The Architecture of Armagh Observatory

John Butler

Preprint Series No. 122, February 1991
To be published in Architectural Survey of Armagh by Ulster Architectural Heritage Society

To the casual observer, the small, picturesque Observatory at Armagh looks not dissimilar to many other country houses in Ireland of the late 18th century. It has a square, rather sparsely ornamented Georgian residence connected by a low wing to an eastern tower. However, on closer inspection, one becomes aware of a building with a number of rare, if not unique, features and one that tells us not only something of its architect, Francis Johnston, but also something of the history of astronomy over the past two hundred years. To understand and appreciate its architectural features it is necessary to know something of the types of building constructed for making astronomical observations in the 18th century in other parts of Europe.

Early observatories in western Europe were often towers, surmounted by a platform, and simply served to raise the astronomer above the noise, smoke and turmoil of the narrow streets. The two great public observatories of the preceding century, Paris and Greenwich, were scarcely more advanced and for both it seems the requirements of aesthetics were paramount to those of practical astronomy. It was remarked by Sir Christopher Wren of his design for the Royal Greenwich Observatory `it was for the observator's habitation and a little for pomp'. The inconvenience to the astronomers at Paris of making their observations through tall windows, and the obstruction by ornamental features of the building, is well documented. Indeed it was concluded by some astronomers and architects that it was just not possible to create a building which was both pleasing to look at and suitable for practical astronomy. Whilst the founding of observatories remained a prestigious preoccupation, only to be undertaken by rich and enlightened patrons, this point of view tended to lead to a dominance of aesthetics over practicality.

The conflict is well illustrated by the necessity for solid and substantial foundations, a point that should have been plainly evident to an architect, but which was neglected in parts of the Greenwich Observatory to the extent that the subsoil below some of the buildings moved and threatened their collapse.

A gradual awareness of the great importance of the stability of foundations for instruments, which began in Scandinavia, early in the 18th century, reached its ultimate fruition in the Observatory of Trinity College, Dublin built at Dunsink in 1785.

A remarkable account was written by its first Director, H Ussher, on why Dunsink was built as it was. It is a model of clarity and simplicity, and it laid the ground rules for successful observatories of the future. Ussher emphasised the importance of the Observatory site and the geology of the surrounding area. He insisted that instruments for measuring the positions of stars should rest on bed-rock and that buildings which housed them should be close to the ground and not on high towers. He also specified that the foundation of the instruments should be independent of the building so that vibrations were not transmitted to the delicate apparatus. Only instruments which were intended to view the whole sky were to be placed on high towers to give them a clear view of the horizon. Though Dunsink was the first building to be entirely conceived and built along these lines some aspects of its design did appear in the earlier observatories at Oxford (Radcliffe 1771), at Kew (King's Observatory 1769) and the `New Observatory Buildings' at Greenwich (1749-50). It seems likely that some of Ussher's ideas derive from the experience of English astronomers he met on his visit to England prior to the building of Dunsink and in particular, Maskelyne, the Astronomer Royal, at Greenwich. Unfortunately, although Dunsink was built in 1785, the delay in the delivery of its major instrument for over 20 years, meant that its superior design could not be tested until many years later. Indeed, Armagh Observatory which was built in 1789-91 received its principal instrument, The Troughton Equatorial Telescope, in 1795, a decade earlier than Dunsink.

For the Observatory at Armagh, several of Ussher's principles were not heeded, in particular his criteria for selecting a suitable site. At Armagh the site chosen was a drumlin hill of compacted boulder clay on the edge of the then City of Armagh. It has been reported that Archbishop Richard Robinson had a town plan for his primatial City and that he wished to place his new buildings in prominent positions so that they could be seen, from his Palace, set against a background of natural woodland. This is in fact the case with the Observatory, and it illustrates once again, the conflicting interests of benefactors in erecting an aesthetic building and those of astronomers in founding a practical Observatory. Later, in the nineteenth century, this point is unlikely to have escaped the attention of the third director, Thomas Romney Robinson, when he announced that he had detected a seasonal movement of the entire Observatory which he attributed to the percolation of water into the underlying boulder clay.

