Telescope: Portable Transit by Brauer (1866)

Made specifically for the Observatory in 1866, this particular telescope was commissioned after Airy had examined a similar one by Brauer that had been brought to Greenwich in 1864 by Colonel Forsch and Captain Zylinski for their determination of differences of longitude by telegraphic means on the Great European Arc of Parallel.

In his 1865 Report to the Board of Visitors, Airy wrote:

‘I have received the sanction of the Government for procuring two instruments of the largest portable class, namely a Transit-instrument with diagonal reflector and eye-view in the axis, and an Altazimuth with similar reflector and eye-view. The former is ordered of Mr. Braun, artist of the Pulkowa Observatory; for the latter Mr. Simms has general instructions, but the details are not yet arranged. The examination of the instruments embodying the principle of eye-view in the axis, which were brought here in the last summer by Colonel Forsch and Captain Zylinski, has removed some of my objections to that construction.’

Airy did not publish a description of the instrument, nor does the instrument appear to have been put to use by him either at the Observatory or in any expedition with which the Observatory was involved. It was however loaned to Captain Alexander Ross Clarke of the Ordnance Survey who in 1868 tested it out along side a copy of modified design that had been made by William Simms who published his assessment of the instrument in his paper On a determination of the direction of the meridian with a Russian diagonal transit instrument..(MMRAS, Vol 37 (1869)). He described the instrument as follows:

‘The Diagonal form of Transit Instrument, namely, that in which the rays of light, instead of passing straight from the object-glass to the eye-piece, are bent at right angles by a prism in the central cube, and so pass out at one of the pivots — is not so well known in this country as in Germany and Russia. The advantages of the instrument are, that the observer without altering his position can observe stars of any declination, that the instrument requires but short pillars , and that the level can remain on the pivots whatever be the direction of the telescope, nor need it be taken off when the instrument is reversed.’

In 1882, the telescope was loaned to Captain Mackinlay for practice at Woolwich in preparation for the forthcoming Transit of Venus expedition. The 1893 inventory records that on 1 April it was in storage in the south wing of the partially constructed New Physial Building (RGO39/10/91). The 1911 inventory records that on 5 May 1914 the instrument was lent on permanent loan to the Science Museum in South Kensington (RGO39/4/21). The 1933 inventory records the same information (RGO39/6/68). Howse (1975) records that it was still there in 1975. Enquiries are presently ongoing as to its present whereabouts and status.

In the Greenwich inventories, the telescope is referred to as the Russian diagonal transit instrument. However, in the Introductions to the volumes of Greenwich Observations it is referred to as the portable transit-instrument with axis-view by Brauer. It has also been referred to as a Brauer portable extra-meridional transit instrument. Such instruments are also sometimes referred to as bent transits or broken transits.

 

Further details of the design from a memoir originally published in 1863 by the Imperial Central Observatory at Pulkovo

‘The Brauer portable extra-meridional transit instrument consists, as to its chief features, of an objective of 2.5 to 3.0 inches aperture and 30 to 36 inches focal length, bearing a power of 150. The ocular being at one end of the axis of revolution, the light for the field illumination enters through the opposite pivot; the total reflection prism within the central cube admits of proper adjustment. To the stationary reticule of five wires is added a pair of close double wires, moved by a fine micrometer screw. The hanging level need never be removed from its position on the pivots. An unusually expeditious reversing apparatus is always in place, and works with scarcely any possibility of detriment to the stability of azimuth or level. This apparatus is so contrived as not to interfere with nadir observations, made by means of a dish of mercury placed on the pier and below the base of the tripod. The three foot screws rest on corresponding blocks, one of which is itself moveable by a horizontal adjusting screw, allowing the instrument to be accurately placed at any azimuth within a range of six or seven degrees. Microscopes and levels for the examination of the pivots are also provided. The excellence of these instruments depends so much upon the convenient arrangement and conscientious workmanship of all the parts, that actual use and the critical study of the results can alone (as it actually does) suffice to persuade one of their superior merits. While the Brauer transit is unusually convenient for use in any vertical plane, it may, of course, be very advantageously mounted in the meridian; it is in this way that the International Commission, having the Valentia-Orsk arc in charge, have decided to employ it, preferring, it would seem, to sacrifice time rather than risk the introduction of unknown errors by the adoption of new methods in a work of such magnitude and importance. That they, in 1863, may have been justified in this discussion will be admitted, but their own, as well as Döllen’s, subsequent experience has removed all serious objections to the methods proposed and advocated by the latter. On the other hand, the experience of Messrs. Thiele and Zylinski has brought out in stronger light a peculiar personal equation that had been long known to exist in the use of meridian transit instruments and of the vertical circle for time determinations. It is found, namely, that the personal equation varies (in eye and ear observations very decidedly, but far less so in eye and hand observations) with the direction of the motion of the star’s image over the retina. This source of error is not eliminated by the reversion of the ordinary direct-vision transit in the Y’s, but may be so if a reflecting eye-piece be properly employed.’

 

Dimensions of the Greenwich instrument

In his paper, Clarke records the aperture of the Greenwich Instrument as about 2⅝ inches, its focal length as about 31 inches and the distance between the uprights as 19 inches.

 

Flexure and other potential sources of error

Although the positioning of the eyepiece gave instuments of this construction great convienience, it came with a  number of potential drawbacks which were examined by Clarke in his paper. The most serious was a greater degree of flexure of the transverse axis that unless corrected, rendered any measurements less reliable. Clarke described the problem as follows:

‘The weight the transverse axis bas to bear is greater than in the case of the ordinary transit instrument, because the weight of the counterpoise of the half telescope is of necessity. considerably greater than the weight of the absent half of the telescope. If now we assume, and it can hardly be considered an assumption, that in any position of the telescope the central cube and the contained prism are simply depressed a certain constant amount, the effect will be that the image of a star in the field is always a certain distance vertically below its proper place, that is, the place it would have occupied if gravity did not depress the transverse axis. This being admitted, every micrometer reading of an object in the field requires a correction - f cos z where f is a constant which may be called the coefficient of flexure, and z is the zenith distance of the object observed.’

In his 1886 paper On the Flexure of Meridian Instruments. William Harkness listed the Coefficient of flexure of 12 Transit Instruments with Broken Telescopes including the Greenwich Instrument by Brauer. The highest had a coefficient of 0.0555”. and the lowest 0.100” The Greenwich Instrument had the second lowest coefficient with a value of 0.211”.

 

Airy’s reason for acquiring the telescope.

Airy never published his reason for acquiring the Brauer or the Altazimuth comissioned from Simms. Although his correspondence with the Admiralty, Struve and Simms is undoubtedly preserved in the archives at Cambridge, quite where is unknown as there is no mention of the telescopes in the catalogue. One can speculate howeverthat they were obtained in order to test out the designs for their suitability for the forthcoming 1874 Transit of Venus expeditions which he began planning for around this time. Since conventional small transit telescopes and altazimuths were subsequently ordered for the 1874 expeditions Since conventional small transit telescopes and altazimuths were subsequently ordered for the 1874 expeditions, it suggests Airy thought designs incorporating conventional telescopes were superior.

As mentioned above, the Brauer Transit never seems to have been used other than by Clarke and for observing practice by Mackinlay. The Altazimuth on the other hand, was used during the 1874 Transit of Venus expeditions and later lent, in September 1903, to Major Watherston of the Colonial Office for longitude observations on the West Coast of Africa (RGO39/10/90).

 

Other transit telescopes of broken design used at Greenwich

The Bamberg Broken Transit Telescope was used at Abinger for time determination from 1947–1955.