The Troughton equatorial, 2-inch aperture, 3-feet focal length, arrived in December 1795 and was mounted, in the English style, under the south dome on two stone piers resting on a massive pillar, round which the staircase of the dwelling house winds. There is no polar axis, but the right ascension circle, 4 feet in diameter, is attached to the polar pivots by four stays. The declination circle is of the same size and is similarly attached by stays to two pivots, which turn in Y's fastened to the right ascension circle. A description of the instrument appears in Dr. Rees's Cyclopaedia,
The equatorial was used for general observations and to measure the declinations of standard stars (see J. Pond, Phil. Trans. Roy. Soc., 453-454, 1806). The correction of observations made with equatorial instruments was the subject of a study by T. R. Robinson the results of which were published in the Trans. Roy. Irish Acad., 15,3, 1825. The instrument was also employed by R. Finlay in the rocket experiments to determine the distance between Armagh and Dublin in 1838 (see T.R. Robinson, Trans. Roy. Irish Acad., 19, 121, 1839).
A comment by T.R. Robinson in 1844 on the condition of the instrument is worth noting: "The West equatorial while in the hands of Mr. Troughton was injured by a frost which condensed on it moisture charged with the sulphorous vapours of the London smoke, and spotted it all over. I fear this change has increased with time, and may at last corrode the brass to some depth. I would therefore suggest that it should be painted and varnished, which has been found sufficient to arrest such an injury in other instruments".
Sketch by Troughton and Maskelyne
Extract from A Short History of Armagh Observatory.
The first major instrument purchased for Armagh Observatory, the telescope made by Troughton of London, is a masterpiece of English instrument-making of the 18th century, only one other instrument of its type exists in the world today. It was purchased by Archbishop Robinson on the recommendation of the Astronomer Royal of England Nevil Maskelyne) who was very impressed with its novel design.
As explained earlier, the simplest method of measuring the coordinates of stars and planets was to record the time at the instant they crossed the meridian using a transit circle. However the major difficulty with this technique is that observers had only one opportunity per night to make such a measurement - when a star crossed the meridian. Troughton and some of the other instrument makers of his time realized that, if instead of using a horizontal east-west axis, they could mount an instrument on an axis that pointed to the celestial pole, it would be possible to measure the coordinates of a star at any time of the night when that star was visible. This would be particularly valuable if one were trying to obtain a sequence of planetary observations in a poor climate when clouds obscure the sky for much of the night. The astronomers were somewhat dubious that the instrument makers of the day could accomplish the high standard of stability and accuracy, in this more complicated design, than in the simple and well tried meridian and transit circles. Manufacturers, and particularly Troughton, felt they could. To improve stability Troughton used massive stone pillars for support and for rigidity used conical brass tubes to support the central telescope ring.
In the final outcome, although many useful observations were made by this telescope by J A Hamilton, the astronomers were proved right, in that the telescope could not match the accuracy of the simple transit and meridian circle. It was an expensive mistake.
See also: Edward Troughton brother of John Troughton.
Last Revised: 2009 November 5th