Telescope Construction

The Casting of the Mirror

After his success with the 36 inch telescope, Lord Rosse turned his attentions to the building of a reflecting telescope with a mirror of 72 inches. The Great Telescope took over three years to construct, costing around 12 thousand pounds at the time, which is equivalent to over 2 million Irish pounds today. In the words of the then Director of the Armagh Observatory, Dr. Thomas Romney Robinson:

"Lord Oxmantown is about to construct a telescope of unequalled dimensions. He intends it to be 6 feet aperture and 50 feet focus......his character is an assurance that it will be devoted, in the most unreserved manner, to the service of astronomy, while the energy that could accomplish such a triumph, and the liberality that has placed his discoveries in this difficult art within reach of all, may justly be reckoned among the highest distinctions of Ireland."

In the middle of the 19th century, the technology to cast large metal coated glass mirrors did not exist and Lord Rosse was forced to use "speculum" metal instead. This consists of bronze, a mixture of copper and tin and the greater the proportion of tin, the more reflective (but also the more brittle) the mirror becomes. To enable the casting of a metal mirror, three crucibles, each 24 inches (0.61m) in diameter and weighing 1/2 ton, were used. The fuel used to heat the metal was turf, which was easily obtainable from the local large areas of bogland. More than than 2000 cubic feet of turf was burnt in the 15 hours it took to melt the speculum ingredients. Robinson describes the scene as follows:

"The sublime beauty can never be forgotten by those who were so fortunate as to be present. Above, the sky, crowed with stars and illuminated by a most brilliant moon, seemed to look auspiciously on their work. Below, the furnaces poured out huge columns of nearly monochromatic yellow flame, and the ignited crucibles during their passage through the air were fountains of red light, producing on the towers of the castle and the foliage of the trees, such accidents of colour and shade as might almost transport fancy to the planets of a contrasted double star. Nor was the perfect order and arrangement and everything less striking: each possible contingency had been foreseen, each detail carefully rehearsed; and the workmen executed their orders with a silent and unerring obedience worthy of the calm and provident self possession in which they were given."

A report in the Bristol Times newspaper and published in 1867 recounts that:

"I saw the Earl, the telescope maker himself, not in state with his coronet and ermine robe on, but in his shirt sleeves, with his brawny arms bare. He had just quitted the vice at which he had been working and, powdered with steel filings, was washing his hands and face in a coarse ware basin placed on the block of an anvil, while a couple of smiths sledging away on a blazing bar at another were sending a shower of sparks about his lordship which he little regarded as though he were a "Fire King". This was in a spacious, rude, smithy which almost occupies one side of the court yard of the castle and in which, not only were swing bridges and force pumps, and tackle for scientific instruments constructed, but common and everyday articles in the shape of agricultural gates, sub-soil ploughs etc, for use on his farms....As he drew on his coat....the Earl looked an intelligent foreman..."

Although the initial casting was completed with no major problems, the new mirror broke during grinding. The mirror was almost immediately recast, ground,and polished successfully. Lord Rosse devised a system of "secondary heating", which helped the molten mirror cool slowly and reduced the possibility of localised cooling, allowing the mirror to be cast as a single unit. Another three attempts were made to cast a second mirror and on the sixth attempt, Lord Rosse had himself a spare mirror, to be fitted to the telescope when the first mirror needed re polishing.

Grinding & Polishing of the Mirrors

In contrast to many of his telescope making predecessors and contemporaries, who sometimes jealously guarded their trade secrets, the third Earl was quite open and published all his important findings to, in own words:

"Simplify the process necessary for the manufacture of a good reflecting telescope....so that any person of ordinary mechanical skill who may think it worthwhile to erect the necessary machinery, will be enabled to obtain with certainty the same results."

Lord Rosse then used a steam-driven machine to grind the mirror to the necessary shape, that of a parabola, which brings the light reflecting off the mirror to a focus further up within the telescope tube. Polishing was then required to maximise the reflective potential of the speculum metal. The 3 ton mirror then needed a mirror cell to support its weight evenly and an initial 27 points of support was later increased by the Earl to 81 triangular areas.

The spare mirror was re polished on a machine set up in the castle courtyard and then transported on a wooden trolley to the telescope tube. The turntable and rails on the north side of the observatory greatly assisted in the manoeuvring of the extremely heavy mirror. The telescope was then hoisted to the vertical position and the tarnished mirror removed and replaced with the "new" mirror. The mirrors were polished once in every one to two years.

George Johnstone Stoney, one of the astronomers at Birr Castle wrote of this process:

"The mirror with the three tiers of levers on which it rested (and from which it could not be disturbed) and with their carriage, weighed about 7 tons (6350 kg). The whole had to be lifted from the polishing machine and transferred to a large truck on which it was slowly dragged by twenty five or thirty men for a distance of about 1/4 mile (0.4 km) to the observatory."

After the epic work involved in casting the primary mirror, the rest of the optical train - the secondary plane mirror and magnifying eyepieces - were relatively easily completed.

