Annual Meeting of the Deep Sky Section, 2011 March 12

 

Astrophotography in the 1980s: Why I didn't blow myself up

Mr Johnstone explained that he had first become interested in trying astrophotography for himself after seeing images in Sky & Telescope magazine in the early 1980s. Around this time he had bought his first large telescope, a 240mm f/5 Astrosystems Newtonian, which he had co-mounted with a 150mm f/7.5 Newtonian for use as a guidescope during long exposures. He went on to explain that a guidescope had been an especially important part of any system used for astrophotography at the time, since the available drive systems had been crude by modern standards. His own, for example, had produced a marked wobble of the field with each turn of its worm wheel, and whilst modern mounts could correct for such problems with periodic error correction, the technology had not existed in the 1980s.

To use the system, he had begun by visually locating and centring his chosen target in the field of the primary Newtonian. He had then turned to the eyepiece of the guidescope to select a suitable guide star within its field of view, and adjusted the guidescope's mount to centre the chosen star on the guidescope's cross-hair. This having been done, he could replace the eyepiece of the primary telescope with a camera and begin an exposure. Throughout the duration of the exposure – typically 20 to 50 minutes – he would sit at the eyepiece of the guidescope, visually monitoring the position of the guide star and nudging the mount to keep it centred. He recounted that his observatory had been housed in a rotating summer house, and that space had been limited; it had often been an uncomfortable squeeze to stay at the eyepiece as the telescope tracked across the sky. On at least one occasion he had found himself pushed hard against the wall and had had to cut his exposure short.

Turning to the question of choice of film, the speaker explained that he had used many different films, but most often either Kodak Tri-X – best suited for sharp images of stars – or Kodak Technical Pan 2415 – better suited for nebulosity. He added that although the latter had only had a nominal ISO rating of 25, nominal ratings such as this in fact meant little when the film was used for astrophotography; all film emulsions suffered an effect known as reciprocity failure: they were optimised for use at a particular length of exposure – in the case of 2415, around 1/250th second – and were less responsive when used in long slow astronomical exposures. However, his interest had been piqued when he had heard reports that hydrogen gas could be used to hyper-sensitise the film, as he had had industrial experience of working with the gas early in his life, growing bacteria samples in a pathological laboratory.

To hyper-sensitise the film, he had needed to place it in a vessel containing hydrogen gas at a temperature of 40°C for 7–8 hours. In industry, he had been able to create similar conditions by first connecting a sealed vessel to a vacuum pump to extract the air, before feeding a regulated quantity of hydrogen into it using a rubber balloon-like bladder which he had filled from a hydrogen cylinder. The speaker had reproduced similar conditions at home in a stainless steel developing tank, which he evacuated using a bicycle pump with inverted valve. He had produced hydrogen gas in a milk bottle, by mixing zinc with hydrochloric acid, and had piped it into the developing tank using rubber tubing from a home brewing kit. He had then heated the tank to the required temperature of 40°C using a fish tank heater.

He added that whilst his work had gained some notoriety among Section members for its hazardousness, hydrogen gas was no more dangerous than other widely-used flammable gases and only became explosive if allowed to mix with oxygen. Had it been available, using forming gas – an inert mixture of hydrogen and nitrogen – would have eliminated the risk altogether.

Mr Johnstone closed his talk with a series of photographs which he had taken using the method described, including images of M33, M51 and the Horsehead Nebula. He added that one of the main difficulties of film photography, as compared to modern CCD imaging, was the limited achievable dynamic range: it had been very difficult, for example, to avoid burning out the nuclei of globular clusters if the outer cluster members were to be visible at all.

Following the applause, the meeting broke for tea, after which the Director invited Dr Nick Hewitt to speak.

Ashburn

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39.04°N
77.49°W
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