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Telescope: The Thompson 9-inch Photoheliograph (1891)

The Thompson Equatorial with the Thompson 9-inch Photoheliograph (extreme left) on the Thompson 26-inch Refractor. Postcard published by Henry Richardson c.1908

The Thompson 9-inch Photoheliograph was created from a 9-inch Photographic Refractor gifted to the Observatory by Sir Henry Thompson in 1890. It was initially mounted on the Lassell 2-foot Reflector on the Lassell Equatorial. The following year, when the 12.8-inch Merz Refractor was substituted for the Reflector, it was remounted on that. Following the decommissioning of the Lassell Equatorial in 1895, the Photoheliograph was placed in store prior to being mounted on the Thompson 26-inch Refractor in 1897. During the rebuilding of this instrument in 1935, it was transferred to the Thompson 30-inch Reflector. From 1896, it was occasionally removed on a temporary basis for use on eclipse expeditions. It was then referred to as the Thompson Coronograph. On one such expedition, it became stranded in Russia for the best part of ten years.

The history of the Photoheliograph after 1936 is obscure and requires further research. Its last mention anywhere in any of the volumes of Greenwich Observations or annual reports seems to be in 1936. It is said by Howse (1975) to have been dismounted in 1939 at the outbreak of war and subsequently transferred to Herstmonceux. It was never brought into use at Herstmonceux and its fate is currently unknown.

Origins of the telescope

Born in 1820, Sir Henry Thompson was a distinguished and wealthy surgeon who, in the late 1880s, built himself an Observatory at Hurstside House, his country residence in West Molesey, Surrey, (Hurstside was demolished in the 1930s and redeveloped as Balmoral Crescent: click here for map). It housed a 12-inch refractor by Cooke, together with a new star spectroscope by Hilger. The telescope was erected in 1887 and brought into use in 1888. It was in that year, that Thompson became a fellow of the Royal Astronomical Society.

Sir Henry Thompson by Lock & Whitfield. Woodburytype from Volume 6 of Men of Mark, Sampson Low, Marston, Searle and Rivington (1882)

At some point, it’s not clear when, Thompson decided to add a 9-inch Photographic Refractor which he planned to mount on the Cooke to create a twin equatorial, a class of instrument that was much in vogue at the time. Ordered from Grubb, the first record of the Photographic Refractor comes in July 1888 when it is mentioned in a report of a visit to Grubb’s works in Dublin published by the Institution of Mechanical Engineers, where it is listed amongst the instruments either under construction or whose construction was about to be put in hand.

For whatever reason, Thompson appears to have abandoned his own observatory almost as soon as it had become operational. By July 1890, the 12-inch Cooke had been removed from Hurstside (RGO7/37). The Photographic Refractor was never mounted there. Thompson arranged instead for it to be sent to by Grubb to his agent W Watson and Sons in High Holborn, London, to be sold on.

In the summer of 1890, rather than sell the telescope, Thompson decided instead to offer it to William Christie, together with a 9-inch diameter prism by Hilger (mounted so that it could be attached in front of the object glass), for use at the Royal Observatory. Following consultations with the Admiralty, Christie was given permission, on 2 December, to accept the telescope and prism. Arrangements were immediately put in hand for them to be collected by the Greenwich carriers Gridley & Ford on Monday 8 December and delivered to the Observatory.

Finding a use for the telescope

Although Christie had been offered the telescope as early as July 1890, it took until 18 November for him to write to the Admiralty to seek permission to accept it. When Thompson made his offer, the 13-inch Astrographic Telescope had just been delivered and work on the 28-inch Refractor was in hand. Since the former was a photographic telescope and the later had been designed so that if could be used either photographically or visually, Christie had no particular use for a smaller photographic instrument. In light of this, he came up with a scheme to mount the instrument as a photoheliograph in order to obtain ‘more perfect’ large scale photographs of the Sun than could be obtained with the Dallymeyer Photoheliograph. He told the Admiralty that with it, the Royal Observatory would ‘be in a position to compete successfully with foreign observatories in a field which the Greenwich series of Solar photographs commenced in 1873 has been adopted as the standard’ (RGO7/37).

Refiguring the object glass and tube

Once the telescope tube and Object Glass had arrived at Greenwich, Christie carried out a series of tests. The telescope however could not be brought into focus as the tube was around an inch too long. On making enquiries, it transpired that Grubb had intentionally supplied Thompson with an oversize tube on the basis that it would be cut to the required size at the time of the telescope’s installation. This he agreed to do for Christie at no charge. When writing to tell Christie this he also told him:

‘I thing it very likely you will find the chromatic correction of this object glass not quite satisfactory, judging from my subsequent experience. At the time that that object glass was made, I had only made 2 others of that type; one the Nasmyth obj glass for D^r Gill and the other for Brussels, both of which were considered satisfactory at the time, but the idea then was to make the correction very like the ordinary photo^c. lens, ie: - approximately co-incident for chemical and visual rays. In the later object glasses, attention is given to the photographic, irrespective of the visual’ (RGO7/37).

Grubb offered to correct the lens without charge. Unfortunately for him, this entailed more work than he had envisaged. He had been hoping to refigure just one of the four surfaces, which he originally anticipated would slightly shorten the focal length (which probably wouldn’t have been a problem). When he tested the object-glass, he found that instead of becoming shorter, the focal length would in fact have been increased by around 2¾ inches which by then was not acceptable to Christie. As a result, Grubb anticipated having to refigure at least two and possibly all four of the surfaces. On returning the refigured object glass to Christie for further testing, Grubb also made the offer of swapping it for a 9-inch photographic object glass that he was in the process of finishing for an American Observatory.

The task of designing a mounting for the photoheliograph was given to Simms (of Troughton & Simms), who was contacted by Christie about the matter on 16 February 1891.

