The 12" Grubb Refractor at Dunsink Observatory
by Professor Patrick Wayman
Among the many benefits of Dublin Millennium year (1988) was therenovation of the historic telescope made by the firm of Thomas Grubb ofRathmines in the 1850's. This Dunsink instrument, with a 12-inch lens ofFrench manufacture, was important in the sequence of astronomicaltelescopes 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 as21 refractors of 13 ins. aperture or greater, as well as a number ofreflecting telescopes of moderate size and many smaller telescopes. Thelargest of the refractors was the 27-inch Vienna telescope completed in1878 for the Imperial Royal Observatory of Austria-Hungary, now theastronomical observatory of the University of Vienna.
The telescope at Dunsink uses a lens donated to Trinity College in 1863by Sir James South, F.R.S., a Victorian astronomer of private means whoengaged in many heated controversies with his contemporaries, but whomaintained a friendship with the Third Earl of Rosse and T. RomneyRobinson at Armagh Observatory. The mounting made by Grubb's was notoriginally intended to carry South's lens but was exhibited in the DublinIndustrial Exhibition of 1853 on Leinster Lawn, Merrion Square. Themodification to take the Cauchoix lens and the proposal to purchase themounting for Dunsink Observatory was probably carried out at the urgingof Romney Robinson. He saw the need for a new instrument, following thelong tenure of Sir William Rowan Hamilton as Andrew's Professor ofAstronomy. F. F. Brünnow was appointed to the chair in 1865 andimmediately set about replacing the original equipment of the 1780's madeby Jesse Ramsden. Hamilton had pursued theoretical astronomy andmathematics with great energy but had only taken a marginal interest inpractical astronomy, and had left all such matters to his assistant formany years by the time of his death.
The telescope that Grubb provided was a successor to a similarinstrument made in the 1830's for Edward J. Cooper of Markree Castle,Co. Sligo, one of the distinguished private individuals in 19th centuryIreland who contributed to astronomy quite effectively. This instrumentwas sold to a Jesuit seminary in Hong Kong in the 1930's, where itsuffered damage by bombing in World War II, when it is said that it wasmistaken for a gun by a Japanese pilot. The 14-inch lens, also byCauchoix, survives to this day in Manila in the Philippines Islands.Grubb telescopes exhibit three or four interesting characteristics. Theyuse large-diameter bearings for good stability, and the weight on thebearings is relieved by pressure of counterbalancing rollers. The flexureof the main tube, which at Dunsink is of tinned iron, is reduced byinternal longitudinal ribbing. The equatorial driving clock, powered bya falling weight and controlled by a centrifugal governor, is carefullydesigned to give an adjustable accurate drive rate.
Later designs used visual devices to read the silveredposition-scales remotely, either at floor level or at the eye end of thetelescope, but the Dunsink instrument is defective in having singularlyinaccessible verniers for reading hour angle and declination.
The refinements that followed the Dunsink instrument and wereincorporated on the Vienna telescope included provision for mechanicalcontrol of the drive rate by a pendulum, thus increasing the potentialaccuracy of drive by an order of magnitude. In practice, this accuracywas only achieved in the late 1880's when the photographic era oftelescope use became established and it became possible to incorporateelectrical control. The ingenious Grubb telescope control, produced byThomas Grubb's son and worthy successor Howard Grubb (later Sir HowardGrubb F.R.S.) survived virtually unchanged in Grubb telescopes up to the1930's. By this date valve amplifiers, synchronous motors, andelectrically maintained tuning forks were available.
Modern instruments that derive from Grubb's of Dublin include theproducts of a daughter firm, Sir Howard Grubb, Parsons & Co founded inNewcastle-upon-Tyne in 1925 and continuing there until 1985. Among manyinstruments produced in Newcastle, four telescopes have been erectedsince 1980 on the Spanish island of La Palma, where Ireland has a sharein 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 theRoyal Greenwich Observatory and now considerably re-equipped, the verynew 4.2 metre William Herschel Telescope, the "Jewel in the Crown" of theisland observatory on La Palma, and finally the Danish-UK CarlsbergAutomatic Transit Circle, probably the most productive classicalinstrument for stellar positions in the world. Other modern Grubb Parsonstelescopes are the tube and optics of the Anglo-Australian telescope atSiding Spring, New South Wales, one of the most successful of thesouthern hemisphere telescopes, and the UK Schmidt at the same site.