Another crucial factor in Ussher's account which was ignored by Francis Johnston, and which later came to haunt Armagh astronomers, was the siting of the main instruments to the east of the residence. The hot air from the chimneys of the residence was carried by the prevailing west wind over the astronomical instruments where it caused the star images to quiver in the telescopes. This undoubtedly limited the accuracy of some measurements of star positions.

For Ussher, in his description of Dunsink, there could be no compromise of the practical requirements of an observatory for the sake of architectural elegance. However, at Armagh, Francis Johnston achieved a fair degree of compromise and as a result produced a building of great charm. As one of his earliest commissions it includes his first attempt to use features that were often to recur in later, much grander, buildings. For instance, the frequent use of curves where normally rectangular features would be used, such as the rounded corners for some rooms, the curved chimney breasts, and the curved architraves around the ground-floor windows. Another example is the unique cupola dome over the staircase which serves the double purpose of enabling the astronomer to check the sky before opening the dome and to illuminate the first floor landing.

No more pertinent example of the difference in approach between the design of Dunsink and Armagh Observatories could probably be found than by comparison of the well proportioned and well-lit spiral stairwell around the central telescope pillar at Armagh, which rises from the foundations through the ground and first floors to the base of the dome. At Dunsink, a much more substantial central pillar was used, which left space for only a narrowly confined and ill-lit staircase between the pillar and the outer wall. At Armagh, Francis Johnston, was forced by the lack of rigidity of the clay substrate, to abandon another of Ussher's principles here, namely the independent support of the telescope. Apparently, he tried to mitigate this shortcoming of his design, by placing a wooden staircase on top of the stone staircase, thereby reducing the transmission of vibrations from the stairs to the instrument above.

Francis Johnston is well known for the minimal decoration of the exterior of his buildings. In contrast, in his interiors, his use of decoration, though restrained, incorporated much fine detail. Both of these characteristics are evident at Armagh Observatory, particularly in the fine beaded woodwork in the ground floor apartments.

The original dome on the south side of the building is one of the earliest astronomical domes to survive in the world. Moveable domes were less common in 18th century observatories than in the 19th century, when, mainly because of the rise in popularity of equatorial instruments, they became ubiquitous. Many of the early domes that were built have been removed or replaced over the intervening years, including those at Greenwich and Dunsink. The Armagh 1790 dome remains in its original state with its original instrument, the Troughton Equatorial Telescope, still in place. It is particularly interesting to see how Francis Johnston has attempted to strengthen, what is inevitably a weak and flexible type of structure, namely a hemisphere with a slice taken out of it. A hemispherical dome, if supported evenly, would be a relatively strong structure, however, when a segment is removed to allow the enclosed telescope to view the sky, it becomes flexible and liable to warp. Domes are notoriously difficult to design, and to this day, many observatory domes, even new ones, function badly. One of the points Ussher raised with respect to astronomical domes was the necessity for good thermal insulation, so that the instrument was not heated unduly by the sun. This would give rise to differential flexure and expansion of the instrument as its various parts cooled at different rates during the following night. To overcome this problem and also to improve the rigidity of the dome, Francis Johnston lined the outer copper dome with light horizontal reeded lathes. The dome, after disuse for well over a hundred years, was restored to full working order in 1995.

To the east of the main building lies a low, single storey, building which, though now connected by a passageway, was possibly originally separate. This building housed the transit instrument and the Observatory's famous Earnshaw clocks. It appears to have been built along the lines suggested by Ussher, with independent stone piers for the clocks and instruments. About halfway along this building is a two storey tower, known as the `Sector Tower', which was built in 1841, probably over the existing building. This is a rather late date for such a tower as by then sectors had been superseded for measuring zenith distances of stars by a type of instrument which Armagh already possessed, namely a mural circle. Thomas Romney Robinson built the Sector Tower to house the Sisson Zenith Sector, one of the instruments from the King's Observatory at Kew, which had been presented to Armagh Observatory by Queen Victoria. Possibly Robinson was embarrassed by the gift of valuable, but outdated instruments, and wished to make some conspicuous sign that he could put them to use. In fact he was quickly defeated by its antiquated design and rejected it for accurate measurements. The tower now supports an anemometer by Monroe (1870) which was erected at Armagh as part of an automatically recording weather station set up by the Board of Trade. The Cup Anemometer, of which this is an early example, was, in fact, developed and first put to use at Armagh Observatory, by Robinson, its inventor.

