Telescope: The five Dallmeyer Photoheliographs (1873)

 

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Dallmeyer Photoheliograph No. 2 undergoing testing in hut D at Greenwich in either 1873 or 1874. Reproduced under the terms of a Creative Commons Attribution-Non-Commercial (CC BY-NC) licence courtesy of Cambridge Digital Library (see below)

All five of the 4-inch Photoheliographs by Dallmeyer originally constructed for the British 1874 Transit of Venus expeditions were subsequently transferred by the British Government to the Royal Observatory. From 1875 until the end of 1976, one was always on hand to take a daily photograph of the sun. As well as the five instruments mentioned above, Dallmeyer made at least three others of similar design which were used to observe the Transit in the colonies. One of these (originally ordered by De La Rue for his own personal use) was used to observe the Transit in India (more on this below). Two more were used in Australia. One was purchased for use in Melbourne, the other (which was supplied without a stand) was purchased for the Sydney Observatory and is now in the Powerhouse Museum.

 

The decision to photograph the Tranist of Venus and the ordering of the photoheliographs

The 8 December 1874 Transit of Venus was the first to occur since 1769. Initial planning for a British expedition began with the publication of a paper by Airy in 1857 (click here to read). In 1868, Airy began a correspondence with the Hydrographer of the Navy about funding an expedition. Although there had been discussion of the application of photography, photographic methods were excluded from the estimate for the expenditure of a sum of £10,500 that was put before a parliamentary committee and approved on 6 August 1869 (click here for Hansard report). Not everyone was happy at the exclusion of photographic observations, particularly the Royal Observatory’s own Board of Visitors who at their meeting on 5 June 1869 had resolved:

‘that in their opinion it would be desirable to make provision for the Photographic record of the phenomena in addition to the preparations already contemplated.’ (ADM190/4)

and as a result, at their annual meeting on 4 June 1870, the Observatory’s Board of Visitors resolved that

‘Mr warren De la Rue be requested to confer with the Astronomer Royal with the view of organizing a plan for photographic observations of the Transit of Venus, and for preparing an approximate estimate of the probable expense.’ (ADM190/4)

At their next meeting on 3 June 1871, they went further and

‘Resolved, that as this Board deem it most important that Photographic be combined with eye observations at the approaching Transit of Venus, an opinion in which the Astronomer Royal fully concurs, the Chairman [Edward Sabine] apply to the Lords Commissioners of Her Majesty’s Treasury to sanction a grant of Five Thousand Pounds (£5000) for the purpose, a sum which it is considered will cover the cost of photographic apparatus and observations for all stations.’ (ADM190/4)

Following further correspondence with Airy, the Admiralty agreed to provide the additional £5000. At this point, an order was paced with Dallmeyer for five photoheliographs, their construction being overseen by De la Rue.

 

Dallmeyer Photoheliograph No. 1 undergoing testing in hut B at Greenwich in either 1873 or 1874. Reproduced under the terms of a Creative Commons Attribution-Non-Commercial (CC BY-NC) licence courtesy of Cambridge Digital Library (see below)

A closer view of Dallmeyer No. 1 undergoing testing at Greenwich. Note the makers plate carrying the words J H Dallmeyer, London, which can be seen at the top of the stand Reproduced under the terms of a Creative Commons Attribution-Non-Commercial (CC BY-NC) licence courtesy of Cambridge Digital Library (see below)

A third view of what is presumed to be Dallmeyer No.1 taken from the other side. Reproduced under the terms of a Creative Commons Attribution-Non-Commercial (CC BY-NC) licence courtesy of Cambridge Digital Library (see below)

A close up of Dallmeyer No.4 undergoing testing at Greenwich in hut E. Reproduced under the terms of a Creative Commons Attribution-Non-Commercial (CC BY-NC) licence courtesy of Cambridge Digital Library (see below)

 

The setting up of a photographic and spectroscopic department for Greenwich

Prior to 1872, Airy had shown no interest in instigating a programme of systematic observations of the Sun at Greenwich. This would probably have remained the case if it hadn’t been for: a) others exerting pressure on the Government for more publically funded science including a dedicated astrophysical observatory, b) the presence (since 1864) of De la Rue on the Board of Visitors, and c) the cessation of his programme at Kew.