Finding telescope

The Observatory had in its possession a number of small portable telescopes. These were listed in the inventories as ‘Detached Telescopes’. The Photoheliograph was initially fitted with Detached Telescope No.1 of 4-inches aperture by Simms as its finder (RGO39/10/77). This appears to have been later changed to a 2.5-inch telescope (RGO39/6/23) whose object glass was taken from Detached Telescope No.5 (RGO39/5/26).

Christie’s description of the photoheliograph

The following text is taken from the introduction to the 1891 volume of Greenwich Observations:

‘The Thompson Photographic Telescope by Sir H. Grubb is a refractor of 9 inches aperture and 8 feet 10 inches focal length, presented to the Observatory by Sir Henry Thompson. It is corrected for the photographic rays and is provided with a prism of 8 [changed to 9 in later editions] inches diameter, by Mr. Hilger, which can be mounted in front of the object-glass for photographing spectra of stars. It is mounted below the Merz Refractor on the Lassell Equatorial and is in regular use as a photoheliograph, 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 image of the Sun in the primary focus is about one inch in diameter, and this is enlarged by the Dallmeyer doublet to nearly 8 inches on the photographic plate, the whole length of the instrument being about 12 feet 4 inches. The exposure is given (as in the Dallmeyer photoheliograph) by a shutter at the primary focus, having a slit in it of adjustable width; but whilst the shutter of the Dallmeyer photoheliograph travels in a groove, and is drawn downwards by the pull of a strong spring, in the Thompson photoheliograph the shutter, which is mode of aluminium for the sake of lightness, turns about a pivot 18 inches from the exposing slit, under the action of a spring placed about midway between the pivot and the slit and giving a very rapid motion to the latter.’

Some later alterations to the photoheliograph


	1923	‘New tinplate tube fitted 18/12/1923 to replace old tube lost on Russian Eclipse Expedition 1914 Aug.’ (RGO39/4/44)

	1926	New enlarging lens by Ross (No.108292) working at F/2.8 fitted. (Annual Report 1927)

The Thompson’s contibution to the Greenwich Series of sunspot photographs

The Thompson Photoheliograph made a major contibution to the Greenwich Series of plates until 1912, when the Dallmeyer Photoheliograph was upgraded (annual report 1913). After that date, its use for this purpose, was much less frequent (it was unavailable from 1914-1923). The last photographs taken with it, that are included in the Greenwich series, date from 1936.

The Thompson Coronograph

When taken on eclipse expeditions, the Thompson Photoheliograph was converted into a coronograph. This was referred to as the Thompson Coronograph. Other telescopes used as a coronograph included the Dallmeyer Photoheliographs and the Astrographic Telescope. By way of explanation for other members of staff, the 1911 inventory contained the following note: ‘These coronographs do not exist as separate entities. They are made up as required’.

According to the inventory (RGO39/4/38, 112 & 114) the Thompson Coronograph consisted of the following parts, which were normally kept or stored in different locations:

The object glass and tube of the Thompson Photoheliograph
A double symmtrical negative lens of 4-inches aperture and 16 inches focus
A mahogany camera for 15 by 15 inch photographic plates, complete with 8 dark slides with hinged shutter
An adaptor for the lens
A Flap shutter

The instrument was used in conjunction with a 16-inch coelostat.

The following description of the instrument in use is taken from Dyson’s preliminary account of the 1901 eclipse:

‘The instrument used was the Thompson photographic telescope with object glass of 9 inches aperture and 8 feet 6 inches focal length, belonging to the Royal Observatory, in combination with a concave telephoto lens by Dallmeyer, of 4 inches aperture and 16 inches focus, fitted as a secondary magnifier, to give an image of the sun 4 inches in diameter, with a field (for full pencils) of 14 inches. The total length of the coronagraph was 12 feet, the equivalent focal length being about 36 feet. The focus was determined by the method used by the Astronomer Royal at the eclipses of 1896, 1898, and 1900, by means of the image of a gauze net in the plane of the plate reflected from the plane mirror of the coelostat. In the determination of the focus, which was done at night, Mr. Curtain, warrant officer, rendered great assistance. A coelostat, with 16-inch plane mirror (made by Dr. Common), was employed to reflect the rays into the coronagraph, which pointed downwards to the mirror at an angle of 3° or 4°, and was in the azimuth 18° north of east on the day of the eclipse. The adjustment of the coelostat was readily made by observations of the Sun with the attached theodolite in the usual manner.’

Eclipse expeditions


Year	Location	Result	Comment

1896, Aug 9	Japan	Cloudy

1898, Jan 22	India (Sahdol)	Successful

1900, May 28	Portugal (Ovar)	Successful

1901, May 18	Sumatra	Successful

1905, Aug 30	Tunisia (Stfax)	Successful	Dismounted from Thomson Equatorial on 21 or 22 July. Remounted 1 October

1912, Oct 10	Brazil (Passa Quatro)	Heavy rain	Dismounted from Thomson Equatorial on or soon after 26 June

1914, Aug 21	Russia (Minsk)	Successful	Instrument stranded in Russia due to the outbreak of war. Stored at Pulkowa Observatory. Returned to Greenwich, minus tube, on 17 April 1923

Historic film clips

Visit of the King of Afghanistan to England. Shot in 1928, this film consists of 7 reels in total. The King’s visit to the Royal Observatory is on reel 4 of (7). Reel 4 starts with the Royal Party travelling by boat downstream to Greenwich. Observatory footage starts at 3min 5 sec. The King and the Astronomer Royal Dyson are seen entering the 28-inch telescope dome and examining the telescope. They are then shown examining the telescopes in the Thompson Dome in the South Building. The footage ends with the royal party on the South Building. Running time: 3 minutes 18 seconds.