Telescope: The Lassell 2-foot Reflector (1845)

The Lassell Telescope in its dome at Greenwich. Sketched from life by an artist from the Pall Mall Gazette on 27 January 1888 (RGO7/29). The lower of the two observers is looking though the eyepiece of the Corbett 6½-inch refractor which was attached in January 1887 as both a guiding telescope and for the observation of occasional phenomena. Originally published by the Pall Mall Gazette on 30 January 1888, the drawing was reproduced by Cudworth on p.321 of his book: Life and Correspondence of Abraham Sharp (London, 1889)

This large equatorial, which dates from 1845, was constructed by William Lassell for use at his private observatory (Starfield) near Liverpool. Lassell served on the Board of Visitors of the Royal Observatory, Greenwich, from 1872 until his death in 1880. In 1883, the telescope was given to the Observatory by his daughters.

In his book The Victorian Amateur Astronomer, Allan Chapman writes that it was ‘the world’s first large reflector to have full motions in the equatorial plane, and to be possible for a single individual to use’. In 1852, Lassell took the telescope to Malta. In the mid 1860s, he bought an estate at Maidenhead near London and set about building an observatory in the grounds. It was there, at Ray Lodge, that the telescope remained for the rest of his life (click here for map). Famous as the telescope with which Lassell discovered Triton (the largest moon of Neptune) and Hyperion (a moon of Saturn), the telescope turned out to be an expensive disaster at Greenwich. It was effectively scrapped in 1895. 

At Greenwich, it was mounted in a new purpose built dome that connected with Magnetic Office 7 at the southern end of the Observatory site in a position now largely occupied by the west wing of the South Building. The telescope came with two primary mirrors, which were interchanged to allow polishing to take place.

The driving clock, differential slow motion and polar axis supports were destroyed by Dyson in November 1929 at the time when a big clear out took place and some of the obsolete instruments (for example Airy’s Altazimuth) were presented to the Science Museum (RGO39/4/124). Surviving parts include the dome which was reused and now surrmounts the South Building and the two 2-foot mirrors, one of which is owned by the National Maritime Museum (Object ID AST0878). The other was presented to Liverpool City Museum in 1956. It is currently (2014) on display in the World Museum in Liverpool. An unsubstantiated report suggests that additional parts (including the mirror polishing machine) survived until the 1950s and that they were dumped in ‘the wilderness’ in the Park prior to the Observatory giving up the site following its move to Herstmonceux.

The Lassell Telescope and Dome. The Magnetic Offices to which the Dome was attached are on the right. From: A Handbook of Descriptive and Practical Astronomy, Volume 2, Fourth Edition (London 1890) by George F Chambers


Description of the telescope from the 1892 volume of Greenwich Observations pp.xvii–xviii

‘The Lassell reflector equatorially mounted, in a new dome in the South Ground. This instrument was constructed by the late Mr. William Lassell, F.R.S., and, some years after his death, was presented by his daughters, the Misses Jane, Caroline, and Charlotte Lassell, to this Observatory in 1883 February. A description of the instrument is given in the Memoirs of the Royal Astronomical Society, Vol. XVlll, p.15, &c. (Description of a machine for polishing Specula, &c.). It may be sufficient here to state that the metal speculum is 24 inches in diameter and of 20 feet focal length, and that the telescope is mounted equatorially on the plan devised by Mr. Lassell, the tube swinging between two “uprights”, each 7 feet long, bolted to the large inverted cone which forms the polar axis. There are two large mirrors available for use. Various alterations have been made in the instrument since it was mounted here, amongst which the following may be specified:- An improved slow motion in R.A., which can be worked by means of a cord from the observing stage, without putting the clock out of gear, and a new slide for gearing the driving screw into the hour-circle; a firm declination clamp which can also be worked from the observing stage; an improved edge-suspension for the large mirror (consisting of a steel band which encircles the mirror and is suspended freely from brackets at six equidistant points of the circumference); a new and firmer mounting for the small mirror, and later a new small mirror (silver on glass) made by Mr. Calver; a stronger mounting for the eye-piece, which allows of its being tilted in any direction, for optical adjustment. The framed iron base which supports the instrument has been bricked up and filled with concrete, and this with other alterations has greatly increased the stability of the telescope. The teeth of the hour-circle have been re-cut and a number of minor repairs have been required.