In 1870, a Royal Commission had been set up to inquire into Scientific Instruction and the Advancement of Science. Often referred to as the Devonshire Commission, it sat from 1870–75 and produced a large number of reports.

In 1872, as the debate on the provision of a dedicated astrophysical observatory was reaching its climax, Airy made the pragmatic decision to try and fend off such an eventuality by suggesting to the Board of Visitors that the Observatory should undertake a continued series of observations of solar spots and possibly engage in solar spectroscopy as well. The Board endorsed his view at their meeting on 1 June as did the Admiralty. By the end of the year, construction of a dome to house a photoheliograph at Greenwich was under way.

Click here to read more about the photoheliographic programme and the debate of 1872.

 

The early years of the photoheliographic programme at Greenwich (1873–75)

In this view, which dates from 1873 or 1874, the wooden building housing the Kew Photoheliograph is seen on the left standing on its brick foundations. To its right are huts for two of the Transit of Venus photoheliographs, the one on the right being destined for Station D (New Zealand). The viewpoint is from near the south-west corner of the Observatory site, looking across the South Ground towards the north-east. The low building behind the two huts on the left is the suite of seven Magnetic Offices. The low level fencing running from the bottom left hand corner marks the edges of a path. An engraving made from the photograph was published in The Graphic on 27 June 1874. Reproduced under the terms of a Creative Commons Attribution-Non-Commercial (CC BY-NC) licence courtesy of Cambridge Digital Library (see below)

Conscious as always that government money could be saved, rather than order a new photoheliograph specifically for the Royal Observatory, Airy planned in the long run to use one of the Dallmeyer Photoheliographs ordered for the Transit of Venus expedition. These however were not going to be permanently available for use at Greenwich until 1875 following their return from overseas. Once he was confident of parliamentary approval for the funding of the new area of work, he wrote in his capacity as Astronomer Royal to the secretary of the Royal Society, (George Stokes, who was also a member of the Observatory’s Board of Visitors), explaining the position and asking if he might borrow the Kew Photoheliograph in the interim. The letter was written on 19 October 1872 and his request discussed at the next meeting of the Society’s Council on 31 October. Since he was the Society’s President at that time, Airy himself was in the chair. After his letter had been read, Airy explained that he had previously been in touch with De la Rue who was happy for such a loan to take place. It was therefore resolved that his request be acceded to.

In preparation for its arrival at Greenwich, a small wooden building with a rotary dome had been erected on the south side of the Magnetic Offices (close to the centre of where the north wing of the South Building now stands). It had a connection though to Magnetic Office No. 3, which served as a dark room. Alterations were also made to Office No.2 so that it could serve as a chemical room. Work commenced in November 1872 with the brick foundations being in place by the 18th of the month (RGO6/26) and the photoheliograph becoming operational in February 1873. A fire-proof chemical room was later erected nearby and fitted with a gas-stove for keeping the photographic chemicals at a suitable temperature thoughout the winter.

Although funding was available for an extra member of staff, the new assistant, E Walter Maunder, was not appointed until later, taking up post on 6 November 1873. Meanwhile, a regular programme of recording sunspots had commenced on 1 June 1873, with photographs of the Sun being taken with either the Kew Photoheliograph and one or more of the Dallmeyers. Unfortunately, the published daily observations do not indicate which photoheliograph was used to obtain them. In preparation for the 1874 Transit, instruction on the photographic process was initially given by Reynolds who had attended the 1860 eclipse at Rivabellosa with De La Rue and the Kew Photoheliograph, and subsequently under Captain Abney (Airy autobiography).

The Kew Photoheliograph was dismounted by Dallmeyer’s staff on 22 September 1875 and replaced by the Transit of Venus Photoheliograph that had been sent to New Zealand (RGO6/784, 1876 BofV Report p.9 & ADM190/4/402). It was subsequently returned to Kew on 5 January 1876.

 

Airy’s 1873 description of the Dallmeyer Photoheliograph

As was his normal custom Airy wrote a description of the Dallmeyer Photoheliograph that was published in the introduction to each of the annual volumes of Greenwich Observations. The text below is transcribed from the 1873 volume which is the first volume in which it appeared.

