History of the Northumberland Telescope

The Northumberland Telescope is familiar to the many members of CUAS who have used it for observations, as well as those who have used it on public observing evenings. While some aspects of its history (such as how it was used by Professor James Challis to fail to discover Neptune) are well known to its users, others are not, and are not reflected in the standard account of this telescope on the IoA web site.

Most notably, it seems to be little known that Airy, who designed the telescope, wrote a book about it, Account of the Northumberland Equatoreal and Dome, attached to the Cambridge Observatory, privately distributed in 1844. (Although the distribution of the book on its own was private and it did not go on general sale, it was also bound with the Cambridge Observations for 1843 (volume XV) as Appendix II, and the Cambridge Observations were sold to subscribers.) This contains nineteen plates with sixty-seven figures showing almost every part of the telescope and its original dome in exquisite detail, down to individual nuts and bolts, with some of the fine details drawn being as little as one eight-hundredth of an inch across, and thirty-nine pages of text explaining these figures and the construction of the telescope and dome in great detail. (The diagrams are not yet available from the online version here.)

I have placed a photocopy of the book in the South Annexe (ante-room C in Figure 2 in the book) of the dome for the use of those using the telescope. Some of the beautiful fine detail has been lost in the copying process, but the copy should be adequate for most purposes when actually using the telescope. I will endeavour to ensure that, when the diagrams are made available online here, they do reflect the level of detail of the diagrams in the book.

The telescope cost 15000 francs for the object-glass and £1938. 9s. 2d. for the rest, according to Airy’s autobiography (which also mentions his book about the telescope, although the autobiography seems somewhat better known).

Extensive contemporary papers concerning the construction of the telescope and the writing, production and distribution of the book are preserved among Airy’s papers in the Royal Greenwich Observatory Archives, in RGO 6/157 (for the telescope itself) and RGO 6/158 (for the book), and may be studied in the University Library.

Among the facts about the telescope that are not well-known are:

• The flint-lens (the original object-glass having been a doublet, with one part of crown-glass and one of flint-glass) was nearly destroyed by an accident in Cauchoix’s workshop, and needed to be reground before it could be sent to Cambridge. This is also described in Airy’s autobiography, as well as in his account of the telescope.
• The original dome (replaced in 1932) was not anything like the current one; rather, it was a wooden structure moving on six cannon-balls.
• While the common story about the replacement of the dome is that the old dome had been irreparably damaged in a severe gale, the evidence, including Annual Reports of the Observatory Syndicate and the Report of the Buildings Syndicate on the repair of the Northumberland Dome dated 28 April 1931 (Cambridge University Reporter, 1930–1931, pp 964–965), does not support this story; no specific gales are mentioned there, and it seems more plausible from the reports that it had gradually fallen into a decrepit state rather than being catastrophically destroyed overnight.
• With the use of microscopes and calibrated and vernier scales, along with declination rods, the telescope could be moved with great precision to any position in right ascension and declination; such precision being essential in work such as that of Challis to attempt to find Neptune. Now, the declination rods appear to have been lost, and a garish tape measure has been fitted (rather than restoring the missing parts and relearning the skills of their use, as Airy’s book provides all the information necessary to do).
• Originally, the observer never needed to leave the observing chair while observing. From the chair, he could move the chair-frame around the dome; the chair up and down the chair frame; the telescope; and the dome itself, as well as making other adjustments.
• It was possible to observe very near to the celestial pole, and there was a chair that could be placed within the polar axis to do so. (It might still be possible to do this, but for the lack of this chair, if the plastic dew-cap were removed.)

One story which does, however, seem to be at least somewhat accurate, is that on setting up the telescope Airy made some adjustments, then tightened some bolts and said that no-one should ever loosen them. No-one ever has, and those bolts have since been painted over; the use of the bolts is explained in his book. However, the correspondence between Airy and Challis shows that much of the adjustment was done by Challis.

Documents available here:

• G. B. Airy, Account of the Northumberland Equatoreal and Dome, attached to the Cambridge Observatory, 1844. (Diagrams not yet available online.)
• I originally made these documents available in 2001–3 and am not now working on expanding the online set of documents. Some relevant images may be found in the IoA collection in Apollo.

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