The 28-inch photo-visual refractor

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'How many ecstatic hours does the astronomer spend at the eye-end, high up on his scaffold-like observing chair, communing with other worlds during the darkest hours of the night? No wonder, then, that the making of colossal 'equatorials' should be replete with wondrous incident, and the details of their history almost beyond belief.'
Fitzgerald, Strand Magazine, 1896

28-inch refracting telescope at the Royal Observatory28-inch refracting telescope at the Royal Observatory 

The 28-inch Greenwich refracting telescope is the largest of its kind in the UK and the seventh largest in the world. Completed in 1893 as a replacement for the 12¾-inch Merz that originally occupied its position, it was commissioned in 1885 by William Christie, Astronomer Royal between 1881 and 1911. It was designed to keep the Royal Observatory at the forefront of contemporary astronomy and more active in the growing disciplines of astrophysics and photography. The job of constructing the telescope was given to Howard Grubb, an Irish optical manufacturer who was then the world leader in the field.

Although the telescope was removed from Greenwich to Herstmonceaux in 1957, it was used for research into double star systems throughout its working life until its retirement in the late 1960s. It was returned to Greenwich in 1971, and has become a central part of educational programmes at the Royal Observatory. With the recent addition of a computer-aided guidance system and CCD camera, it continues to work as an excellent visual aid to observing the night sky.

The 28-inch lens

'...the finest pair of discs of their size which have ever been made.'
H. Grubb

The south-east dome with the shutter openThe south-east dome with the shutter open, c. 1897 (B304). 

It is ironic that the one part of the telescope the visitor never sees is the most important. The 28-inch lens, recessed inside the telescope tube, was rarely seen even by those who used it. Yet this invisible component was so difficult to make that there were only two glassmakers in the world capable of producing it; the Chance Brothers in Birmingham and Charles Feil in Paris. Such were the problems involved that delivery time was estimated in years.

The 28-inch lens was to be of revolutionary design, allowing the telescope to perform as an observational and photographic instrument - which normally required two telescopes. With the help of the mathematician George Gabriel Stokes - a friend of both the Astronomer Royal and Howard Grubb - a lens was designed which was capable of being reversed and repositioned within the telescope tube, according to the type of observation being undertaken. This repositioning would achieve the necessary optical corrections. However, although Stokes's theory was sound, it proved flawed in practice. After detailed testing showed this to be the case, the proposed role of the 28-inch telescope was changed to double star measurement and over the next 70 years it completed many successful observation programmes. In 1896, Howard Grubb mounted a 26-inch refractor at the Observatory, specifically designed for photographic observations.

The job of casting the lens was given to the Chance Brothers of Birmingham and late in 1888, almost three and a half years and 15 individual casting failures later, the blanks were ready for polishing and finishing. Although it is usually referred to as one lens there are in fact two separate elements; the crown and the flint. When Chance finally delivered the unfinished blanks they were received with great enthusiasm by Howard Grubb, who said they were 'the finest pair of discs of their size which have ever been made'.

The polishing process went through several stages. First, the very rough surface irregularities were taken off with the abrasive combination of sand and water. Then progressively fine emery was applied to the emerging lens. Next, a long series of polishes with jewellers' rouge almost completed the surface. Finally, perhaps the most critical stage began, as Grubb himself completed the 'local touching', taking into account any small irregularities. By this stage the glass was being painstakingly monitored, inch by inch, under a microscope. In between each of the polishing stages it also underwent a continuous and rigorous series of tests, numbering in thousands by the time the lens was ready for use.

The completed lens, weighing 200lbs, was finally put into position in 1893, over eight years after it was originally commissioned

The mount

'The great stability of the English form of mounting, therefore, commended it very highly to Airy, and he designed the great Northumberland equatorial of the Cambridge Observatory on that plan, as well as one for the Liverpool Observatory at Bidston, and in 1858 the S.-E. equatorial at Greenwich.'
E. Walter Maunder, The Royal Observatory Greenwich - a glance at its history and work (1900)

undefinedThe 12¾-inch Merz - illustration from The Midnight Sky by Edwin Dunkin (1891) 

The mount on which the 28-inch telescope now sits was built for an earlier instrument and predates it by over thirty years. The original instrument, the 12¾-inch Merz, was originally used for planetary observations and, later, as a guide telescope to the new 26-inch Thompson, housed in the New Physical Building in the late 1890s. Its design is similar to that of the Northumberland telescope at the Cambridge observatory. This is no coincidence as both were designed by the then Astronomer Royal, George Biddell Airy.

The mount is an 'English equatorial' type, with the telescope mounted on an axis which is tilted so as to be parallel to the Earth's axis of rotation. The telescope can then follow a star from east to west across the sky merely by rotating the mount. The telescope's clock drive, in the base of the mount, moves the telescope at the same rate as the rotation of the Earth, keeping it fixed on the same celestial coordinates - a valuable asset when long observations are required. The original clock drive was powered by falling water and was prone to freezing in the winter months. A more reliable electric drive was installed early in the 20th century.

The design of the mount allows the telescope to be moved to almost any postion in the sky. The shape of the telescope dome prevents the telescope from observing below about 20 degrees above the horizon, while the north pier of the mount obscures the region of the sky around the North Celestial Pole.

The telescope

'As regards the need for a large refractor, I have the honour to invite the attention of their Lordships to... the enclosed report to the Board of Visitors and to the accompanying list of large refractor telescopes, from which it appears that there are no less than 30 at national or private observatories which are larger than that at the Royal Observatory Greenwich, six of them being more than twice the diameter...'
Christie, letter to the Admiralty, 17 June 1885

The 28-inch telescope and mount returning to the Observatory in October 1971The 28-inch telescope and mount returning to the Observatory in October 1971

The telescope tube is over 28 feet long and passes between the central spokes of the mount. The mount was not originally intended for such a large telescope, however, and it was necessary to taper the middle section of the tube in order to attach it. This is why the telescope has the curious characteristic of being round at each end but rectangular in the middle.