In 1887 January, some further additions were made to this instrument. A delicate slow motion in R.A. (with differential wheels) and a firm N.P.D. clamping arm with fine motion in N.P.D. were applied; and the Corbett 6½ inch refractor was mounted below the tube of the reflector and parallel to it, to serve as a directing telescope in taking photographs, and also for observation of occasional phenomena, [the Corbett refractor was replaced by the newly acquired Thompson 9-inch Photographic Telescope in 1891].

In 1891 November the reflecting telescope was dismounted, and the Merz Refractor was substituted [with the Thompson 9-inch Photographic Telescope mounted beneath it].

The Dome was constructed by Messrs. T. Cooke and Sons, of York, and was completed in March 1884. It is hemispherical, of 30 feet diameter, and is covered with papier-mâché, on a framework of angle-iron. It is carried by nine wheels or rollers (each 2 feet in diameter) fixed to the iron-curb of the Dome and running on a flat rail, horizontal rollers being provided to prevent excessive lateral motion from the force of the wind or other cause. The Dome can be turned with great ease either from the floor, or by means of an endless rope from the observing stage, which is attached to the Dome on the following side of the opening, and is carried with it in its motion. The shutter opening extends from beyond the zenith to the horizon, and is closed by a single curved shutter 3 feet 6 inches wide at the zenith and 6 feet wide at the horizon), which turns about a point in the dome-curb opposite to the opening, and runs on guiding rails at the horizon and near the zenith, the curved shutter being continued by an open framework to complete the semi-circle.’


Parts listed in the 1911 inventory

The following parts of the original telescope are listed in the 1911 inventory (RGO39/4/124). Most were stored in the lower museum in the South Building.

2 concave mirrors (2-foot)
1 flat mirror
2 finder telescopes (4-in reflector & 3-in refractor, with brackets)
2 eyepieces, positive
1 driving clock
1 differential slow motion
polar axis supports

Of these parts, the two eyepieces were later put to use elsewhere, whilst the driving clock, differential slow motion, and polar axis were destroyed in November 1929 as part of a general clearout.


Extracts from the Annual Reports of the Astronomer Royal to the Board of Visitors

The Annual Reports were read at the annual visitation of the Royal Observatory – normally on the first Saturday of June each year. These extracts illustrate the changing fortunes of the telescope at Greenwich.


2 June 1883

The presentation of the Lassell two-feet reflecting telescope by the Misses Lassell, of which further mention will be made, has necessitated some alterations in the buildings and grounds. It has been mounted in the South Ground, and a circular building 30 feet in diameter has been erected in preparation for the dome. The dip and deflexion instruments have in consequence been moved from Magnetic Office 7 to the New Library.

A very valuable addition has been made to the instruments of the Royal Observatory by the gift of the Lassell two-feet reflecting equatorial, which has been generously presented by the Misses Lassell. The exceptional qualities of this fine telescope (with which Hyperion was discovered in 1848) are well known, and there could be no hesitation in accepting on the part of the Admiralty the offer of such a valuable gift. The instrument was moved from Maidenhead early in March, and has been erected in the South ground, where it commands a nearly unobstructed view of the sky to within 5º of the horizon. A circular building 30 feet in diameter has been erected for the Lassell telescope, and the construction of a suitable dome is authorized. There are two large mirrors available for use, and I contemplate taking advantage of the firm mounting and perfect clock movement of the South-east equatorial to mount the spare mirror on this instrument, attaching it to the tube of the refractor, so as to have on the same mounting a refractor and reflector with their axes parallel. The former would be available for eye observation, whilst the latter could be used on the same object for physical work, spectroscopic or photographic. The Lassell telescope itself would be well suited for observation of faint satellites and comets which are beyond our present instrumental means. 


7 June 1884

The new dome for the Lassell telescope was completed by Messrs. T. Cooke and Sons at the end of last March, and is in every respect satisfactory. It is 30 feet in diameter and covered with papier mâché on an iron framework, and turns with great ease. The shutter-opening extends from beyond the zenith to the horizon, and is closed by a single curved shutter (3ft. 6in wide at the zenith and 6 feet wide at the horizon), which turns about a point in the dome-curb opposite to the shutter-opening, and runs on guiding rails at the horizon and near the zenith, the curved shutter being continued by an open framework to complete the semi-circle. This arrangement appears to leave nothing to be desired as regards ease of manipulation. After the completion of the dome, the carpenters’ work on the flooring &c. of the building and the attachment of the observing stage (which is fixed to the dome) have necessarily occupied much time, and the building is hardly yet complete in all details.

