The Pavilion was built to house the new Altazimuth Instrument proposed by Christie in 1892. Of solid construction, and of two floors, it was erected in the only space that Christie had available, which was on the South Ground just to the north of the Magnetic House. To avoid excessive interference with the magnetic observations, the site selected was on the magnetic meridian of the declination magnet and 90 feet to the north of it. The centre of the building is 141 feet east and 166 feet south of the Airy Transit Circle.
Like the South Building, it is constructed of red brick and terracotta with a ring of porthole like windows to light the observing floor inside. Christie sometimes referred to the telescope as a universal transit circle, and to this end, as well as the main telescope, there were collimating telescopes at the north and south ends of the building which could be swung into position from beneath the movable roofs that covered them.The building has tunnel like entrances to the north and south that sit beneath the supporting masonry for the collimators. Most of the internal space is taken up by the massive supporting column for the main telescope. This sits on a six foot deep concrete foundation on which the whole of the building rests.
The terracotta decoration was limited by Crisp to the porticos surrounding the two entrances. The dome was built by Sir Howard Grubb and opened in a quite different way to any of the others at Greenwich. It had no shutters. Instead, the two halves of the dome could be slid outwards to create a 4½ foot wide opening. The weather vane represents Halley’s Comet and was added in June 1901.
The Altazimuth Instrument, which was mounted 20 feet above the ground, was never a success. It was dismantled by Spencer Jones at the start of WW2 and replaced with a new instrument to the observatory – a small transit instrument by Cooke Troughton and Simms that had been made for time determinations in Singapore, but never used for that purpose. A brick pier was built on top of the existing one and a raised wooden floor installed above the existing floor to make the telescope accessible for the observers.
After the announcement had been made in 1946 that the Observatory would be moving to Herstmonceux, the Ministry of Works, to whom the site was to revert on behalf or the Crown, drew up plans to return the southern half of the Observatory site to the Park. Under this plan, only two buildings were to escape demolition, the South Building Store which was destined to become a public lavatory and the Altazimuth Pavilion which was scheduled to be adapted as a shelter by the opening up of the windows on both the east and the west side. Its future looked in doubt when during a ministerial visit that took place on 22 April 1958; Hugh Molson (the Government Minister with oversight for the project) indicated that he wanted it demolished.
In the end, the building was retained and passed into the care of the National Maritime Museum where it was adapted for use both as a shelter and as a new home for the Sheepshanks Equatorial. To this end, the staircase was retained and only the western half adapted as a shelter. Although the terracotta around the southern doorframe was renewed, the rest of the terracotta on the southern end was not. A new concrete pier was constructed on top of the base of the brick pier that had been built for the small transit instrument. The dome was re-clad in fibreglass somewhere around 1965.
The Sheepshanks Equatorial was used by local groups who were given access to the Observatory at night. It remained in place from 1963 until 1982, when it was replaced by the 4-inch Dallmeyer photoheliograph and the Newbegin 6¼-inch refractor following their discontinued use at Herstmonceux. Nick Sidle has two fabulous wide-angle images of the telescope and interior of the dome taken in 1986 which he has put on flickr. They can be viewed by clicking here and here. The photoheliograph was used with schools and the public until around 2004. It was dismounted and placed in storage on 6 October 2017 in order to make way for two modern telescopes.
The dome was (and still is) opened and turned by hand. There does not appear to have been any physical mechanism to prevent the wind from blowing it shut, or causing it to rotate. Whilst undoubtedly irritating, this would not have damaged the original Altazimuth Instrument, but the photoheliograph and the Newbegin refractor were too large for the dome and vulnerable to being knocked by the internal structure, not only if the wind caught the dome, but also when they were being manipulated into position. This was particularly problematic with the photoheliograph, which, because it was on a German Equatorial mounting, needed to be flipped over at around midday in order to observe on the opposite side of the meridian. Also problematic was the safety of the public as a result of the raised floor. There were three potential hazards, all of which were managed effectively over the years. Firstly, the steel beam supporting the dome had to be ducked under in order to gain access to the raised floor. Secondly, there were no safety railings at its edge, though in practice, the rail of the dome, which was around waist height, performed this role. Thirdly, there was the danger that members of the public might get their hands or hair caught during the turning of the dome.
During the construction of the Peter Harrison Planetarium, the ground floor of the Altazimuth Pavilion was remodelled and the windows on the west side reinstated. To improve health and safety, a new less steep staircase was installed together with a new raised floor of considerably smaller diameter. Although the latter removed the possibility of individuals getting their hands caught by the dome as it was turned, it created several new problems that hadn’t been properly considered when the changes were planned. Firstly, it became much more difficult to access the lens caps safely and manipulate the telescope and the dome. Secondly, the reduced floor area available at the eye-end of the telescope made if difficult or imposible to observe. These factors together with changing priorities in the Museum, brought the regular use of the photoheliograph to an end.
Alterations were also made to the level, direction of slope and the surface finish of the ground surrounding the building. This resulted in water flowing off the newly paved surface directly into the brickwork at the southern end of the building (and above where a damp course might be expected to be). In order to try and cure the problem, a drainage gulley was later installed adjacent to the building.