Telescope, photoheliograph, metal / glass, designed by Janssen and De La Rue, made by J H Dallmeyer, London, England, 1873-1874, used at Sydney Observatory, Sydney, New South Wales, Australia
"I do not suggest that photographic observations should displace eye observations; on the contrary, I think that both eye and photographic observations ought to be made." Warren de la Rue 1873
This is one of the most significant telescopes held in the Powerhouse museums collection. While not officially part of the British Government's observation program this telescope was purchased by Sydney Observatory and sent to Australia in 1874 along with attachments to photograph the Transit of Venus. A special stand was constructed in Sydney to support it during the observations. The telescope was made by J.H. Dallmeyer and was among the first to be used for astronomical photography in Australia.
For the observation of the Transit of Venus this telescope was set up at Woodford in the Blue Mountains at the residence of A. Fairfax. There were seven observers present for the occasion: P. F. Adams Surveyor-General; Hirst a well known amateur astronomer; Mr. Vessy of the Trigonomical Survey; Mr. Du Faur of the Survey Department; Mr. Bischoff the photographer and two unnamed carpenters.
The telescope and attachments are the same as those at five other observatories who were part of the Royal Observatory Transit of Venus program. The others went to Honolulu, Mokkatam, Rodriguez, Kereguelen and Burnham. It was made by J. H. Dallmeyer to accommodate a special piece of photographic apparatus designed by Janssen and de la Rue which took 6.5 inch circular photographic plates (H10213 & H10379).
Unfortunately of the 14 Janssen plates taken at Woodford none have survived. Twelve of the resulting Jansen photographs (60 on each plate), and 36 normal plates were sent to the Royal Observatory in Greenwich and have since been lost. The whereabouts of the other two is not known although they may have found their way into the New South Wales Government Printing Office. Only one of the unexposed Janssen plates has survived (H10379). One reason the plates which were sent to England were not well cared for is that, like the other photographs sent in from observatories around the world, the plates proved to be less than successful.
The reasons for this were described by George Airy, Astronomer Royal at the Greenwich Observatory, in 1881, "After laborious measures and calculation it was thought best to abstain from publishing the results of the photographic measures as comparable with those deduced from telescopic view. The consideration which led to this decision are that, however well the Sun's limb on the photograph appeared to the naked eye to be defined, yet on applying to it a microscope it became indistinct and untraceable"
While the photographs proved less than successful the observations themselves played an important part in the official report made by Captain Tupman to the British Government. Of the 61 reliable reports of Venus crossing the sun which were recorded at points around the British Empire 22 were form Australia.
Geoff Barker, August, 2007
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