news > sections > deep-sky > observing report (2004 March 24)

Observing Report

2004 March 24

Auke Slotegraaf

It's been too many days since my last deepsky session. I've been doing lots of astronomy but precious little observing. A couple of weeks ago, I gave a star show to a group of school kids and enjoyed their enthusiasm at seeing the rings of Saturn and the moons of Jupiter. A few days later, while glancing over an IAU Circular, I noticed that comet NEAT Q4 was now around 7th magnitude. I'd forgotten about it, as I am inclined to do about solar system objects. I picked it up in binoculars that evening. Whoa, I thought, as I studied its unfamiliar presence in Tucana. Comets always surprise me, in the same way that waking up and finding a life-size statue of George Bush next to my bed would raise an eyebrow. The next day, I showed the comet to friends. From a site darker than my back yard, the comet's appearance was neatly bracketed by two nearby globular clusters, 47 Tuc and NGC 362.

This has been the extent of my recent observing. Then, I emerged from a world of colour-magnitude and vector point diagrams long enough to notice Venus and the slim crescent Moon setting with the Sun. This jolted me - deeptime would soon end with the return of the Moon. That evening, I started watching the most important thing a deepsky observer should pay attention to - the weather charts. A wiggly blue line was approaching the coast, which means cloud over Stellenbosch that night. And so it was. The next day, another blue line, but this one was further into the Atlantic. It was a done deal.

Thursday evening by 19:30 I was up at the abandoned Rifle Range, the unwieldy 6-inch f/8 on its German equatorial set up, charts and eyepieces arrayed. Anticipation. Time to sit back, roll a cigarette (a skill in the dark) and sip a long espresso. Espressos are meant to be drunk quickly (hence the name). But a long espresso is an entirely different thing. If they served Segafredo at mass, even I would attend. I surveyed the heavens in contemplation, and silently toasted the Moon, now lost in the pine trees behind me. The False Cross was at break-neck altitude, which is always a pleasant thought. Soon, Scorpius would rise, but for the moment, Crux occupied my thoughts. As is my custom, I turned the telescope first to Acrux. This is the "Do your worst, you slimy bastard" moment, when the metal telescope tube hasn't cooled properly yet, but sufficiently enough to tell if the skies are good. If there is little shimmering this early, things are going to kick ass. Putting in the lowest power eyepiece, I slew in the direction of the bright glare. Adjust the focus, so. Seeing Acrux and its dim companion always makes me grin. Then the high-power eyepiece. At 300x, Acrux is a w i d e binary, while the bright component is itself double. Exquisite.

I wait patiently for the tube to cool down. Over the next while, I check the progress. Acrux drifts out of the field of view and after a few moments is replaced by a deepsky object. This little cluster, NGC 4609 (the Coalsack Cluster), must be one of the easiest to find. Just point at Acrux and wait. In the low power eyepiece - which I refer to as the sweeper - the cluster is an obvious bowtie shaped collection of 9th magnitude stars with a bright unrelated star just to the southeast.

Some more espresso, and I see a brilliant glare rising above the eastern mountains. Not quite as bright as the aeroplanes that cross overhead on the way to Cape Town, but having a similar yellowish colour. Jupiter, the show-off of the solar system. We regard each other in cold silence. Three porcupines shamble past.

Not long after, I slew the telescope in the direction of Vela, and find m Velorum, the starting point of tonight's journey. I star hop southward, using the Millennium Star Atlas chart number 962. Just over a degree away, in the field with a few 8th magnitude stars, lies ESO 213 G 2. But the 6-inch isn't up to it. Although its position can be established accurately, I can only pretend that I see a dimness there. Immediately west of it lies QX Vel. Wondering if it would be reddish, since it is a variable star, I pay close attention. No particular colour; I later see that it is an eclipsing variable.

