Telescope: Sheepshanks Equatorial (1838)

The Sheepshanks Equatorial has also been known as the Sheepshanks 6.7-inch Refractor, the Southern Equatorial, the South-East Equatorial and the East Equatorial. Erected in 1838, it was the first major instrument to arrive at the Observatory following the appointment of George Airy as Astronomer Royal in 1835. Although larger than the existing and rather unsatisfactory Shuckburgh Equatorial, its own limitations meant that the Shukburgh continued to have a useful role. It was the Observatory’s largest equatorial until the arrival of the 12.8-inch Merz in 1859.

From 1838 until 1963, the Sheepshanks Equatorial was mounted under the South Dome (renamed as the South-East Dome in 1845, then as the East Dome in 1850 and finally as the Sheepshanks Dome in the early twentieth century). This was located in what is now generally referred to as the Meridian Building. When first erected, the telescope had a good view of the whole of the sky, uninterrupted by nearby buildings. This changed in 1858 when the view to the east was partly obscured by the Great Equatorial Building which was erected nearby for the 12.8-inch Merz. The Sheepshanks Dome suffered damage during the Second World War, rendering the telescope unuseable. In 1951/2 it was put back into full working order and the telescope reconditioned in the Observatory workshop.

The Sheepshanks Dome in 1839 when it was still known as the South Dome. The telescope was housed there for 125 years. From a drawing by Elizabeth Smith, 11 February 1839

According to Howse (Greenwich Observatory, London, 1975, p.90), the telescope was transferred to the National Maritime Museum in 1963 (Object ID: AST0912) at which point it was moved and remounted in the Altazimuth Pavilion with the aim of opening it up for public use. A few years earlier, the Museum and Ministry of Works had begun drawing up plans to refurbish the Meridian Building and open it to the public for the first time (it opened in 1967). Although building plans drawn up in 1959 (WORK17/374) indicate that consideration was given to preserving both the Sheepshanks dome and its pier, the pier was regrettably demolished and the potential for displaying the telescope in its original location lost for ever. The telescope has been in store since 1982 when it was replaced in the Altazimuth Pavilion by the Newbegin telescope which the National Maritime Museum had recently acquired from Herstmonceux.

 

Background to the telescopes acquisition

Before he came to Greenwich, Airy had been at Cambridge where he had entered Trinity College as an undergraduate in 1819, becoming a fellow of his college in 1824, Lucasian professor in 1826 and Plumian Professor and Director of the new Cambridge Observatory in 1828. It was while at Cambridge, that he began a lifelong personal and professional friendship with the Rev. Richard Sheepshanks who was seven years his senior and had been a fellow of Trinity since 1817. Deeply interested in astronomy, Sheepshanks was able to devote his life to scientific pursuits having received a substantial inheritance from his father in the mid 1820s. In the 1830s, he acquired, what for the time was a large object-glass of diameter 6.7 inches, presenting it to Airy in his capacity as Astronomer Royal in 1837. Made by Cauchoix, of Paris, it was of a good, though not perfect quality (see below). The Admiralty readily agreed to pay for the lens to be incorporated into an equatorial telescope. Writing to John Quincy Adams in 1839, Airy stated its cost as £205. Although Airy designed most of the telscopes he introduced at Greenwich, the Sheepshanks was not one of them.

 

Location of telescope and mounting

The size of the telescope was naturally enough dictated by the diameter of the lens and its focal length (which was a fraction over eight feet). The type of mounting on the other hand was determined by the fact that Airy planned to mount it on the redundant pier which had originally been constructed for the Shuckburgh Equatorial. Back in 1811 when that instrument was given to the Observatory by Shuckburgh’s heir, it had been planned to mount it not as an English Equatorial as constructed, but as an altazimuth. The decision to mount it as an altazimuth meant that only one rather than two piers were constructed. The plan did not work out. Despite the pier’s massive construction, insufficient thought had been given as to how the Shuckburgh would need to be mounted to give it sufficient stability as an altazimuth. Because there was only one pier, it was not possible to mount the Shuckburgh there as an equatorial. For the same reason, it would not have been possible to mount the new instrument as an English Equatorial. Airy therefore opted for a mounting similar in general form to that of the Dorpat telescope (a configuration that later became know as a German equatorial) with the telescope being on one side of the axis, counterpoised by weights on the other.

Thomas Grubb of Dublin was commissioned to make the telescope and mounting. Like the later 12.8-inch Merz Refractor, the telescope tube was made of wood. Clockwork was attached to the instrument, to carry the telescope with a motion equal to the diurnal rotation of the earth on its axis. The whole instrument was mounted on a rectangular stone pillar whose extreme breadth from East to West was 1 foot 4 inches and from North to South 5 feet 4 inches. This sat on top of the existing pier which was in the form of a truncated cone, having a diameter of about four feet at the level of the floor of the dome and nearly twice that at the level of the ground floor which was two stories below. A set of plans of the buildings drawn by Airy in 1846 (ADM140/426) and another drawn in 1959 (WORK17/374) imply that the north side of the rectangular pillar was flush with the edge of the existing pier and that south side overhung it.

