Thomas Romney Robinson, the third director of the Observatory, was, by all accounts, a remarkable man of many interests. He was also a child prodigy – able to read poetry by his third year and to write verse by his fifth. He published a book of poems in his thirteenth year.
As a schoolboy his technical and scientific interest developed rapidly and in his poem ‘The Triumph of Commerce’, which he wrote in his ninth year, he combined his practical interests with his artistic abilities to praise his friend Mr. W. Richie the founder of the Belfast Shipyards. Robinson continued his brilliant academic career by taking his BA at sixteen years old and becoming a fellow of Trinity College, Dublin at 21.
In 1823 he was appointed director of Armagh Observatory a post which he retained for a total of 59 years and a world record for an observatory director which still stands today. Whilst at Armagh he retained his living as rector of Carrickmacross which supplemented his rather meagre income as Observatory director. He was a brilliant orator and was popular amongst the learned societies of Ulster where his rousing speeches were met with rapturous applause. He was a founder member of the Armagh Natural History and Philosophical Society, President of the Royal Irish Academy in Dublin and President of the British Association for the Advancement of Science. He married twice, first Elizabeth Rambaut, and secondly, Lucy Jane Edgeworth, a sister of Maria Edgeworth the author.
After the death of Archbishop Richard Robinson, (no relation to TRR), in 1794, his successors, who had no interest in science, made little provision for the improvement of the Observatory. To be truthful this may have well have partly resulted from the lack of enthusiasm of the second director, W. Davenport (1815-1823), whose contribution to astronomy was practically nill and who, after eight fruitless years, committed suicide in his study. This could easily have been the end of the story for Armagh Observatory but, it was not to be so; and as the young and energetic Thomas Romney Robinson took up his office he quickly dispelled any remaining doubts.
T R Robinson was fortunate in that, by the time of his appointment, Primate John George Beresford, was created Archbishop of Armagh. He was a rich and generous person who obviously took a great deal of interest in science and in the Observatory. He contributed several thousand pounds of his own money to refurbish the Observatory with the best instruments available.
T R Robinson made it plain in his report to the Governors that in his opinion ‘the job of an observatory was to furnish accurate observations of the stars (positions of stars).’ Theoretical astronomy, by which he meant principally celestial mechanics, could be carried out anywhere, and by anyone, with sufficient mathematical knowledge.
In spite of its novel design, the Troughton Equatorial Telescope never fullfilled expectations and Robinson, quite rightly, saw no alternative but to return to the more straight-forward transit and meridian circle type of instrument. By this time, (late eighteen twenties), the old meridian quadrant design used in the 18th century, had been superseded by the mural circle – where the telescope and its assemblage of divided circle and microscopes were fixed to a stone wall of massive construction. This design provided the rigidity and stability that is required for careful measurement of star positions. Both the Mural Circle and the Transit Instrument were built by Jones of London whose reputation as an instrument maker was rapidly increasing at that time.
Together the two instruments were a powerful combination and, though similar pairs were set up in UK and Commonwealth countries during the nineteenth century, those at Armagh are the only pair known to survive to this day on their original site.
The tedious work of measuring the positions of stars occupied astronomers at Armagh Observatory for most of the nineteenth century. The computations required to correct measurements for small errors introduced by the alignment of the telescope, refraction and various other effects, were extremely laborious in those days of hand calculation. Nevertheless, eventually, Robinson was able to compile the first Armagh Catalogue of stars which he published in 1859. It is a monument to the enthusiasm and perseverance of himself and his dutiful assistants and it established the position of Armagh Observatory as a scientific institution of national and international importance. The stellar positions recorded in this book were ultimately combined with those from other observatories around the world to form a fundamental catalogue of stars which defines ‘the reference frame of the Universe’ against which the movements of the planets are measured.
The Development of the Reflecting Telescope
Thomas Romney Robinson was a close friend of the Dublin instrument maker Thomas Grubb; the founder of one of the most important telescope building companies in the nineteenth century. In 1834 Thomas Grubb built for Armagh Observatory a 15 inch diameter reflecting telescope which incorporated several revolutionary innovations which have subsequently become widely accepted in telescope design.
- It was a Cassegrain telescope, rather than a Newtonian, as were almost all other reflecting telescopes built in Britain or Ireland at that time. The Cassegrain design had never previously been used in a large telescope.
- It was the first large reflecting telescope to be mounted on a polar axis with a clock drive.
- It incorporated a novel lever support system for the primary mirror.
Whilst, regrettably, this unique telescope was dismantled and largely broken up in the 1920’s its place in the history of telescope design is ensured.