I continue southwest, in search of NGC 2972. Almost at my destination, I'm distracted by a pair of pair of 10.5 mag stars. Each pair is 1 arcmin apart, and the two pairs are 3 arcmin apart. It's an obvious grouping in the wide field of the sweeper. A check on the star chart shows no special object, just the two pairs. Nevertheless, I pop in a higher magnification. At 96x and 133x, it's interesting enough to sketch (Figure 1). The eastern pair is now joined by two more stars, about 12th mag, forming a rough square. In total I plotted 10 stars down to about 12.5 mag. (The field is centred on 09h 41.7m, -50°13')

Figure 1. The anonymous scattering of stars centred on 09h 41.7m, -50°13'. The sketch has North up, East to the left and the circle measures 5 arcminutes across. Various eyepieces (48, 96, 133x) were used.

V magnitudes, converted from Tycho-2, are as follows: eastern-most pair V=10.3 (top), V=11.0 (bottom). western pair V=10.8 (top), V=10.4 (bottom). The star due east of V=10.8 (top) is V=11.6. The dimmest star in the Tycho catalogue, V=12.1, is the middle star in the bottom east-west row of three.

NGC 2972 lies in the same field, to the southwest. Star hopping to where the cluster should be, I find a 9.5 mag star, and then I see the cluster itself, a nuance further west. At 48x, it is an elongated (1:3) mottled glow oriented NW-SE. It is an elegant, soft object, in contrast to the now-bright 9.5 mag star to its east. A short line of stars can be made out, forming the SE edge of the cluster. At 96x, at least a dozen stars appear, forming a stretched-out 5 arcmin affair. The NW edge is marked by a nice double star. A second shorter chain penetrates into the centre of the grouping, coming from the direction of the 9.5 mag star. The impression of two star chains crossing each other is engaging.

Two and a half degrees east brings me to Collinder 213. Set in the middle of two bright stars 1 degree apart, is a roughly 20-arcmin, not-much-denser scattering of 11th mag and fainter stars. I defer to Dr Collinder's judgment that this is a cluster; visually, the 6-inch doesn't convince me.

I continue eastward (now on MSA 961) to Ruprecht 87. After seeing "Dirty Rotten Scoundrels", Steve Martin's zany character "Ruprecht" has forever sullied my impression of Ruprecht clusters - I can never take them seriously. 96x shows here three brighter and 5 or so fainter stars, in a coarse group.

Two degrees north, MSA 961 shows a tempting fat oval - ESO 213 G 11 - with a tiny star on its southwestern edge. I studied the area closely, and like Tantalus, was not rewarded. The star was obvious enough, being 10.5 mag. One or two dimmer stars nearby, more or less where the galaxy could be expected, appeared as soft glows, so I certainly would not say that I saw it.

Blatantly obvious, though, was NGC 3201 due north. This wonderful globular cluster is as tantalizing as ESO 213 G11. The cluster is 5 arcmin across, and doesn't have a distinct smaller nucleus. Instead, the entire disk is sprinkled with tiny stars. Looking directly at lower power the stars lurk just below the threshold of visibility. But with averted vision at 96x they snap into view, star-studding its entire 5 arcmin extent. Particularly interesting is that the cluster appears to be lob-sided, with the stars denser and brighter on the eastern side. Dimmer stars seem to extend, loosely scattered, to the west, exactly as if someone took the central portion and shifted it eastward a notch. Two 9th mag stars, shown on MSA 961 as touching the cluster's northeast edge, are well-distant when seen with the 6-inch.

I had reached the eastern limit of my planned session, and took a break. Fresh bread, cheese and espresso, enjoyed under a starry sky next to a telescope. Good stuff. Looking around, its obvious that the skies here aren't particularly dark. The observing site, an abandoned rifle range on the lower slopes of Stellenbosch Mountain, is surrounded by pine trees. Cape Town 50km west, and Stellenbosch just over the hill see to it that the mountain is well light. Particularly irritating is the stray light from farms in the valley that reaches between the trees, preventing proper dark adaption. However, the site is less than 5 minutes from my house, and with my naked eye (and through slightly grubby glasses), I can usually see V=5.8 at the pole, sometimes V=6.1. In binoculars, V=10.5 is a reasonable limit.

View from the Paradyskloof rifle range looking towards Cape Town. Table Mountain and Devil's Peak can be made out on the distant horizon.

Keeping a mindful eye on the weather (the cold front was approaching the Cape), I looked out over the valley towards the southwest. There, I saw a band of cloud stretched out on the distant horizon. My time was limited.