 

Defects

Although Airy valued the telescope (perhaps, in part, in deference to his friend), he was well aware of its shortcomings. These he explained in an address to the Board of Visitors in 1855 when pitching for a new and larger equatorial:

‘First, the object-glass, though very good (especially for sidereal purposes) when used with care, is somewhat irrational, as well as veiny, and is not such as ought to be exhibited to a foreigner as the best telescope for general purposes in the National Observatory. Secondly, the mounting, which necessarily carries very small circles, makes it unfit for accurate determination of the place of a celestial body.

The determinations of place, rather than the observations requiring no graduated circles, must, I think, always be considered as the proper object for instruments in this Observatory. We are compelled, therefore, to rely on the Shuckburgh Equatoreal for what I consider to be, here, the normal applications of an Equatorial.’

Elsewhere, he described the object-glass in the following terms: ‘Its definition is good: a small quantity of colour from the secondary spectrum, and a diffusion of light from brilliant objects, being the principal defects.’

 

Observations

Donati's Comet with Arcturus. From a drawing made with the Sheepshanks Equatorial on 5 October 1858 by Wyvill James Christy, who was one of the Observatory's computers. From Walter Bryant's History of Astronomy, (London, 1907)

The first published observations were those of Encke's Comet made on 29 October 1838. Click here to view. In 1839–40 it was used in conjunction with the Shuckburgh Equatorial to make detailed observations of Galle’s First Comet (click here to view the published observations). Regular mentions of the telescope being used were published in the volumes of Greenwich Observations until the late 1920s. As well as being used for the observation of comets, it was also used to observe solar and lunar eclipses, transits of Mercury. It was also used with other instruments in a regular programme in which eclipses, occultations, and transits of Jupiter’s Satellites, compared with the Nautical Almanac; and occultations of Stars by the Moon, with the equations deduced from the occultations. There are also records of it being used for observations of the planet Mars.

 

Clock

The telescope was used with the regulator known as Earnshaw. Originally purchased by the Board of Longitude, Earnshaw was transferred to the Observatory when the Board was abolished in 1828. Originally used with Pond’s Great Zenith Tube, it was moved to the Sheepshanks Dome in 1838. Although removed from time to time for other short term programmes, the clock was always returned to the dome, where it remained until 1938 when it was sold.

 

Preparations for large scale photographic work

A hitherto little known part of the telescope’s history was its role in trialling scaled down versions of the object glasses of the 13-inch Astrographic and the 28-inch Refractors. In this role, it was used as a directing telescope.

In June 1887, immediately before the trials started, the Sheepshanks was completely dismounted, thoroughly cleaned, its object glass re-centred and the slow motions repaired and improved. The first photographic object glass to be experimented with was a 4-inch object-glass belonging to one of the Dallmeyer photoheliographs. This was mounted in a light wooden tube and firmly attached to the side of the telescope in order to carry out experiments on the field available on plane and curved plates respectively, the latter being moulded by Messrs. Chance to a radius of 22 inches.

In April 1888, the Dallmeyer object Glass was replaced with one by Howard Grubb of 6 inch aperture and 6 feet focal length as a preliminary trial piece for the object glass for the 13-inch Astrographic. This was replaced in the summer of 1889 by a four-inch experimental object glass to trial the design for the yet to be constructed 28-inch refractor. The lens design had been proposed by Stokes (one of the Visitors). By reversing its crown glass element and altering its separation from that of the flint glass it was designed to work for both photographic and eye-observation.

More can be read about the photographic trials with the Sheepshanks in the 1888–90 Reports of the Astronomer Royal (1888, 1889, 1890).

 

Loan of the object glass

The object glass was removed and lent to the Joint Pemanent Eclipse Committe for the observation of the Solar Eclipse of August 1914. In 1916/7 it was lent to the Inspection Department at Woolwich Arsenal where it joined a number of object glasses already on loan from the Observatory. It remained at Woolwich until the end of the war.

 

Airy’s description of the Sheepshanks

A brief description of the instrument was published in the introduction of each of the editions of Greenwich Observations from 1838 to 1908. The text below is from the 1879 volume and is significantly longer than those of later years. Originally published as a single paragraph, it is broken down here into shorter sections. Click here to read it as originally published.

‘This instrument was erected in the year 1838. The aperture of the object-glass is about 6.7 inches, and its focal length about 8 feet 2 inches. The object-glass was made by M. Cauchoix, of Paris, and was presented to the Observatory by the late Rev. R. Sheepshanks. Its definition is good: a small quantity of colour from the secondary spectrum, and a diffusion of light from brilliant objects, being the principal defects.

It is mounted in the Eastern Dome, 38 feet East and 6 feet South of the old Transit-Instrument, or 19 feet East and 1½ feet North of the Transit-Circle; with a mounting similar in general form to that known as the “German Equatoreal.”

A stone pier is erected, whose extreme breadth from East to West is 1 foot 4 inches, and from North to South 5 feet 4 inches: the southern or principal portion of the upper surface of this pier is sloped so that its plane produced passes through the celestial pole: upon the surface is fixed a cradle of cast iron, having the lower bearing of the polar-axis in a projection from its lower end, and the upper hearing at its upper end: the distance between the two bearings is 3 feet 7 inches.