Between the East Tower and the Sector Tower lies the Mural Circle Room, which was built to house a very fine, large, instrument for measuring star positions. The Mural Circle was mounted on a stone wall of massive construction, which unfortunately is no longer in place. Nevertheless, a wooden replica of the wall has been made and now serves to support the partly restored instrument. It was with this instrument that the bulk of the observations associated with the Armagh Catalogue of Stars, published in 1859, were made.

At the eastern extremity of the Observatory rises a round tower, shorter, but nevertheless rather reminiscent of those enigmatic bell towers of medieval Irish monasteries. However, instead of a conical roof, it carries a second copper dome, very similar to the earlier dome on the south side of the residence. The tower was built in 1827 and may possibly also be the work of Francis Johnston who died in Dublin two years later. It belongs to the second main phase of building at the Observatory which followed the appointment of John George Beresford as Archbishop of Armagh. This second dome, though it was originally built to house a Herschel reflector, became in 1834-35 the home of one of the most original and interesting telescopes that has been built in the British Isles. To understand its originality one must follow the development of telescope design through the 18th century to modern times.

In the early 18th century, refracting telescopes suffered from two severe limitations: firstly from chromatic aberration, which leads to a different focus for red and blue light, and secondly from spherical aberration which leads to a different focus for light from different parts of the lens. In order to mitigate these effects, refracting telescopes in the early 18th century were made extremely long, sometimes well over 100 feet in length. The great physicist, Isaac Newton, decided that the only remedy was to build telescopes with mirrors, i.e. reflecting telescopes. However, in the mid 18th century, Dolland showed how chromatic aberration, at least, could be overcome by combining lenses made of different types of glass. These are known as achromatic lenses, and their use, together with other improvements, enabled the length of refracting telescopes to be reduced. As the century progressed, astronomers demanded not only shorter and more convenient telescopes but also wider apertures to collect more light. The creation of large aperture refracting telescopes, (greater than 10 inches diameter), was just not possible at this time and in Britain and Ireland, through the influence of Newton and Herschel, the reflecting telescope became increasingly popular. Sir William Herschel made progressively larger reflecting telescopes, culminating in his great 40 foot reflector which had a mirror of 48 inches diameter. It was with another of his reflecting telescopes that Herschel discovered the planet Uranus in 1781.

However, Herschel's telescopes suffered from two principal disadvantages. Firstly, their altazimuth mountings, though ingenious in design, were cumbersome and difficult to maneuver; it required two people, winding winches at non-uniform rates, for the telescope to follow a star as it moved across the sky. Secondly the observer was situated at the prime focus at the top of the telescope, precariously perched, high above the ground, in the pitch dark and exposed to wind and the elements. Both of these difficulties were first overcome in the telescope once housed in the East Dome of Armagh Observatory. This instrument, a 15 inch reflector, was mounted on a polar axis around which it was driven at a constant rate by a clock. The principle had been used, just a few years earlier by Fraunhofer, at Dorpat in Germany, for a refracting telescope, but the Armagh telescope is believed to be the first large reflector in which this principle was employed. The simple clock-drive counteracted the Earth's rotation and allowed the telescope to follow the star with minimum human intervention.