‘The Dallmeyer Photoheliograph has an object-glass of 4 inches aperture and 5 feet focal length, forming an image of the Sun half an inch in diameter; this image is enlarged by a secondary magnifier to 4 inches on the camera screen, where the sensitive plate is inserted, the whole length of instrument being about 8 feet. The exposure is given by a shutter, having a slit of adjustable width, which is carried by a spring across the primary image. At the principal focus cross-wires are placed, which give facilities for determining the position-angles of spots on the photographs. The instrument is equatoreally mounted, though this is not absolutely necessary to its efficient action, as the exposure is practically instantaneous, amounting only to a few thousandths of a second in ordinary cases.’

 

The Dallmeyer Object Glasses

The Dallmeyers were unusual in that the flint element of the doublet was on the outside. This is stated in both the 1888 Report to the Board of Visitors (p.10) and in the description of the Melbourne Dallmeyer given by Clark and Orchiston in their 2004 paper The Melbourne Observatory Dallmeyer Photoheliograph and the 1874 transit of Venus. The OG of the Melbourne instrument is described as having an aperture of 102 mm and a focal ratio of f/15. The polar axis of the Melbourne Dallmeyer also carries the date 1869. It would be interesting to know if any parts of the three Royal Observatory Dallmeyers now owned by the National Maritime Museum have an equivalent marking.

 

The formal transfer of the Transit of Venus instruments to the Observatory

All five of the British Transit of Venus instruments were formally transferred to the Royal Observatory between 2 June 1877 and 16 May 1878 (precised date currently unknown). Their location in May/June 1878 is recorded in both RGO7/67 and the minutes of the Board of Visitors (ADM190/4/402). They give the locations of the photoheliographs at that time as follows:

A  Lent to the Cape Observatory
B  Lent to the South Kensington Museum
C  Lent to Col. Tenant in India [Dehra Dun]
D  In use at the Royal Observatory
E  Packed away in the Great Shed at the Royal Observatory

Despite the fact that the same infromation is repeated twice in the archives, it is believed to be an inaccurate record (see below).

 

The arrival of the Thompson Photoheliograph in 1891

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 brought into use in 1891. From then until 1936, both the Thomson and a Dallmeyer were used at Greenwich. Initially, the Thompson appears to have been the instrument of choice, (possibly because it was the superior of the two, but possibly because of its more convenient location or as a gesture towards its donor). The Dallmeyer being used when it wasn't available. This included the period from 1892 until 1898 while the Thompson was dismounted during the construction of the New Physical Building, times when the Thompson Equatorial was out of use for repair or maintenance work and times when the Thompson was sent overseas for eclipse expeditions. The Dallmeyer became the instrument of choice in 1912 when the Thompson was sent to observe a solar eclipse in Brazil in 1912 followed by another in Russia in 1914 from which it failed to return until 1923 because of the outbreak of war. Although it was eventually remounted at Greenwich in 1924, it played second fiddle to the Dallmeyer which had been upgraded with a new OG fitted in 1911. The last Thompson photographs measured for the published observations were taken in 1936. During the period 1924–36, the Thompson tended to be used on days of good definition or during the winter months when the Sun’s disc was reddish.

 

Alterations to the ‘Greenwich’ Dallmeyer

The specific Dallmeyer used at Greenwich varied over the years (more on this later). The information here refers to the ‘generic’ Dallmeyer in use.

 
Date
Alteration / comment
1884, April 2 (4?) New secondary magnifier. This increased image diameter from 4 to 8 inches and length of instrument from about 8 feet to 9 feet 7 inches. Altered in the same manner as the photoheliograph lent to India.
1909 New camera, increasing length of instrument to 11 feet 2 inches. At about same time, a similar camera was fitted to the ‘Cape Dallmeyer’ and the Thompson so that all three produced images of the same size as those of Dallmeyer No.4 which by then was at Kodaikanal.
1911 New OG by Grubb (supplied 19 July). Focal length two inches less than Dallmeyer allowing the camera to be shortened (not specified how this was done).
1912 New 4-inch refractor with solar diagonal eye-piece mounted in place of counterweight. This used the OG and micrometer from Airy’s Altazimuth which had been dismounted to make way for the Dallmeyer.
1927, Jan 23 New enlarging lens by Ross (No.110705) working at f2.4 supplied following supply of a similar lens for the Thompson which was designed to give better definition of the Sun's limb.
1949, May 2 Moved to Herstmonceux and mounted on the Newbegin Refractor (without the 4-inch refractor?).
1982 Returned to Greenwich and mounted on pier of sheepshanks telescope in Altazimuth Pavilion.

