IAS logo

The 12" Grubb Refractor at Dunsink Observatory

by Professor Patrick Wayman

Among the many benefits of Dublin Millennium year (1988) was the renovation of the historic telescope made by the firm of Thomas Grubb of Rathmines in the 1850's. This Dunsink instrument, with a 12-inch lens of French manufacture, was important in the sequence of astronomical telescopes built in Dublin by Grubb's, many of which are still in use. Following the building of the Dunsink refractor, Grubb's made as many as 21 refractors of 13 ins. aperture or greater, as well as a number of reflecting telescopes of moderate size and many smaller telescopes. The largest of the refractors was the 27-inch Vienna telescope completed in 1878 for the Imperial Royal Observatory of Austria-Hungary, now the astronomical observatory of the University of Vienna.

The telescope at Dunsink uses a lens donated to Trinity College in 1863 by Sir James South, F.R.S., a Victorian astronomer of private means who engaged in many heated controversies with his contemporaries, but who maintained a friendship with the Third Earl of Rosse and T. Romney Robinson at Armagh Observatory. The mounting made by Grubb's was not originally intended to carry South's lens but was exhibited in the Dublin Industrial Exhibition of 1853 on Leinster Lawn, Merrion Square. The modification to take the Cauchoix lens and the proposal to purchase the mounting for Dunsink Observatory was probably carried out at the urging of Romney Robinson. He saw the need for a new instrument, following the long tenure of Sir William Rowan Hamilton as Andrew's Professor of Astronomy. F. F. Brünnow was appointed to the chair in 1865 and immediately set about replacing the original equipment of the 1780's made by Jesse Ramsden. Hamilton had pursued theoretical astronomy and mathematics with great energy but had only taken a marginal interest in practical astronomy, and had left all such matters to his assistant for many years by the time of his death.

The telescope that Grubb provided was a successor to a similar instrument made in the 1830's for Edward J. Cooper of Markree Castle, Co. Sligo, one of the distinguished private individuals in 19th century Ireland who contributed to astronomy quite effectively. This instrument was sold to a Jesuit seminary in Hong Kong in the 1930's, where it suffered damage by bombing in World War II, when it is said that it was mistaken for a gun by a Japanese pilot. The 14-inch lens, also by Cauchoix, survives to this day in Manila in the Philippines Islands. Grubb telescopes exhibit three or four interesting characteristics. They use large-diameter bearings for good stability, and the weight on the bearings is relieved by pressure of counterbalancing rollers. The flexure of the main tube, which at Dunsink is of tinned iron, is reduced by internal longitudinal ribbing. The equatorial driving clock, powered by a falling weight and controlled by a centrifugal governor, is carefully designed to give an adjustable accurate drive rate.

Later designs used visual devices to read the silvered position-scales remotely, either at floor level or at the eye end of the telescope, but the Dunsink instrument is defective in having singularly inaccessible verniers for reading hour angle and declination.

The refinements that followed the Dunsink instrument and were incorporated on the Vienna telescope included provision for mechanical control of the drive rate by a pendulum, thus increasing the potential accuracy of drive by an order of magnitude. In practice, this accuracy was only achieved in the late 1880's when the photographic era of telescope use became established and it became possible to incorporate electrical control. The ingenious Grubb telescope control, produced by Thomas Grubb's son and worthy successor Howard Grubb (later Sir Howard Grubb F.R.S.) survived virtually unchanged in Grubb telescopes up to the 1930's. By this date valve amplifiers, synchronous motors, and electrically maintained tuning forks were available.

Modern instruments that derive from Grubb's of Dublin include the products of a daughter firm, Sir Howard Grubb, Parsons & Co founded in Newcastle-upon-Tyne in 1925 and continuing there until 1985. Among many instruments produced in Newcastle, four telescopes have been erected since 1980 on the Spanish island of La Palma, where Ireland has a share in the operation and use of the UK Optical Telescopes, the so-called "Isaac Newton Group". These are the 1-metre Jacobus Kapteyn reflector, the 2.5 metre Isaac Newton Telescope originally erected in 1967 at the Royal Greenwich Observatory and now considerably re-equipped, the very new 4.2 metre William Herschel Telescope, the "Jewel in the Crown" of the island observatory on La Palma, and finally the Danish-UK Carlsberg Automatic Transit Circle, probably the most productive classical instrument for stellar positions in the world. Other modern Grubb Parsons telescopes are the tube and optics of the Anglo-Australian telescope at Siding Spring, New South Wales, one of the most successful of the southern hemisphere telescopes, and the UK Schmidt at the same site.