The second innovation in this telescope was the use of a `Cassegrain' focus. In this design a curved secondary mirror at the top of the telescope reflects the beam of light back down into the telescope tube, through a hole in the primary mirror, to a focus that is easily accessible from the floor of the dome. This type of focussing, which had never previously been used in a large telescope, had the added advantage that it allowed the telescope to be made sufficiently short that it could be housed inside a dome. Another original feature of the Armagh 15 inch was the mirror support mechanism, which has been copied and improved upon in many later instruments. All of these factors made the 15 inch telescope one of the most practical and convenient to use of its time. It was the first to be made by the world-famous telescope builders, Grubb of Dublin, (who later became Grubb-Parsons of Newcastle), and the features it incorporated were used in many subsequent telescopes, right up to the modern era. One of the most famous of these was the `Great Southern Telescope', built by Grubb of Dublin, for the state of Victoria in 1869. The third director of Armagh Observatory, Thomas Romney Robinson, who chaired the Committee of the Royal Society responsible for the design of the telescope, frequently referred to his experience with the Armagh reflector. Regrettably, this unique telescope was largely broken up in the early 20th century, but sufficient parts of its drive, mounting, optics and mirror support survived for it to be reconstructed by the Sinden Optical Company in the early years of the 21st century. The last large reflector in the long line of telescopes built by Grubb, which began with the 15 inch in Armagh, was the giant `4.2 metre William Herschel Telescope', erected in the Canary Islands, just before the company went into liquidation a few years ago.

To the south of the East Dome, at some distance from the main Observatory building, are two low-lying domes, one of which houses the Robinson Memorial Telescope, built in 1885 by Grubb and the other an 18 inch Calver telescope that was later converted into a Schmidt telescope for celestial photography. The design of the Robinson Memorial Dome is of interest to architectural historians as an early example of a prefabricated building. The dome itself is carried on a circular rail mounted on a series of vertical struts which are held in position by diagonal tensioned bars. The principles employed bear some similarity to those of a tent and may derive from mid-19th century portable military buildings.

The Robinson Memorial Telescope was widely used by Dreyer, the fourth director, to check his earlier observations of nebulae made with Lord Rosse's great six-foot telescope at Birr. Ultimately, these observations were complied in his famous New General Catalogue of Nebulae and Clusters of Stars, first published in 1888 and still in print today. It is the order in which they occur in this catalogue which gives the nebulae their well-known NGC number. This world-famous book is probably the most important contribution to astronomy to have come from Armagh Observatory. The Robinson Memorial Telescope, or `Ten-Inch', as it is widely known, was also used more recently (1967), by Dr A. D. Andrews and colleagues in a remarkable observation of a huge flare (explosion) on a star, which was seen simultaneously from Armagh and with the giant 250 foot, radio telescope at Jodrell Bank. This type of work, now largely done with satellites, has provided a fruitful line of research for Armagh astronomers over the past two decades.

Beyond the immediate precincts of the Observatory, there are several buildings of architectural interest. Firstly, the small gate lodge at the entrance with its pointed window; an early use by Francis Johnston of the gothic style which featured in many of his more famous later buildings, including the Chapel Royal, Dublin Castle, Glanmore Castle, Co Wicklow and Charleville Castle, Co Offaly, and of which he is known as one of the pioneers in Ireland.

Further afield are the Meridian Marks, which were built to provide a stable reference point on the horizon with which the instruments could be checked for any disturbance. It was from observations of one of these that T.R. Robinson detected an earthquake in 1846. The earlier meridian marks were built of stone.

One, the northern mark, at Tullyards, just off the Armagh-Loughall Road, is an arch-shaped structure surmounted by two pinnacles. These served to provide the initial alignment of the transit instrument, with finer adjustment provided by a small copper disk with a triangular hole in its centre. A second meridian mark of similar design was built to the south of the Observatory at Corkley but was destroyed in the 19th century. Both of these marks were built in or around the early 1790's and were almost certainly the work of Francis Johnston. A second, southern, meridian mark which still survives was built at Ballyheridan, just beyond the southern extremity of the Palace Wall. It is a square neoclassical column and is probably also the work of Francis Johnston. Adjacent to the northern stone mark stands a cast iron obelisk in the Egyptian style. It is the work of Gardner's foundry at Armagh and was erected in 1864 when the Jones Mural Circle was converted by Robinson to serve as a meridian circle. With his modified instrument, Robinson was able to measure both coordinates of stars, (i.e. right ascension and declination), with the one instrument and a single observer, where previously he had required two. The fine adjustment of the instrument relied on an adjustable pointer which is hidden in the apex of the obelisk so that it was visible from the Observatory, but not to the vandals that so frequently tried Robinson's patience. Both of the northern markers have been recently restored by the Department of the Environment and may be visited by a short excursion from the City.

Last Revised: 2011 November 11th