Annual Meeting of the Deep Sky Section, 2007 March 3

 

A Pinch of SALT

Mr Clark explained that the "SALT" in his title was the South African Large Telescope; he would be describing a 10-day observing expedition to South Africa upon which he and Mike Cooke had embarked in 2006 August, hoping to find skies better than those of his native Manchester – perhaps England's cloudiest corner. His destination had been Sutherland, a remote settlement in the midst of the high-altitude South African desert, 80 miles from the nearest town. Posting queries online had put him in contact with the Astronomical Society of South Africa (ASSA) – a ProAm society who had proved an invaluable contact. He had learnt that Sutherland had roughly 50% completely clear nights, 75% spectroscopically clear, and 25% cloudy, which would prove roughly in line with what he was to experience.

He had flown to Cape Town and arrived in pouring rain, but as he travelled inland, the weather had greatly improved – this was apparently quite usual; such weather systems clung tightly to the coast. Along the final 80 miles of road from the nearest civilisation to Sutherland, he had not passed a single other car. His destination had turned out to host a few B&Bs, a trade which it apparently tried to make from its vicinity to the South African Astronomical Observatory (SAAO); the town nicknamed itself the "Gateway to the Universe".

On his first night, he had taken advantage of the lack of traffic to observe from the road, 3 km outside Sutherland; he had been able to set up his 4" telescope on the tarmac undisturbed. As darkness had fallen, the Milky Way had stretched across the sky from north to south, with its brightest part – Scorpius and Sagittarius, containing the Galactic Centre – roughly overhead; this remarkable sight had been the focus of this entire night. A pair of 15×50 Canon image-stabilised binoculars had proven a remarkable tool for observing it; the speaker could not recommend them more highly.

Subsequently, he had gained access to the instruments in the public observatory on the SAAO site – a 16" Schmitt and another 14" – about 200 m from the SALT itself; he had had five nights of observing with them. Presently, he would arrange his observations from them into a mythical night's observing, though his images were in practice taken over five. He remarked that the instruments had proven to be quite poorly maintained; he had been able to substantially improve their collimation, and he wondered whether this show of expertise was responsible for the staff's subsequent willingness to give him free access to them. Their lack of dew shields had been an initial source of anxiety, not entirely understood by the locals; he had later realised that on a site with 20-30% relative humidity, such worries were quite alien.

At sunset, he had been able to see the green flash quite easily, followed by the rising of a beautiful pearly-red Belt of Venus in the east. The sight had been quite special in such clear air. As darkness had fallen, the most obvious sight had been that of the Milky Way – the η-Carina nebula perched on the southern horizon, the Coalsack Nebula just above it in Crux, the Sagittarius clouds overhead, the Scutum star cloud and the Wild Duck Cluster (M11) further north still, and the Dumbbell Nebula (M27) on the northern horizon. This whole complex of objects – all naked-eye-visible – had been littered with dark clouds and dust lanes – a very rich sight. The speaker remarked how astoundingly bright a truly dark sky was – whilst he couldn't have read by the light of Sagittarius, it had been quite bright enough to cast shadows; they had been visible moving up the wall of the observatory as the night progressed.

The zodiacal light/band had been readily visible, about 20-30° across, as well as the gegenschein; using the Bortle scale3, the speaker rated Sutherland considerably better than an 'excellent dark site'.

Within the Milky Way, there had been relatively little colour as compared to the warmer hues of the zodiacal light, but it had appeared remarkably broad; elements of its visual extent had seemed to stretch right out as far as the Large Magellanic Cloud (LMC). Sweeping the 4°-field of his 4" into its path, the flooding of stars into view had been a memorable sight. Around it had been 8-10 naked-eye globular clusters – both Messiers and some of which he had never before heard of. M5, 15, 2, 30 and 22 had all been visible to the unaided eye. Telescopically, even many lesser-known galaxies and clusters had often been quite breathtaking.

The ω-Centaurus cluster (NGC 5139) had been a remarkable telescopic sight, seeming to overflow the eyepiece with a sprinkling of diamonds – pin-points of light everywhere. By comparison, the Jewel Box open cluster (NGC 4755) had been almost a disappointment – perhaps the speaker was just more of a galaxy observer. As the night had drawn on, the Milky Way had begun to sink into the west, making other objects more inviting. In the southern pinwheel galaxy (M83), bright knots, dark dust lanes, and the beautiful spiral shape, had all been accessible with minimal effort. The Hamburger Galaxy (NGC 5128; better known as the radio source Centarus A) had been another inviting target. Barnard's Galaxy (NGC 6822) had passed virtually overhead later in the night, and lying back, looking through the 16", the speaker had found its granular nature, emission nebulae and the structuring around, to be all clearly visible; it had been incomparable to the meagre sight seen from the UK.

As the night had drawn on further, some degree of darkness had been achieved as the Milky Way had begun to set further, and the southern galactic pole in Sculptor had opened up, bringing with it a rich cluster of galaxies. In the Fornax Group, NGC 1365 had seemed to rival M51 in magnificence, having a very rich zig-zagging spiral structure. Visiting a few more familiar areas, the speaker had found the Lagoon Nebula (M8) and Trifid Galaxy (M20) to be well-placed – quite a contrast to their skirting along the southern horizon in the UK. All that he could say was how disappointing their appearance had seemed upon his return.

He closed by mentioning the Magellanic Clouds, which had been best observed later in the night. In the SMC, the 47-Tucanae globular cluster was a rival in greatness to ω-Centaurus, and quite a contrast to it, having a tight central condensation. The LMC offered a rich array of clouds and star clusters; the Tarantula Nebula (30 Doradus) had proven especially memorable.

In addition to his observing, the speaker had also toured the research instruments on the SAAO site, including 1.4- and 1.9-m infrared instruments, the southern SuperWASP exoplanet search instrument, and, of course, the SALT itself. He remarked upon how the 91 hexagonal mirrors of the SALT's 10-m primary had appeared quite grey with dirt; he presumed that the cost of cleaning them was simply not economic. Amateurs with grubby optics might take some comfort from seeing that the mirrors of a premium professional telescope were no better.

Following the applause, a member asked whether this opportunity to use the SAAO instruments was open to all amateurs. The speaker replied that when he had flown, he had not expected that he would have access to the 14" and 16" instruments – they were not generally available, but he had struck very lucky in making contact with the ASSA and convincing them of his competence. Dr Moore asked of the security situation in South Africa. The speaker replied that remote areas such as Sutherland tended not to have a problem; it was built-up areas which tended to have no-go districts.

After a break for lunch, the Director invited Dr Richard Miles, BAA President, to speak.

Cambridge

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42.38°N
71.11°W
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