Stretching my legs and rolling my neck, like an athlete preparing for a burst of activity, I reviewed the next stage of my observing. As I walked over to the charts, I glanced up at Jupiter. I thought, what the hell, I've got the telescope set up, a moment to spare, and I'll never admit it, but I'll look at a non-deepsky object. When I was a school kid, I was fortunate enough to have access to a 15.5-inch f/9 equatorial Newtonian. I spent many, many nights observing with it, and saw a great variety of things. Jupiter was a favourite, and I enjoyed sketching its wonderful features. With the eyepiece sometimes 2.5 metres above ground level, the telescope had a hefty, lofty, ladder. It's a good thing my 6-inch doesn't need a ladder, because I would surely have fallen off of it this night. Jupiter was gaspingly beautiful. I picked it up with the wide-field eyepiece, and was delighted to see all four Galilean moons arranged on one side. I focussed carefully to get them to tiny points, and then switched my attention to the planet. Because I can't hope to describe it, I can only say that I've never seen such a spectacular view, with so much detail in the equatorial belts. For the longest time, I fought with the desire to phone friends and tell them to get over here right away. Eventually, I decided not to - I knew the clouds were coming, and it would take a while to dark adapt after they left. With the image of Jupiter still clear in my mind, I set about arranging the charts for the next session.

I started off by briefly viewing NGC 5128, Centaurus A. This showpiece object deserves more time than I had at the moment, but it was as marvellous as Jupiter, in the same way that Pink Floyd is as marvellous as Bach. The galaxy's dark lane is always a pleasure to see. In one sense, one sees a brightish star (10 mag) from which blooms a broad nebulous fan of soft light, extending northwestward. The furthest extent of this oval nebulosity is snipped off, the decapitated piece clearly separated from the main body by a broad dark patch.

But my target was the Centaurus galaxy cluster, so I hurried on to nearby NGC 5090. MSA chart 933 shows it just southwest of a brightish star. With some attention I could make out the galaxy as an almost stellar, very faint, smudge.

Further west, sharing a field of view with bright stars, lies NGC 5026, which is quite challenging but distinct, appearing as little more than a soft round glow.

I made it all the way across to the heart of the Centaurus cluster, four degrees away, before I noticed that all was not well. NGC 4696, quite obvious in a diminished star field, was a soft glow, much like a bloated star. I looked up from the eyepiece, and saw the first wisp of cloud move in. It was just after midnight.

I abandoned the galaxies, and turned the telescope on IC 2602 in Carina. I use this cluster as a magnitude check, and found that V=9.7 in the 6-inch was looking decidedly dim! No worries, I thought. I've got a rich collection already, and happily packed the whole shebang back into the car and, after a last espresso, headed home.

Notes on some objects:
ESO 213 - G - 002: Discovered, here in South Africa, by R.T.A. Innes and reported in the Union Observatory Circulars (Nos. 45-76, p374) as a new nebula, "small, round, brighter middle, 12 mag., 15 arcsec diameter. Is 10° 15arcsec from an 11th mag star."
NGC 2972: Spotted by Sir John Herschel, who noted it as "a small pretty compressed cluster; irregular firgure." He cross-referenced it to James Dunlop's No. 397 "a very small faint round nebula .. with two or three exceedingly small stars slightly involved in it..."
Stewart (1908, Ann. Harv. Coll. Obs., 60(6)), reviewing photographic plates of the region, decided it wasn't real: "No cl., but S* near." Trumpler (1930, Lick Obs Bul, Vol 14, No. 420) decided it was genuine, and listed the diameter as 4.5' and the class as 1 2 m. A R Hogg (1965, Mem. Mnt Stromlo Obs.) recorded: "Definitely an open cluster." Vogt & Moffat [1972A&AS....9...97V] as part of their study of southern open clusters, wrote: "The colour-colour diagram of this faint cluster shows three groups: one single B-star, a group of A-stars and 5 late-type stars. The cluster is real only if it consists of the A-star group. ... A very tentative solution is ... d = 1.18kpc, earliest spectral type A0. NGC 2972 may contain three red giants..." Magda Streicher (personal comm.) saw it as resembling the letter 'A', or the Taurus region of Aldebaran and Hyades.