The above table is believed to omit a trial where two substitutions were made in the same year to the enlarging lens. Further information concerning the source of this information is currently under investigation.

The succession of alterations meant that as time went on, fewer and fewer of the original Dallmeyer parts remained. By 1949, only the tube is thought to have been an original part though the OG cell and lens cap may also have been.

 

The ever changing location of the ‘Greenwich’ Dallmeyer
 
Date
Location of Photoheliograph / comment
1875–1891 Hut originally built for Kew Photoheliograph in South Ground.
1891–1894 Mainly in Kew Photoheliograph hut, but moved in the winter months when the Sun was obscured by the New Physical Building which was then under construction. Moved to first floor of New Museum onto the stand of another Dallmeyer (in the New Physical Building) 9 September 1891, returning to hut 6 April 1892. Moved again to same location between 9 September 1892 and 7 April 1893. Moved to 28-inch Refractor in September 1893 (the first floor of new building not being available), then to roof of south wing of New Physical Building in April 1894.
1894–1899 Roof of south wing of New Physical Building from April 1894. Dismounted 13 Oct 1898 and placed in the upper floor of the museum.
1900 Mounted on 26-inch refractor from 9 March while Thompson sent to Portugal.
1901 Mounted on 26-inch refractor during eclipse expedition to Sumatra.
1902–1906 Generally not known, probably roof of either the south or west wing of New Physical Building. Mounted on 26-inch refractor from 22 July 1905, then transferred to to its own mounting on roof of west wing on 31 July 1905. Remounted on 26-inch on 10 August remaining there until 30 September when it was gain dismounted.
1907–1911 Roof of south wing of New Physical Building.
1911–1949 Dome previously occupied by Airy's Altazimuth Telescope
1949–1982 Solar Dome at Herstmonceux (the first of the domes to be built there).
1982–present Altazimuth Pavilion, Greenwich.

In this photo of the partially completed South Building in the summer of 1897, the hut that was placed on the roof of the south wing in April 1894 to house the photoheliograph can be seen centre left. Photograph reproduced by permission of the Greenwich Heritage Centre (see below). A copy of the photo was published in the 17 July 1897 edition of the Illustrated London News

The photoheliograph soon after it had been moved into the Old Altazimuth Dome. The photo appears to have been taken during 1911 before the counter weight was replaced with a second telescope. From Sun-Storms and the Earth by E Walter Maunder. Harper's Monthly Magazine (Sept 1912)

 

Christie’s 1884 description of the ‘Greenwich’ Dallmeyer following its first modification

The first modification to the Photoheliograph took place in 1884, when a new secondary magnifier was fitted. Information about this was included by Christie in the description of the instrument that appeared in the 1884 volume of Greenwich Observations. The full description is transcribed below.

‘The Dallmeyer Photoheliograph has an object-glass of 4 inches aperture and 5 feet focal length, forming an image of the Sun half an inch in diameter; this image is enlarged by a secondary magnifier to 4 inches on the camera screen, where the sensitive plate is inserted, the whole length of instrument being about 8 feet. From 1884 April 2 a new secondary magnifier has been used by which the image is enlarged to 8 inches diameter on the camera screen, the whole length of the instrument being increased to 9 feet 7 inches. The exposure is given by a shutter, having a slit of adjustable width, which is carried by a spring across the primary image. At the principal focus cross-wires are placed, which give facilities for determining the position-angles of spots on the photographs. The instrument is equatoreally mounted, though this is not absolutely necessary to its efficient action, as the exposure is practically instantaneous, amounting only to a few thousandths of a second in ordinary cases.’

The photoheliograph showing the new secondary magnifier that was added on 2 April 1884. Photo by E Walter Maunder. From The Leisure Hour (1898) p.376

 

Maunder’s 1898 description of the ‘Greenwich’ Dallmeyer and its method of use

In his series of articles for The Leisure hour that were published in 1897/8 Maunder gave a brief description of the Dallmeyer Photoheliograph. This is reproduced below. The text was reused by him in largely unaltered form in his book The Royal Observatory Greenwich a glance at its history and its work which was published in 1900.