The Cassegrain versus the Newtonian telescope design
As telescopes increased in size in the late eighteenth and early nineteenth centuries, the inconvenience of the Newtonian system where the observer perches precariously at the top of the telescope exposed to wind and the elements, became apparent. The Cassegrain telescope design avoids this by using a curved secondary mirror, which projects the image downwards through a hole in the primary mirror, to a focus just below the bottom of the telescope. Here the image is conveniently positioned for an observer on the ground.
Robinson was a life-long proponent of the Cassegrain design for large telescopes and it was after his experiments with the Armagh 15 inch Grubb Telescopethat he pushed this design in preference to the rival Newtonian design for the Great Southern Telescope built by the state of Victoria in Melbourne, Australia in 1870. From that time until the modern era most large telescopes feature a Cassegrain focus.
The polar or equatorial mounting and clock drive
The great telescopes built by Sir William Herschel in England, in the late eighteenth century, dramatically increased the power of telescopes available to astronomers. However they suffered from several serious disadvantages.
Firstly they used an alt-azimuth form of mounting which, in order to reach any particular part of the sky, required the telescope to move both in azimuth and altitude. Once the celestial object had been located it was then only possible to follow the object as it moved across the sky, (due to the earths rotation), by moving the telescope in both azimuth and altitude simultaneously. For a large telescope this required a minimum of two assistants who turned the winches to move the telescope on instructions shouted by an observer at the top.
Robinson and Grubb realised the tremendous advantages and greater simplicity of the alternative, equatorial or polar mounting, such as had been used by Troughton in the instrument he built for Armagh in 1795. In order to keep the star in view, such a telescope required movement in only one direction (about the polar axis) and this movement was at a uniform rate of 4 degrees per minute (or 1 rotation per day). Conversely the movement in altitude and azimuth in the old style alt-azimuth mounting was non-uniform in both directions. The requirement of a uniform rotation about the polar axis could easily be provided by driving a telescope with a clock. This innovation was in fact first used on the great Dorpat Refractor in 1824 by the German astronomer Fraunhofer. The Armagh 15 inch Grubb telescope is, however, the first large reflecting telescope in which this principle was used.
The lever mirror-support cell
Telescope mirrors of the eighteenth and early nineteenth century were made of an alloy of copper and tin called speculum metal. It is an extremely fragile material which shatters with the least disturbance or even application of heat. It was used because of its high reflectivity.
One of the problems encountered by the telescope makers of the day was the support of this very heavy and fragile mirror in the telescope. If it simply rested on a flat bed, small humps on the back surface of the mirror took a disproportionate amount of the weight and, as a result, the mirror bent slightly and the image became distorted. To overcome this problem Grubb invented the lever support system for the Armagh reflector whereby the supporting pressure is evenly distributed over the back surface of the mirror. This design was later incorporated in the mirror cell for Lord Rosse’s Six-Foot Reflector and the Great Southern Telescope at Melbourne and has been copied, in one form or another, in many of the large telescopes built to this day.
Thomas Grubb and his son Howard Grubb were two of the most important telescope makers of their day and the company which bears their name – Grubb Parsons – recently built the 2.5 metre Isaac Newton Telescope and the 4.2 metre William Herschel Telescope in the Canary Islands.
The Great Six-Foot Telescope At Birr
In addition to Thomas and Howard Grubb of Dublin, Ireland produced a third great telescope builder of the nineteenth century, perhaps the greatest of them all; William Parsons the third Earl of Rosse. It was at his country seat in Birr, County Offaly that the world’s largest telescope stood for 70 years, until it was superseded by the 100 inch reflector on Mount Wilson during this century. Thomas Romney Robinson was closely involved in the construction of this telescope and vividly described the casting of the great, 6 foot diameter, speculum mirror.
“On this occasion, besides the engrossing importance of the operation, its singular and sublime beauty can never be forgotten by those who were so fortunate as to be present. Above, the sky, crowded with stars and illuminated by a most brilliant moon, seemed to look down auspiciously on their work. Below, the furnesses poured out huge columns of nearly monochromatic yellow flame, and ignited crucibles, during their passage through the air, were fountains of red light, producing on the towers of the castle and foliage of the trees, such accidents of colour and shade as might almost transport fantasy to the planets of a contrasted double star.”
He was present with Sir James South at the first viewing with the ‘Leviathan of Parsonstown’, as the telescope came to be known, and described many observations of nebulae. This instrument was the first to show the spiral nature of the external galaxies, which, like our own Milky Way, are made up of millions of faint stars.
In 1852 Robinson was asked by the Royal Society to chair a committee to report on the best design for a large telescope to be built in the southern hemisphere. This telescope which came to be known as the ‘Great Southern Telescope’ was built by Grubb in Dublin and incorporated many of the innovations which Robinson, Grubb and Lord Rosse had pioneered. Also, it marked the start of the involvement in southern hemisphere astronomy by Irish Astronomers – an involvement that has continued at Armagh and Dunsink Observatories to the present time.