The Lassell equatorial has required a number of small repairs and general cleaning, some parts of the mounting having been probably strained in process of removal, and the bearings in particular having suffered from wear and subsequent disuse, so that it has been necessary to raise the instrument and re-grind these in several instances. The mirror has been cleaned, and appears to be very good as far as it has been practicable to test it before the mounting of the telescope has been put into proper order. The delay in the completion of the dome has necessarily delayed the work on the instrument, which is now rapidly advancing to completion.


6 June 1885

The new building and dome for the Lassell telescope were completed in all details during the past year. A stove has been fixed in it to dry the interior in wet weather, as it was found that the mirror and ironwork of the instrument were exposed to injury from damp.

The work on the Lassell equatorial has occupied a great deal of attention during the past year, a number of repairs and alterations having been required to get the instrument into proper working order. The driving clock, which was found to drive the instrument at only three-fifths of the proper speed, has been altered, a slow motion in R.A. (to be worked from the observing stage) has been contrived, a new slide of improved construction has been made for gearing the driving-screw into the hour-circle, the teeth of the hour-circle have been re-cut, a firm declination clamp has been applied, and improved edge suspension for the large mirror (consisting of a steel band which encircles the mirror and is supported by brackets at six equidistant points on the circumference) has been contrived, a new firmer mounting of the small mirror has been made, and the eye-piece has been mounted firmly on a plate which allows it to be tilted in any direction for optical adjustment. The framed iron base which supports the instrument has been bricked up and filled with concrete, and this with the other alterations has greatly increased the stability of the telescope, which is now quite satisfactory. Difficulty is, however, still experienced from want of stability of the optical axis of the large mirror, which requires to be readjusted continually, as the telescope is moved. When the mirrors have been properly adjusted the definition appears to be very good, the companion to Vega being shown with remarkable distinctness without any trace of scattered light from the large star.

… Comet  (c) 1884 has been observed on 4 nights, the Lassell reflector or one of the other equatorials being employed. …


5 June 1886

A new plane mirror (silver on glass) has been obtained from Mr. Calver for the Lassell equatorial, and a wooden screen has been fixed at the eye-end to protect the open end of the tube from the heat of the observer’s body, it having been found that the definition was much affected by convection currents at the eye-end, giving rise to an apparent astigmatism which was at first supposed to be caused by tilt of the large mirror. The optical performance appears now to be satisfactory. At Mr. Common’s suggestion a frictional connexion between the clock and the driving-screw has been applied so as to allow the later being turned in either direction (for slow motion in R.A.) without putting the clock out of gear. Divisions to every 5m of R.A. have been painted on the large hour-circle, and an iron handle for turning the instrument in R.A. has been fixed to it.

… Comets have been observed with the Lassell reflector or one of the other equatorials on 29 nights during the year ending 1886 May 20. …


4 June 1887

A lathe, with a set of chucks and turning tools, has been procured and set up in the basement of the Lassell building.

Various additions have been made to the Lassell equatorial with a view to making it available for astronomical photography and for general use. A delicate slow motion in R.A. (with differential wheels) and a firm N.P.D. clamping arm with fine motion in N.P.D. have been applied, the steadiness and general usefulness of the telescope being greatly increased by these additions. The Corbett 6½ -inch refractor has been mounted below the tube of the reflector and parallel to it to serve as a directing telescope in taking photographs and also for observation of occasional phenomena. A camera to take circular plates 8¼ inches in diameter (giving a field of 1º 58’ in diameter) has been mounted at the principal focus of the Lassell mirror and some trial photographs of the Moon, Pyocyon, γ Leonis, and Præsepe, have been taken.


2 June 1888

The Lassell equatorial is in good order. The photographic camera was dismounted at the beginning of this year and the small plane mirror inserted for eye observation. The driving clock has required some small repairs. The dome, 30 feet in diameter, is in excellent order and it has been found that it can be turned completely round (through 360º) in 30 seconds by one person [This was achieved by Turner during an open competition amongst the staff at the Observatory sports day on 21 April! (RGO7/29)].