‘In order to photograph so bright a body as the sun, it is not in the least necessary to have a very large telescope. The one in common use at Greenwich is only four inches in aperture, and even that is usually diminished by a cap to three inches, and its focal length is but five feet. …

In 1909, a new enlarging lens and mahogany camera were fitted to the photoheliograph, increasing its length by 19 inches and probably making it too large for its observing hut. In this photo which was taken on the roof of the south wing of the New Physical Building soon after the camera had been fitted, the imprint left by following the hut's removal can just be made out on the roof near the base of the stand. From The heavens and their story, by Annie & E Walter Maunder

The image of the sun in the principal focus of this telescope is about six-tenths of an inch in diameter; but a magnifying lens is used, so that the photograph actually obtained is about eight inches. Even with this great enlargement, the light of the sun is so intense that with the slowest photographic plates that are made the exposure has to be for only a very small fraction of a second. This is managed by arranging a very narrow slit in a strip of brass. The strip is made to run in a groove across the principal focus. Before the exposure, it is fastened up so as to cut off all light from entering the camera part of the telescope. When all is ready, it is released and drawn down very rapidly by a powerful spring, and the slit, flying across the image of the sun, gives exposure to the plate for a very minute fraction of a second – in midsummer for less than a thousandth of a second.

Two of these photographs are taken every fine day at Greenwich; occasionally more if anything specially interesting appears to be going on.’

 

A problem of identity

The photoheliograph during its time in the Old Altazimuth Dome. The photo was probably taken at some point after the mid 1930s. An unidentified staff member is looking though the eyepiece of the telescope incorporating the object glass of Airy's Altazimuth which was constructed and mounted in place of the counter weight in 1912

Prior to 1891, the five photoheliographs were identified using the letters A to E which had been used to designate the five Transit of Venus observing stations. Sometimes, the name of the Observaing station was used instead. From 1891 onwards, the use of the letters A to E was dropped and the numbers 1 to 5 used instead. Unfortunately, nothing has been identified which definitively shows the correlation between the two systems. Worse, different documentary sources show different correlations.

As well as the photographs mentioned above, the following sources have been used to track the identity and history of each of the instruments: The preamble to the published observations (registration (but no subscription) required), The Reports of the Astronomer Royal to the Board of Visitors, The inventories (1893 (RGO7/67), 1911 (RGO39/4), 1926 (RGO39/5) & 1933 (RGO39/6)), the minutes of the Board of Visitors  (ADM190/4), the Journals of the Astronomer Royal and the First/Chief Asisstants and other material identified via the archive catalogue at Cambridge.

Unfortunately, the information about the identity of the instrument that was being used at Greenwich that is given in the preamble to the published observations is not always consistent with that published in the Report of the Astronomer Royal to the Board of Visitors. This is particularly true during the 20 year period commencing in 1890 when the location of the photoheliograph had to be frequently changed during the construction of the New Physical Building (South Building). The inconsistency is likely to have been caused by the fact that much of the text of both the preamble and of the annual Report to the Visitors did not change from one year to the next and was simply copied verbatim. It seems probable, that due to an oversight that there was a failure to update certain information that had changed.

The earliest images of the photoheliographs show them at Greenwich undergoing testing in their huts prior to being shipped overseas for the Transit. Of the five photoheliographs, photographs of three of them exist from this time There are five images in total, all of which are reproduced above. They can be viewed in still higher resolution at the Cambridge Digital Library. In each of the photographs, the huts containing each of the instruments are identifiable as their component parts are marked with the station identifier (A to E) to which they were to be sent. Likewise, in four of the images the component parts of the telescope are marked with a number that has been stencilled on to them, the same number being used for all the parts of the same telescope. The relationship between letters and numbers is as follows:

Transit of Venus Station Designation letter
Photoheliograph number
Egypt A
Sandwich Islands B 1
Rodrigues Island C
New Zealand D 2
Kerguelen E 4

Based on the information below, it would appear that the combinations of huts and photoheliographs set up for testing at Greenwich were not the same as the combinations that were shipped overseas.

 

The Dallmeyer in use at Greenwich & Herstmonceux – The ‘Greenwich’ Dallmeyer

It seems fairly certain that the Dallmeyer initially in use at Greenwich was the one that had been sent to New Zealand for the Transit. This is confirmed by both its specific mention in the 1876 Report of the Astronomer Royal to the Board of Visitors (p.9) and the minutes of the Board for 1878 (ADM190/4/402).