In 1900 there were about six engineering firms in the world capableof making large astronomical telescopes and Grubb's of Dublin was one ofthose firms. Howard Grubb had travelled to California to discuss theproject initiated in the 1880's by James Lick to place a large telescopeon the summit of Mount Hamilton at an altitude of 5000 feet above sealevel. The instrument that was erected, a 36-inch refractor, was built byWarner and Swasey of Cleveland, Ohio, who entered telescope building, asdid Grubb's, from a background of making machine tools. The Lickrefractor became the premier instrument of its day, being the world'slargest refractor telescope on the world's best astronomical site. It wasbuilt with great enterprise and with that aim in mind. Howard Grubb wasbitterly disappointed that his bid, lower than that of Warner and Swasey,was not accepted; the adopted design owed a good deal to Grubb's ideasand he was paid a fee as a consultant. There followed George ElleryHale's 40-inch refractor at Yerkes Observatory in Wisconsin, relativelyconvenient for Chicago, but much less effective because its site wasseriously inferior to Mount Hamilton.
All large instruments since 1900 have been reflectors, starting withHale's 60-inch and 100-inch reflectors on Mount Wilson in southernCalifornia, right up to the William Herschel telescope of the presentdecade. This telescope is on a very fine site, arguably the best in theworld in some respects, and is available for approved programmes of workto Irish observers through the Panel of the Allocation of Telescope Time(PATT) of the UK Science and Engineering Research Council (SERC). The School ofCosmic Physics at Dunsink Observatory is responsible for theadministration of the Irish participation at La Palma.
The fine tradition of instrument making that the work of Thomas andHoward 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 Rathminesfirm. During World War I, submarine periscopes were made for the BritishRoyal Navy and, for security and logistic reasons, the firm wastemporarily removed to St. Albans in Hertfordshire. By 1925, Howard Grubbwas 81 years old and the firm was closed down. In its early days inDublin, the business of Thomas Grubb had included the design andconstruction of printing machines for producing the banknotes of the Bankof Ireland, whereby hundreds of thousands of identical numbered notes hadto be produced. Starting from the type of machine used by the Bank ofEngland, Thomas Grubb made many important improvements and several ofGrubb's machines were still in use in the 1920's.
Examination of Grubb's work always shows how carefully the designswere worked out. Items such as weight-driven clock drives were scaled upor down according to the size of telescope, with minor modifications asrequired. The distinctive style of Grubb was imposed on every major itemproduced. Generally speaking, massiveness in the major components,elegant finish to brasswork, and proper provision of devices formechanical adjustment were the hallmarks of the designs. If there was acharacteristic fault with the Grubb designs it arose from the hand-made nature of the products, sothat screws might not be interchangeable because they are of differentlengths or clearances. Parts were always dot-marked or numbered, and incertain cases, the order in which parts have to be dismantled couldbe an inconvenience.
Following repair of the dome shutters in 1985, it was decided in 1987to proceed with renovation of the South telescope in order to ensure itspreservation as far as possible in its original form. The work ofdismantling, renovation, repair and re-assembly was carried out veryskilfully by Mr. Jeremiah Daly, Experimental Officer in the School ofCosmic Physics. This was probably the first time that the telescope hadbeen completely dismantled since it was erected in 1868. During therenovation, 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, butgenerally speaking, no changes were made to the original design - ratherthe original design was re-instated and later additions removed, in therespectful spirit due to antiques.
Editor's note: This article by Professor Wayman originally appeared inthe September 1988 issue of "Technology Ireland" and is reproduced bykind permission of the author.