The Observatory and Telescope Tube

Lord Rosse realised that designing a proper mounting system for the telescope involved a compromise, between the conflicting aims of keeping the giant telescope steady in high winds and also attempting to provide the large range of movement necessary to view the constantly moving celestial sphere. Patrick Moore clearly states the dilemma Lord Rosse faced and his mechanical solution to it:

"To swing the huge 58-feet tube around in the manner of a modern telescope would have been impracticable: for one thing , there was wind-force to be considered, and if the instrument swayed around when in use, it would be of no value at all. The telescope would be mounted between massive walls, each 70 feet long and 50 feet high, so that it could be turned only toward that part of the sky which lay near the meridian-that is to say, the north-south line. This would mean a star could be followed for only a limited period. For an object on the celestial equator, the total viewing was about an hour each night; before and after this period, it would be hidden by one or other of the walls. This was unfortunate, but not so nearly disastrous as might be thought, because a star is at its highest when on the meridian, and so is less affected by the unsteadiness of the Earth's atmosphere. Moreover, every object would be brought around to its "viewing position" at some time or other."

Moore goes on to describe the telescope tube and observatory:

"The base of the actual mounting was a very massive joint of cast-iron, similar to a modern universal joint. On it was bolted a cubical wooden box, about 8 feet wide, carrying the speculum. This is turn carried the tube, which when vertical looked not unlike one of the old Irish round towers. The tube itself was 8 feet across in the middle, tapering to 7 feet at either end. It was made from inch-thick deal staves, hooped with iron clamp rings and strengthened by iron diaphragms; at the top was the secondary mirror, which was so heavy it had to be counterpoised. The movement in declination (i.e. up and down) was by means of a strong cable chain fixed to the top of the telescope, passing over a pulley fixed at a suitable height, down to a windlass on the ground operated by two assistants. The quick movement in right ascension (i.e. to and fro) was given from below by a wheel turned by another assistant. For each motion there were fine adjustments, so that small movements to keep an object in the field of view could be made by the observer using the telescope. Thoughtfully, Lord Rosse made provision for screens to protect assistants from "any casualty such as the fall of an eyepiece"

It has been written that anybody who used the great telescope had to be as much of a mountaineer as an astronomer, although there are no reports of accidents over the 60 year period the telescope was in operation. Quoting Dr. Robinson again:

"Though it is rather startling to a person who finds himself suspended over a chasm 60 foot deep, without more than a speculative acquaintance with the properties of trussed beams, all is perfectly safe."

The elaborate construction of the mounting system, with over 40 tons of ironwork castings, (which includes the two mirrors with their levers and trolley), provided smooth movements which were essential to the observations carried out by the telescope. The supporting walls are faced with cut limestone and filled with rubble bedded in lime mortar. They also helped guard the tube against danger from high winds, which had previously overturned the smaller 36 inch telescope in January 1839. Surprisingly however, a "finder" telescope, with a relatively large field of view to guide the observer to the target object, was never installed on the telescope. Instead Lord Rosse favoured a low power eyepiece. To quote Robinson again:

"There is a supplemental eyepiece of very wide field: a slide carries it ; and the holder of the others, so that by a little shift one can be substituted for the other in an instant."

The Performance of the Telescope

Although the famed French scientist and mirror maker Leon Foucault declared that "Le telescope de Lord Rosse est un blague" (Lord Rosses' telescope is a joke), other sources, including those astronomers who actually visited Birr and used the instrument, directly contradict this opinion. Various observers commented on the quality of the Birr telescopes, including Johnstone Stoney:

"The test usually applied was the performance of the mirror on the star of the 8th or 9th magnitude, magnification 750. Such stars are bright in the great telescope. They are usually seen as balls of light, like small peas, violently boiling in consequence of the atmospheric disturbance. If the night is good there will be moments now and then when the atmospheric disturbance will abruptly seen to cease for a fraction of a second and the star is seen for an instance as the telescope really presents it. It is by the opportunities of such moments that the performance of the telescope must be judged. With the best of your father's mirror that I saw, the appearance at such opportunities was like that of the light shining through a minute needle-point in a card placed in front of a flame. I think any practical astronomer will agree with me in the opinion that mirrors of 6 feet in diameter that bore the test bordered very closely indeed on theoretical perfection."

Dr. Robinson later wrote in the Proceedings of the Royal Irish Academy, giving his favourable impressions of the initial tests of the telescope:

"Only one view of the moon was obtained, March 20th, and it was shared with them by several visitors, who, when once in possession of the telescope, were by no means disposed to make way for the astronomers."

Another source of information on the excellence of the optics is a paper published in 1935 by A. Stanley Williams (an experienced planetary observer), commenting on a collection of drawings of Jupiter, made mainly with the smaller 36 inch reflector.

"The longitudes of a large number of spots were derived from these drawings by estimation, and it was an unexpected surprise and a great pleasure to find that the resulting positions were in excellent accordance with those derived from the central meridian transits. Nearly invariably the Rosse longitudes fell almost exactly where they should have done according to my provisional identification of the different spots....It was almost like working from a collection of photographs of the planet"

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