It is also pretty certain that from 1910 onwards, the instrument in daily use was Dallmeyer No.2 . The identity of the instrument used in the period from 1878 up to 1910 is less clear as different instruments are stated as being used in different sections of the published records.   

The table below has been compiled from the preamble to the published results – specifically:

Greenwich Observations

1874-1885 Greenwich Photo-Heliographic Results
Greenwich Spectroscopic and Photographic Results (for each of the years 1878-1900
Greenwich Photo-Heliographic Results (for each of the years 1900-1955)

Bulletins

Royal Greenwich Observatory Bulletins (No. 14 (1956)
Royal Observatory Bulletins (Nos: 26 (1957), 57 (1958), 103 (1959), 132 (1960) & 144 (1961))

Annals

Royal Observatory Annals (Nos: 6 (1962–64), 8 (1965), 10 (1966), 11 (1967), 12 (1968–71) & 13 (1972–76))

It is imported to note that the above results only include infromation on the photographs that were actually selected for measurement.

Year
Dallmeyer Used
Comment
1874–1877 New Zealand Kew Photoheliograph until 17 Sep 1875, then NZ Dallmeyer
1878–1880 Not specified
1881–1898 New Zealand
1899 No Greenwich Dallmeyer photos used
1900–1901 New Zealand
1902 No Greenwich Dallmeyer photos used
1903–1916 New Zealand
1917–76 Not specified From 1917 to 1948, the instrument is referred to as the Dallmeyer Photoheliograph. From 1949 onwards (following the move to Herstmonceux) it is referred to as the 4-inch photoheliograph

 

Two notes in the archives at cambridge (RGO7/70 (E8)) add to the picture (and confusion). The first is a memo on them written by Christie on 14 June 1886:

‘The T.V. Photoheliographs have a No. on the mounting, the letter of the station to which they were sent in 1874 on the dark slides and both No. & letter on camera end of tube.

Photoheliograph 4D was originally mounted at Greenwich for daily photographs.

In the spring of 1884 a new secondary magnifier (to give 8in images of Sun) was adapted to tube 3A and in March 1884 this tube 3A was mounted on the Photoheliograph mount No.4 & has been in regular use from that time.

The tube 4D is in Photographic Office No.3.’

The second is a list of the Photoheliographs at the Observatory on 22 May 1890 (which fails to mention the driving clock from an unspecified instrument (presumably No.2 or No.5) that was in use with the personal Equation Machine).

One complete, mounted in Photoheliograph hut: No.4
Two tubes complete: E.2. & A.5
One stand with clock in the Lassell Dome: No.3.

Whilst the first note links station A to Dallmeyer No.3, the second one appears to contadict it by linking it to Dallmeyer No.5!

The information in the table above is also at odds with that published in the Reports of the Astronomer Royal to the Board of Visitors. They indicate that photoheliographs No.2, No.3 and No.4 were all in use at different times between 1875 and 1910.

According to the Reports, the instrument set up originally was that from New Zealand. The insturment in use was  not then mentioned by name in the reports until May 1891, when the instrument in use is recorded as being No. 4. This is said to have remained in use (though for some of the time on the stand of No.3) until April 1894. No. 3 was mounted on the roof of the south wing of the New Physical Building in April 1894, remaining in use there until at least 1901. In the meantime, No.4 had been sent to the Observatory in Madras where it arrived in April 1895, never to return to England. Inconsitencies in the 1893 and 1894 reports suggest that No.3 may in fact have been brought into use in September 1893 when the photoheliograph was mounted for the winter months on the 28-inch Refractor (see above). This would seem to be supported by the 1893 inventory (RGO7/67) which does not (for what ever reason) seem to mention the presence of No.3 at the Observatory, but does mention No.4 which is recorded as being complete on its stand on the ground [lower] floor of the New Physical Building and destined for Madras

After May 1901, no direct information is given about the identity of the photoheliograph until May 1910 when the working instrument is reported as being No.2 (with its new enlarging lens) on the roof of the south wing of the New Physical Building.