The axis is of cast iron; its form is nearly conical: the end which rests in the lower bearing is a more obtuse cone. The diameter of the axis, where it turns in the upper bearing, is 6 inches: it rests here in a Y upon two plates of gunmetal. At a small distance below the upper bearing, a strong circular plate parallel to the equator, 14 inches in diameter, is fixed to the axis: and close above it, another plate of the same size is mounted, parallel to the former, turning freely round the axis; in that surface of each of these two plates which is nearest to the other plate, a dovetailed groove is turned, and a moveable clamping-piece has one clamp in the groove of one plate, and the other clamp in the groove of the other plate. By means of this clamping-piece, the two circular plates can be fixed together at pleasure, in any relative position. The moveable circular plate is inseparably connected with a long flat arm, or sector, of 24 inches in length (measured from the circumference of the plate) in the plane of the equator: the edge of this sector is cut into teeth, in which works the endless screw carried by the clock, which is fixed to the North side of the pier. Therefore, if the clamping-piece above-mentioned is not fixed to either plate, or is fixed to only one, the action of the clock-work upon the sector, though it turns the moveable circular plate, does not turn the fixed circular plate; but if the clamping-piece is fixed to both plates, the action of the clock-work on the sector carries the moveable plate and the fixed plate (now connected with it), and therefore turns the polar axis, and the telescope, &c., which is supported by the polar axis. Between that part of the clamping-piece which is connected with the fixed plate, and that part which is connected with the moveable plate, there is a slow-motion screw, turned by means of a Hook's joint and a long handle. Immediately above the upper bearing, the polar axis carries the hour-circle, 12 inches in diameter; it is divided on its edge to 1m of time, and is read off to 2s by two verniers. Above this, a square box is firmly fixed to the axis; perforated in a direction parallel to the equator for the insertion of the declination-axis. The diameter of the declination-axis at each of its two bearings is about 4 inches; it is supported at each by a Y. To one end of the declination-axis is fixed the cradle, carrying the telescope; to the other is fixed the declination-circle, and the counterpoise. The diameter of the declination-circle is 11 inches; it is divided on its edge to 15’ of arc, and is read off to 30” by two verniers attached to the square box.

The telescope-tube is of wood; its form is square in the middle, chamfered off towards the ends so as to become octagonal: several stops, or transversal plates with holes of the proper magnitude, are fixed in it; it is very firm, and free from tremors. Upon a large ring which is fixed to the square box there turns with stiff friction another ring, carrying two sector-arms, graduated at their extremities; these graduations are read by micrometer-microscopes, carried, one by the eye-end of the telescope-tube and one by the object-end: the use of this graduated double sector is, to measure small differences of declination (not exceeding 10º) with great accuracy; as, being brought by the hand under the telescope, it is then retained in its position by friction, and is not affected by the motion of the telescope. Upon another ring, which is fixed to the square box, there turns a ring-clamp, or brake, that admits of being fastened to it in any position; with this ring-clamp the telescope-cradle is connected by means of a slow-motion screw, which fixes the telescope in declination, or gives the means of imparting to it a slow motion in declination. The speed of the clock is regulated by two balls, suspended to the ends of a horizontal arm of 4¾ inches in length, which is carried by a vertical spindle; when the velocity is so great as to cause the suspending-rods to make a certain angle with the vertical, small projections, carried by the balls, are thereby made to rub against the lower surface of a fixed horizontal ring, and the friction thus caused prevents the weight which urges the clock from greatly increasing the velocity. This mounting was constructed by Mr. T. Grubb, of Dublin.

Besides several negative eyepieces unfumished with wires, this telescope has a wire-micrometer, a comet-eyepiece with thick wires, and a double-image micrometer constructed on the principles explained in the Memoirs of the Royal Astronomical Society, vol. xv, page 199, and Monthly Notices, vol. x, page 160.’

 

Modifications by the National Maritime Museum

A new finding telescope by Wildey was fitted by the Museum in 1964. The Museum’s website currently contains (June 2015) the ambiguous statement that ‘In 1964 the object glass was replaced by Mr Wildey of the BAA.’ It transpires that the Cauchoix object glass had been removed for testing and that this is simply a reference to it being remounted rather than being replaced with a new one by another manufacturer.

 

Images

For copyright reasons, it has not been possible to show any images of the telescope on this page. The only known contemporary image is very rudimentary and appears in an East-West cross sectional view of the Meridian Building drawn by Airy in 1846 (ADM140/426/Section 10).

The National Maritime Museum's website has a picture of the telescope in pieces in its store and also a photo taken in 1966 showing the telescope in the Altazimuth Pavilion. The Flamsteed Astronomy Society has a number of pictures on its website of the telescope on its mounting which were taken during a visit to the Museum store in 2014. Click here to view them.

 

Further reading

On the Regulator of the Clock-work for effecting uniform Movement of Equatoreals. G.B. Airy, Memoirs of the Royal Astronomical Society Vol 11, pp. 249–268, 13 March 1840