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The 10" Grubb Refractor at Armagh Observatory

by Dr. C.J.Butler, Armagh Observatory

On 28 February 1882, Dr. Thomas Romney Robinson, one of Ireland's most famous 19th Century astronomers, died at the age of 89. He had been Director of Armagh Observatory for 59 years, a record for any observatory in the world that is believed to stand today. During this time, he contributed to many aspects of astronomical and meterological research and became one of the leading scientists of his day. As a memorial to his life and work, an appeal was launched for the erection of a modern instrument at Armagh to replace the by then ageing Mural Circle.

This appeal was rather less than successful in obtaining sufficient funds for the required instrument but, with the help of a government grant, it eventually became possible to consider a purchase. In the intervening period, Dr. J. L. E. Dreyer, Robinson's successor, decided that it would be appropriate for the work he had in mind to buy a telescope with an equatorial mounting which could observe the whole sky. With this, he would be able both to check and extend his work on nebulae, which began with the giant telescope at Birr, Co. Offaly, and also to make micrometer measurements of stars and nebulae.

After careful consideration of different designs, Dreyer opted for a conventional 10-inch refractor to be manufactured by the leading telescope maker of the day, Howard Grubb of Dublin. Other examples of this type of instrument by the same maker are to be found at Dunsink Observatory, near Dublin (1868) and University College Cork (1882). This type of instrument became the standard design for refractors in the second half of the 19th Century and the early part of the 20th Century, culminating in the large (aperture greater than 24 inches) Grubb refractors built at that time for Vienna (1878) and many other observatories of the world. Grubbs of Dublin continued as large telescope makers until the 1st World War when they moved to Newcastle, England, eventually to become Grubb-Parson. In recent years, they built the 150-inch Anglo-Australian Telescope and much of the William Herschel Telescope now in operation at the La Palma Observatory in the Canary Islands. The firm has now ceased telescope manufacture. It was fitting that Grubb was chosen to build the Robinson Memorial Telescope as, during his life, Robinson had been closely associated with many of Grubb's improvements in telescope design.

The new telescope was erected at Armagh on 28 July 1885 and, after some initial adjustment, was soon put to use by Dr. Dreyer. His observations with the new telescope concerned the checking of nebulous objects he had seen with Lord Rosse's giant 72-inch reflector at Birr and other nebulous objects subsequently reported to him. These observations were invaluable to him in his compilation of the New General Catalogue of Nebulae and Clusters of Stars, published in 1888. This book, commonly known as the NGC Catalogue, remains the pre-eminent catalogue of galaxies, nebulae and clusters to this day and is still widely used by astronomers. Dreyer made many other observations with the 10-inch refractor, including observations of the rings and disk of Saturn, the disk and moons of Jupiter, a transit of Mercury in 1914, and of sunspots. He also observed several comets, including the 1910 return of Halley's Comet.

Following the departure of Dr. Dreyer in 1916 to Oxford, Dr. W. F. A. Ellison took over as Director of Armagh Observatory. Together with his son, M. A. Ellison, he made many, many measurements of double stars with the 10-inch telescope and even discovered a new one close to 8 Lyrae. He also made some interesting observations of Saturn's moon Iapetus as it passed behind Saturn's rings.

In later years, the rather small aperture of the telescope, together with the vagaries of the Irish climate, have made observations with this telescope less and less attractive. Nevertheless, on 19 January 1969, Dr. A. D. Andrews and a group of enthusiastic amateur astronomers used the telescope to observe the cool dwarf star YZ Canis Minoris. They were fortunate in witnessing the largest flare ever detected on a star in the solar neighbourhood, an increase of 1.7 magnitude in the visual wavelength, which is equivalent to an increase in brightness of about a factor of five. This dramatic event was believed to have been detected simultaneously at radio wavelengths by the Jodrell Bank Radio Telescope. While Armagh astronomers no longer use the telescope for routine observation, the impetus from this discovery in 1969 has lead to a fruitful line of research at this Observatory in the study of stellar flares. Much of the present work of the Observatory staff concerns the observation of stellar and solar flares using X-ray and ultraviolet telescopes on satellites such as EXOSAT, IUE and SMM, and large ground-based telescopes such as the 74-inch telescope at Sutherland, South Africa and the Very Large Array Radio Telescope in New Mexico. Thus, the humble 10-inch telescope has continued to play its part in the story of Armagh Observatory.