Attached to the outside of the tube are a variety of smaller telescopes. Several of them were used as wide-angle guide scopes. Since the field of view would have been too narrow to find an object easily with the main telescope, these were first used to target an object. Others were used to read the scales on the telescope itself. By magnifying the scales on the axes of the telescope, errors were greatly reduced when positioning it for observation.

The whole telescope is very finely balanced, using a series of counterweights which are added and removed, depending on the equipment being used on the eyepiece. In addition, some of the tubes of the mount itself are filled with lead shot. Since an equatorial telescope is necessarily used in a variety of positions, precise balance and easy manoeverability are essential, particularly for an instrument of this size.

The Onion Dome

'This dome - which has been likened, according to the school of aesthetics in which its critics have been severally trained, to the Taj at Agra, a collapsed balloon, or a mammoth Spanish onion - houses the largest refractor in England, the 'South-east Equatorial' of twenty-eight inches aperture.'
E. Walter Maunder, The Royal Observatory Greenwich - a glance at its history and work (1900)

Royal Observatory, GreenwichRoyal Observatory, Greenwich

The dome built in 1859, to house the 12¾-inch Merz telescope, was a flat-topped 'drum’ . This was a wooden structure, with an opening from the centre of the roof to the edge and down to floor level. Since the telescope could be used in almost any direction, the dome needed to rotate and did so by being mounted on cannonballs, which acted as ball-bearings. This mechanism was prone to problems and was scrapped when the 28-inch was installed.

The new telescope, however, was over eight feet longer than its predecessor, so a innovative dome design was needed to avoid major rebuilding of the supporting brick tower. This was achieved in 1893, in the form of the unique 'onion dome' devised by T. Cooke & Sons, into which 28-inch refractor was moved in 1894. By bulging to a maximum radius of about five feet wider than the supporting tower walls, the dome was able to enclose the telescope while using the same rotation seating as the 'drum dome’. The cannonball bearings were replaced with a modern system.

The original dome construction was a riveted iron frame covered with papier mache. It was damaged by an air raid in October 1940, but was stripped of its covering when a V1 flying bomb fell in Greenwich Park on 15 July 1944. The telescope was removed from the dome in 1947, and the dome was dismantled in 1953. The present dome is a fibreglass replica of the 1893 dome, installed after the 28-inch returned to Greenwich in 1971.

The makers

'An increase in our optical means is required to enable us to carry out satisfactorily the determinations of proper motions of stars in the line of sight with the spectroscope, a work which peculiarly belongs to this Observatory, as supplementing the determinations of proper motions from meridian observations.'
Christie, Report to the Board of Visitors, 1885

Sir William ChristieSir William Christie, KCB (1845–1922)
Eighth Astronomer Royal 1881–1910 

Although there were many people involved in the completion of the 28-inch telescope, just two were ultimately responsible for its design and building; the Astronomer Royal, William Christie, and the telescope manufacturer, Howard Grubb.

Christie wanted to continue extending the work done at the Royal Observatory in the expanding fields of astrophysics and photography. In 1885 he announced plans for a large, new, 28-inch equatorial refractor, to replace the 12¾-inch Merz, then 26 years old.

Appropriately for a instrument of this size, the 28-inch was to be of revolutionary design, allowing it to perform as an observational and photographic telescope. For optical reasons, this was usually the job of two separate instruments.

The job of designing and building the telescope went to Howard Grubb, one of the great optical manufacturers of his age and, as a company, supplier to the Royal Observatory for some 50 years. Since the 1830s, the Grubb family had been a manufacturer of high-quality optical instruments and suppliers of some of the biggest telescopes of that time, including the Vienna 36-inch - the largest in the world when it was built in 1880. They were also renowned for their innovative techniques and attention to the smallest detail.

Double star observing

'This Observatory should possess an equatorially mounted telescope comparable with those of other first-class Observatories, so that we may no longer be prevented by deficient optical means from obtaining complete series of observations of comets and faint satellites.'
Christie, Report to the Board of Visitors, 1885

Astronomers making observations with the 28-inch refractor in 1894Astronomers making observations with the 28-inch refractor, 1894 

The 28-inch telescope was originally intended to be used for visual and photographic observations. This dual role was based on theoretical work by the Cambridge professor, George Gabriel Stokes.

However, this proved to be less successful in practice than on paper. Soon after the telescope was ready for use a new 26-inch refractor was commissioned purely for photographic work. From the mid-1890s, the 28-inch was used in the programme of double star observations, a field of astrophysics that had been developing rapidly in the late 19th century.

Double stars are stars that share a common centre of gravity and are in orbit around that centre. Little was known about the nature of stars in the 19th century and the idea that they could be associated physically as well as visually was new. Today it is known that at least 50% of stars are binary stars in orbit around one another but at the beginning of the 19th century their existence was unknown. The idea was not taken seriously until 1806 by William Herschel.

By attaching a micrometer to a telescope, an instrument that allows fine positional measurements of celestial objects, the relative positions of stars could be measured very accurately. Measurement of the orbits of binary stars allows their masses to be determined by applying Newton's Law of Gravitation. This is the only method by which astronomers can directly measure the masses of stars.

The 28-inch was moved to Herstmonceaux in 1957, but continued to be involved in double star programmes on and off until its retirement from research in the late 1960s.