History of the Northumberland Telescope
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.
Return to my home page
Contact: Joseph Myers
(jsm@polyomino.org.uk)
Last updated:
10 February 2019