1 June 1889

Owing to want of Space in the Observatory buildings the moveable instruments, clocks and other apparatus which are not in actual use, are for the most part stowed away in their boxes in wooden sheds, where their periodical examination and renovation are attended with great difficulty. For the proper care of these instruments it would be advisable that they should be housed in a brick building to serve as an Observatory museum for instruments and apparatus of scientific value or historical interest. A building of one story, about 40 feet long and 30 feet broad, would be suitable for this purpose, and a convenient site would be in the South Ground, the proposed building being connected with the circular Lassell building. In that situation it would not interfere with the use of the astronomical or magnetic instruments, a consideration of much importance.

The Lassell, South-east, Sheepshanks, and Shuckburgh equatorial are in good working order.


7 June 1890

The Admiralty has authorized the building of a storehouse or museum for the better housing of the various portable instruments and apparatus belonging to the Observatory as proposed in the last Report. After careful consideration, it appeared advisable to modify the original idea of a building of one storey occupying nearly the whole of the ground available for future extensions, and to arrange for a building of two storeys, having a smaller area, designed to form the eastern wing of a larger structure with the Lassell Dome as a central portion. The other wing would provide a suitable Computing Room for the Magnetic and Meteorological Branch, and sleeping accommodation for observers charged with observations in the early morning. The Ante-room of the Magnetic Observatory, which now serves as the Computing Room for that branch, is hardly fit for the purpose, as it is built of wood, and is not large enough for the proper accommodation of seven persons. The effective use of the Lassell telescope for the observation of occultations and phenomena would be greatly increased if it were mounted at a greater height, as it would be if the plan here suggested was carried out. In the present position neighbouring trees interfere greatly with the observations of the Moon and Jupiter when south of the equator, and the use of the instrument is much restricted in consequence.

The Lassell, South-east, Sheepshanks, and Shuckburgh equatorial are in good working order….

The 12¾–inch refractor which will be dismounted shortly would be very useful for the observation of comets, occultations, and phenomena, for which it is well adapted. It might with advantage be mounted on the Lassell equatorial in substitution for the much heavier tube and cradle of that instrument, the provision for rotation of the mirror and tube (which necessarily adds greatly to the weight) being dispensed with. The tube of the refractor would, I believe, provide a good attachment for a large mirror, 3m.43 (11ft 3in) in focal length, which Mr. Common has generously offered to make for the Observatory. Reference has already been made to the expediency of mounting the Lassell equatorial at a greater elevation above the ground.


6 June 1891

After much consideration it has been decided that the Museum or Storehouse for the portable instruments and apparatus should be built so as to form the central octagon of a future cruciform structure in the South Ground, which would accommodate the Physical Branch of the Observatory, and would carry the Lassell Equatorial and Dome at such a height above the ground that the neighbouring trees would not interfere with the effective use of the instrument. The building for the Museum (consisting of the two lower storeys of the octagon) was commenced at the beginning of March and is now nearly completed as far as the structure is concerned. …

An essential part of the scheme would be the mounting of the Lassell Equatorial, with the 12¾ -inch refractor substituted for the Lassell reflector, on the central octagon (above the spectroscopic and photographic laboratory) in a position which it could be used to great advantage for equatorial observations of various kinds.

A photographic telescope with 9-inch object glass by Grubb, and a prism of 9 inches by Hilger have been generously presented to the Royal Observatory by Sir Henry Thomson. The telescope has been mounted on the Lassell telescope as a photoheliograph, to give 8-inch pictures of the Sun, a camera with Dallmeyer doublet (from Photoheliograph No. 4), and an exposing shutter, specially designed to give very short exposures, being attached to it.

The Lassell, South-east, Sheepshanks, and Shuckburgh equatorial are in good working order.

As already mentioned, the photographic telescope recently presented by Sir Henry Thompson has been mounted below the Lassell telescope in the place previously occupied by the Corbett refractor, and arranged for taking solar photographs of 8in. in diameter. …


4 June 1892

The Admiralty have authorized the building of the South wing of the proposed Physical Observatory during the present financial year, in order to provide additional accommodation for chronometers and deck watches, under the Chronometer Room and Lower Record Room to be appropriated to chronometers as soon as the South wing is ready to receive the contents of these two rooms. The completion of the proposed Physical Observatory by building three other wings and of the two upper storeys of the central tower (to carry the Lassell Equatorial and Dome) has been postponed for the present, the Admiralty, while recognising the desirability of carrying out the work, being unable to make provision for it in the present financial year. The objects for which the new building is required were set forth in my last Report, and the experience of the past year has shown the urgent need for the accommodation which would be provided by the proposed Physical Observatory.

