Annual Meeting of the Deep Sky Section, 2011 March 12

 

Then and Now: Thirty Years of Section Images

The Deep Sky Section had been founded in 1981, initially under the name of the Photographic Section at the 1981 September BAA Council meeting, before changing its name to the Deep Sky Section only a month later at the following Council meeting.7 Its first Director had been Ron Arbour, now known for his work as a discoverer of supernovae, and from the beginning it had summarised its work in a triannual newsletter, initially titled the Deep Sky Diary, the first issue of which had appeared in 1982 April. The cover of this first issue had shown an image of the globular clusters around M31, captured by Geoffrey Johnstone on Kodak Technical Pan 2415 film using a 10.5-inch f/5 Newtonian. On subsequent pages were photographs of M42, NGC 884 (the double cluster) and M31. Producing satisfactory printed reproductions of images like these had presented a considerable challenge at the time, and whilst the speaker noted that each of these original photographs had represented a considerable technical achievement at the time, he added with regret that the newsletter had done poor justice to them.

Turning to discuss the equipment used, he explained that in the early 1980s, astrophotographers had typically worked with Newtonian telescopes of 20–30cm aperture. Also popular had been Aero-Ektar lenses, often manufactured by Taylor-Hobson and bought as army spares. Of necessity, the targets chosen had been bright objects, usually stellar clusters or galaxies. Exposure times had been limited to around 10 minutes by the lack of automated guidance systems.

By contrast, there was now a trend that many astrophotographers preferred small aperture refractors on account of their high quality optics, though there were others who still used Schmidt-Cassegrain telescopes. For several years, film photography had been entirely superseded by digital imaging, using either digital SLR cameras or dedicated astronomical CCDs, on account of the more immediate feedback provided by the new devices, together with their improved linearity and quantum efficiency – i.e. sensitivity. Longer exposures, often exceeding eight hours, had been made possible by more accurate drive systems and by the advent of automated guiding. The ability of computers to read images directly from modern CCDs had allowed astrophotographers to stack huge numbers of short exposures using automated image stacking programs such as Registax. This meant that the need for drift to be completely eliminated over the whole length of the exposure was now much less pressing.

Dr Moore noted that the rise in length of exposures was at first sight surprising, given the greatly improved sensitivity of modern detectors. It was apparent that much fainter and more challenging targets were being chosen. He added that there had also been a rise in the use of narrow-band filters, especially among those in areas severely affected by light pollution, and that the inherently low transmission of such filters also necessitated longer exposures.

Turning to compare some of the images that he had recently received with those in the Section's archive, Dr Moore identified galaxies as objects that had been particularly difficult to photograph in the 1980s, on account of their low surface brightnesses. In an image of the interacting pair M81 and M82 from the early 1980s, it was tricky to make out the galaxies at all. The change was much more slight among images of open star clusters, which had often already been readily accessible in the early 1980s, as was illustrated by an early 1980s image of M45, recorded by Alan Dowdell on Kodak Tri-X film. The only dramatic change was that modern colour images could record star colours, whereas the earlier images were invariably black-and-white. An audience member remarked that it was a shame that Ron Arbour's absence from the meeting prevented a re-opening of the historical debate between 2415 and Tri-X film.

Images of globular star clusters had vastly improved, as a comparison of new and old images of M13 revealed. Here, the difficulty for film photographers had been the large dynamic range between the dense cores of clusters and their faint outer members; the reciprocity failure of photographic film had severely limited the available dynamic range.

Assessing which objects had been the most popular photographic targets in the 1980s, Dr Moore observed that the Andromeda Galaxy (M31) had been a particular favourite, likely because its brightness had singled it out as the only galaxy within which significant detail had been within easy reach at the time. The Section archive also had plentiful images of M33, M51, and more exotic targets such as Stephan's Quintet, though it was apparent that their low surface brightnesses had posed considerable challenges at the time. Orion's Horsehead nebula seemed almost unique in having retained a constant popularity throughout the past 30 years, probably because it had always been a significant photographic challenge, yet it was tantalisingly just within visual reach. For reference, the speaker showed a 10-minute exposure taken by John Fletcher in 1985 on Boots 1000 film, and a 51-minute exposure taken by Martin Mobberley in 1986 on Tri-X film.

The speaker closed by noting that images of planetary nebulae had been vastly improved by the advent of CCDs; old film images were almost invariably burnt out, showing little detail and appearing rather like out-of-focus stars. By contrast, modern images often revealed so much detail that the objects didn't resemble their visual monikers, such as the Swan, at all. The speaker expressed his thanks to Patricia Wainwright for having scanned the archival images used in his talk, and added that she was hoping eventually to scan the entire Section archive, which would then be made available on CD-ROM. Dr Moore then welcomed David Arditti to talk about his experiences using the f/2 HyperStar System.

Ashburn

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39.04°N
77.49°W
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