rim giving a weakly annular appearance. A very faint 15th magnitude star was just off the NNE edge and a 16th magnitude star occasionally popped into view at the west edge.
Abell 51 is a relatively tough planetary in Sagittarius. At 115x and OIII filter it appeared very faint, fairly small, round, ~35"-40" diameter, very low surface brightness. I couldn't quite hold this object continuously with averted at 160x using a UHC filter.
One of the stranger appearing planetaries and one of my favorites is Sharpless 2-71 in Aquila. At 140x and an OIII filter, Sh 2-71 appeared fairly faint, fairly large, elongated at least 3:2 N-S, ~1.6'x1.0'. It appeared sharply defined with a straight border along eastern edge which runs N-S edge. The south side has a lower, irregular surface brightness and this is the faintest section though it appears to extend just as far as the north end. Without a filter the 14th magnitude central star was easy and a fainter star was close north of the central star, appearing to be a double. A trio of mag 10.5-12 stars extend beyond the planetary off the western side. On an earlier observation I also noted that at the SE corner a brighter knot bulges out slightly from the main glow.
Aquila is home to a number of NGC planetaries including NGC 6741. I picked it up at 115x by blinking with an OIII filter. It appeared light blue and soft at this magnification, making the identification as a planetary evident. At 323x a small, crisp-edged disc was fairly bright, ~8" diameter. Excellent view at 565x and the planetary appeared slightly elongated ~E-W with a faint, very thin outer envelope with total size of ~10"x8".
NGC 6751 in Aquila is a pretty small object though showed lots of structure at 257x. The central star was easily seen and the rim appeared brighter and the center slightly darker. There also appeared to be a thin outer shell. A mag 14.5 star is close off the east edge and two mag 14/15 stars off the west side. At 435x, the planetary is possibly slightly elongated ~E-W and one or extremely faint stars occasionally sparkled with one possibly on the west edge. The stunning carbon star V Aquilae is in the same low power field 30' NW.
Abell 55 is one of the better Abell planetaries and also resides in Aquila. At 115x and OIII filter it appeared fairly faint, round, fairly small. Just visible without a filter once identified thought does not appear as crisp-edged. Also, 160x and a UHC filter provided a good view. Without a filter a few very faint stars were visible at the edges on the N and NW sides. The surface brightness was pretty uniform and the disc was slightly elongated SW-NE.
NGC 6772 is another relatively bright planetary in Aquila. With the good seeing and transparency, there was no problem using 323x with a UHC filter. The planetary appeared moderately bright, round, fairly large, ~60" diameter. The rim was sharply defined and slightly brighter with a subtly darker center giving a weak annular impression.
I pumped up the power on NGC 6778, a tiny planetary in Aquila. At 115x a fairly bright but small, round disc was visible with a blue-grey color. Excellent view at 435x with an irregular shape, slightly elongated ~E-W, ~25"x20". A brighter knot at the west end gave the appearance of being double or bipolar. At 565x the surface brightness was noticeably irregular and an extremely faint star was occasionally visible at the west edge and also fleeting glimpses of a centrally located star.
At 435x, NGC 6852 appeared moderately bright, fairly small, elongated 3:2 NW-SE, ~25"x18". A mag 14.5 star was close off the SE end. At this magnification the planetary was occasionally bipolar with a well-defined, very small bright knot just NW of the mag 14.5 star and a second less defined knot that forms the NW end. The two knots appeared resolved though I never had a steady view of both simultaneously. Located less that 5' ENE of a mag 7.5 star.
Abell 65 is another one of the better Abells and is found in a rich star field in eastern Sagittarius. At 115x and OIII filter a fairly large, moderately bright elongated glow was easily viewed, extending at least 2:1 NW-SE, ~1.6'x0.7'. A mag 13 star is attached at the SE end and a mag 12 star is off the NW end.
I'm looking forward to returning to this site again, possibly in October, with a new list of galaxy clusters.
Steve