It is possible that No. 2 was in fact brought into use in 1904 as the Report for 1905 states that Nos. 2, 3 & 5 were at Greenwich, that the O.G. of No 5, was with Turner (and had apparently been so since 1898) and that Photoheliograph No.3 had been exchanged for a similar one at Mauritius (the Mauritius instrument having then become known as Dallmeyer No.3). The original No.3 was dispatched to Mauritius in 1904 (see below), but was in use at Greenwich from 12 September to 16 December 1903 while repairs were being carried out to the Thompson Dome preventing the Thompson Photoheliograph from being used. It is also possible that it was not No.2 that was mounted on the roof to replace Greenwich No.3, but the Dalmeyer from Mauritius as no photos are believed to have been taken with a Dallmyer at Greenwich between 25 February 1904 and 21 July 1905.

 

A second Dallmeyer No.3  – The exchange of instruments with the Royal Alfred Observatory in Mauritius

For the days when the Greenwich Instruments were unable to take a solar photograph, plates from other observatories were used. From 1878 until 1909 some of these were obtained from the Royal Albert Observatory in Mauritius which had a Dallmeyer Photoheliograph similar to the ones that were in use at Greenwich.

By 1903, the Royal Albert Dallmeyer was in a poor state of repair. In August that year, its Director, Thomas Claxton, started a correspondence with the Astronomer Royal at Greenwich asking if he might loan them one so that theirs might be sent back to the makers for repair. Christie was happy to oblige, but suggested that as an alternative to a loan, time and money could be saved if the Royal Albert Dallmeyer was simply exchanged with one from Greenwich. It was agreed that the Royal Albert Dallmeyer would not be packed and dispatched until the Greenwich instrument had been received and was working so that there would not be a break in the sequence of photos taken at the Royal Albert. It was also agreed that the Colony would pay for repairs and alterations to ‘their’ photoheliograph as well as the cost of carriage of both instruments. The instrument sent from Greenwich, was Dallmeyer No.3 which underwent a number of alterations for the purpose. These were carried out at the Observatory. The instrument was packed into two cases for shipping. It is believed that neither the stands nor the drives were part of the exchange. By 28 September 1904, following a slight adaption to its mounting brackets once it had arrived at the Royal Albert, No.3 was mounted and ready for use. The Royal Albert Dallmeyer arrived at Greenwich on 17 February 1905 from which point onwards it too was referred to as Dallmeyer No.3. For further details on the specific alterations to the two photoheliographs, RGO7/42 should be consulted.

 

The Dallmeyer sent to the Cape

Edward Stone, the former First Assistant at Greenwich and HM Astronomer at the Cape of Good Hope spent part of 1875 in England, visiting the Observatory at Greenwich at various times between August and October (Journals of the Astronomer Royal & his Chief Assistant, RGO6/26 & RGO6/784). During that time, it was arranged that one of the Transit of Venus photoheliographs would return with him to the Cape. According to the minutes of the 1878 meeting of the Board of Visitors, the photoheliograph that was sent was Photoheliograph A – the instrument that had been used in Egypt. However, according to an entry in the Journal of the Chief Assistant (William Christie), Photoheliograph C, complete with its hut, was taken to Deptford Dockyard on 18 October 1875 for transmission to the Cape Observatory (RGO6/784). This was the instrument that had been sent to Rodrigues Island for the Transit. From 1894 onwards, the Astronomer Royal began to refer to the Cape photoheliograph as Dallmeyer No.1. Given that there is photographic evidence of hut C being at the Cape (see images below), it seems most likely that the instrument was in fact Photoheliograph C as stated by Christie in his journal rather the Photoheliograph A (as stated in the Board of Visitors minutes (and also in RGO7/67)) or Photoheliograph B as indicated by the photographs above of Dallmeyer No.1 while it was on trial at Greenwich. The first time that Airy told the Visitors that a photoheliograph (unspecified) had been loaned to the Cape was in his 1877 Report.

Writing to Airy on 10 March 1876 (RGO15/97/164), Stone told him that the instrument was brought into use for the first time at the Cape on 12 February 1876, going on to say that the hut was not fit for purpose as the converging ribs which support the shutters convenient prevented photographs being taken when the altitude of the Sun was high except with a very serious loss of time. He went on to say that the temperature of the dark room in the Cape climate made it ‘fearful to work in’. To compound his problems his third assistant had handed in his notice as the pay was so low.