The 13-inch Merz refractor of the South-East equatorial is now being mounted in place of the Lassell 2-foot reflector, which for this purpose was dismounted on April 20.

The Thompson photographic-telescope will be carried on the tube of the 13-inch refractor instead of on the tube of the Lassell reflector.


3 June 1893

The building of the South wing of the proposed Physical Observatory, in order to provide additional accommodation for chronometers and deck watches, was commenced on November 24 last, but was interrupted on March 4, on arriving at the first floor by a failure in the supply of terra cotta.  Building operations are now about to be re-commenced. The need for further accommodation is being more urgently felt every day in all departments of the Observatory. A number of staff are at present housed in the Octagon room, which is part of my official residence; and the inconvenience of this arrangement was strikingly demonstrated during the last winter. In view of this the Admiralty have now authorised the building of the North wing of the Physical Observatory and the completion of the central octagon by the addition of a storey and the erection of the Lassell Dome over it, a much needed extension, the postponement of which, since it was first brought forward in 1891, has caused serious difficulty in carrying on the work of the Observatory, both as regards observations and computations.

The 12¾ - inch Merz refractor of the South-east equatorial was mounted last May in place of the Lassell 2-foot reflector, the Thompson 9-inch photographic-telescope being carried on the same mounting. It has been used assiduously by Mr. Lewis since February for observations of double stars. In all he has made 545 measures of position angle and 609 of distance of 85 pairs; 32 pairs being less than 1” apart, 26 between 1” and 2”, 8 between 2” and 3”. This work has a special value at the present time, as giving information as to the performance of the instrument and as to the modifications in the mounting which may be advisable before erecting it in its proposed position on the Physical Observatory. In its present site much of the sky is cut off by the adjacent building, and important observations such as those of the conjunction of Saturn and γVirginis, are thus often lost.

The photographic telescope presented by Sir Henry Thompson, which has been mounted on the Lassell equatorial has been in regular use as a photoheliograph since January 1893. …


2 June 1894

The South wing of the Physical Observatory, intended to relieve the pressure in the Chronometer room, was at length completed after many delays on April 20, and is now being prepared for occupation. The building of the North wing and completion of the central octagon by the addition of a storey and the erection of the 30-feet Lassell Dome over it, which was authorized more than a year ago, has not yet been commenced, but it is understood that tenders for the work have been invited, and it is hoped that there will not be much further delay in providing this much needed extension. The details of the building have required careful consideration, and the plans have been skilfully worked out by Mr. Crisp under the supervision of the Director of Works for the Navy to meet the special requirements.

A valuable gift has been made to the Observatory by Sir Henry Thompson, who has generously offered a sum of £5000 to provide a large photographic telescope with accessories, which could serve as the compliment of the 28-inch visual telescope just completed. This munificent offer was readily accepted by the Admiralty, and after careful consideration and discussion, a photographic telescope of 26 inches aperture and 22 feet 6 inch focal length, equatorially mounted, was ordered of Sir H. Grubb on May 5, the instrument to be completed in 18 months. This telescope will have exactly double the dimensions (aperture and focal length) of the Astrographic Equatorial which has proved so successful, and it will be mounted on a very firm stand which will allow of complete circumpolar motion without the necessity for reversal on the meridian, which has been felt a drawback in the Astrographic Equatorial. It will be erected on the central tower of the new Physical Observatory, under the 30 feet dome, which is shortly to be placed there, and will carry the 12¾ - inch Merz refractor as a guiding telescope and the Thompson 9-inch photoheliograph. It will thus be mounted under very favourable conditions for work, and will be in every respect a most effective instrument.


6 June 1896

The building of the North wing and Central Dome of the Physical Observatory have been delayed by failures in the supply of terra cotta and by labour disputes, and is still unfinished as regards plastering and carpenters’ work. The Lassell dome, which was erected in the South Ground in 1884, was taken down in August [1895] and has been re-erected on the new building. The completion of the Physical Observatory by the building of the East and West wings has been sanctioned, and provision has been made in the Navy estimates for commencing the work during the present year.

Although it’s dome was taken down in 1895, the equatorial mounting appears to have remained in situ until October 1897 when it was eventually removed.