In 1900 there were about six engineering firms in the world capable of making large astronomical telescopes and Grubb's of Dublin was one of those firms. Howard Grubb had travelled to California to discuss the project initiated in the 1880's by James Lick to place a large telescope on the summit of Mount Hamilton at an altitude of 5000 feet above sea level. The instrument that was erected, a 36-inch refractor, was built by Warner and Swasey of Cleveland, Ohio, who entered telescope building, as did Grubb's, from a background of making machine tools. The Lick refractor became the premier instrument of its day, being the world's largest refractor telescope on the world's best astronomical site. It was built with great enterprise and with that aim in mind. Howard Grubb was bitterly disappointed that his bid, lower than that of Warner and Swasey, was not accepted; the adopted design owed a good deal to Grubb's ideas and he was paid a fee as a consultant. There followed George Ellery Hale's 40-inch refractor at Yerkes Observatory in Wisconsin, relatively convenient for Chicago, but much less effective because its site was seriously inferior to Mount Hamilton.

All large instruments since 1900 have been reflectors, starting with Hale's 60-inch and 100-inch reflectors on Mount Wilson in southern California, right up to the William Herschel telescope of the present decade. This telescope is on a very fine site, arguably the best in the world in some respects, and is available for approved programmes of work to Irish observers through the Panel of the Allocation of Telescope Time (PATT) of the UK Science and Engineering Research Council (SERC). The School of Cosmic Physics at Dunsink Observatory is responsible for the administration of the Irish participation at La Palma.

The fine tradition of instrument making that the work of Thomas and Howard Grubb represents was not limited to astronomical telescopes. Surveying instruments of ingenious design, magnetic recorders, seismographs, and at least one tower clock were built by the Rathmines firm. During World War I, submarine periscopes were made for the British Royal Navy and, for security and logistic reasons, the firm was temporarily removed to St. Albans in Hertfordshire. By 1925, Howard Grubb was 81 years old and the firm was closed down. In its early days in Dublin, the business of Thomas Grubb had included the design and construction of printing machines for producing the banknotes of the Bank of Ireland, whereby hundreds of thousands of identical numbered notes had to be produced. Starting from the type of machine used by the Bank of England, Thomas Grubb made many important improvements and several of Grubb's machines were still in use in the 1920's.

Examination of Grubb's work always shows how carefully the designs were worked out. Items such as weight-driven clock drives were scaled up or down according to the size of telescope, with minor modifications as required. The distinctive style of Grubb was imposed on every major item produced. Generally speaking, massiveness in the major components, elegant finish to brasswork, and proper provision of devices for mechanical adjustment were the hallmarks of the designs. If there was a characteristic fault with the Grubb designs it arose from the hand-made nature of the products, so that screws might not be interchangeable because they are of different lengths or clearances. Parts were always dot-marked or numbered, and in certain cases, the order in which parts have to be dismantled could be an inconvenience.

Following repair of the dome shutters in 1985, it was decided in 1987 to proceed with renovation of the South telescope in order to ensure its preservation as far as possible in its original form. The work of dismantling, renovation, repair and re-assembly was carried out very skilfully by Mr. Jeremiah Daly, Experimental Officer in the School of Cosmic Physics. This was probably the first time that the telescope had been completely dismantled since it was erected in 1868. During the renovation, only two parts had to be re-made because of wear-and-tear; one or two items had to be supplied because they were missing, but generally speaking, no changes were made to the original design - rather the original design was re-instated and later additions removed, in the respectful spirit due to antiques.

Editor's note: This article by Professor Wayman originally appeared in the September 1988 issue of "Technology Ireland" and is reproduced by kind permission of the author.