Taken at the Cape Observatory in about 1900, the photographs shows the Transit of Venus hut (left) that was originally used to house its photoheliograph. Image courtesy SAAO Archives

The experimental Huggins coronagraph on the Dallmeyer mount in Transit of Venus hut C at the Cape Observatory. The photo is thought to have been taken in about 1885. Image courtesy SAAO Archives

The Cape photoheliograph on the Troughton & Simms mount in 2009. Photo courtesy of Ian Glass

When Stone resigned in 1879 to take up the position of Radcliffe Observer, the photoheliograph fell out of use and remained so throughout the period while David Gill was in office (1879–1907). How much this was to do with the personal animosity that came to exist between Gill and Christie is unknown. Gill did however claim that ‘the stand was by no means satisfactory’. It was only under Gill’s successor Sydney Hough (1907–23) that it was brought back into use. In the meantime, in 1885, the tube had been dismounted by Gill and replaced with the experimental Huggins coronagraph.

In 1909, provision was made for ‘the daily photograph of the Sun in order to supplement the series of Greenwich photographs’. To this end, the photoheliograph was dismounted and forwarded to England for overhaul in March 1909. According to the 1909 Report of the Astronomer Royal to the board of Visitors, it was sent with all but 'the heavier parts of the mounting'. Christie's original intention was to send Photoheliograph No. 2 in its place rather than return No.1 to the Cape after adapting it to take photographs of the Sun on the 8-inch scale.

In the event, so we learn from the 1910 report, it was decided to return the upgraded No. 1 to the Cape and retain the No.2 at Greenwich. We also learn, the both photoheliographs (No.1 and No.2) were fitted with identical new enlarging lenses and cameras to take photographs of the Sun on the 8-inch scale or as Christie then went on to explain, that more precisely, on a scale of one decimetre to the solar radius at mean distance. At the same time, a new camera was made for the worn out one on the Thompson photoheliograph, but at the same scale as Dallmeyer Nos. 1, 2 and 4 (which was at the Kodaikanal Observatory).

The upgraded instrument minus the equatorial section of the mounting and the driving clock arrived back at the Cape in December 1909. A suitable mounting was then made locally for attaching it to the tube of the Cape Astrographic Telescope. After a few initial experiments, it was brought into regular use on 1 March 1910.

The photoheliograph remained on the Astrographic Telescope until 1929 when it was transferred to the dome from 1849 that was originally built for a Merz 7-inch, on a Troughton & Simms mount dating from the 1870s. The bottom section of the originally Dallmeyer stand is still thought to exist at the Observatory.

 

The Dallmeyer sent to India

 

The Dallmeyer Coronograph

 

Loans to the south Kensington Musuem

 

Destruction of unused / unwanted parts

Scattered though the inventories are notes on the locations of different parts of the photoheliographs as well as the destruction of some of them. The tube, finding lens and Camera of Dallmeyer No.3 were all condemned in January 1930 (RGO39/5/27).

 

Public programmes with the Greenwich & Cape Dallmeyer photoheliographs

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Contemporary accounts and further reading

Account of observations of the transit of Venus, 1874, December 8, made under the authority of the British government: and of the reduction of the observations

The Transit of Venus Enterprise in Victorian Britain. Jessica Ratcliffe, (Routledge, 2008)

Report on the preparations for, and observations of the transit of Venus, as seen at Roorkee and Lahore, on December 8, 1874. JF Tennant, (Calcutta, 1877)

The Greenwich Photo-heliographic Results (1874 – 1976): Summary of the Observations, Applications, Datasets, Definitions and Errors. D. M. Willis, H. E. Coffey, R. Henwood, E. H. Erwin, D. V. Hoyt, M. N. Wild & W. F. Denig. Sol Phys (2013)

The Greenwich Photo-heliographic Results (1874 – 1885): Observing Telescopes, Photographic Processes, and Solar Images. D. M. Willis, M. N. Wild, G. M. Appleby & L. T. Macdonald. Sol Phys (2016)

Re-examination of the Daily Number of Sunspot Groups for the Royal Observatory, Greenwich (1874 – 1885). D. M. Willis, M. N. Wild &  J. S. Warburton. Sol Phys (2016)

 

Image licensing information

Special thanks are due to the Greenwich Heritage Centre for permission to reproduce the 1897 photograph of the partially completed South Building.

The image of the Kew Photoheliograph hut and the five images of the Dallmeyer Photoheliographs undergoing testing are reproduced in compressed form under the terms of a Creative Commons Attribution-NonCommercial 3.0 Unported License courtesy of Cambridge Digital Library (RGO6/276).