Telescopes: The proposed Horizontal Transit Instrument & Mirror Transit Circle

In the early 1940s, the adoption of quartz clocks in place of pendulum clocks as the Observatory’s time standards lead to a much improved precision in the time service, and with it, a requirement for an improvement in the precision of the observations made with the transit telescopes, particularly those associated with time determination. In 1943/4, the Admiralty agreed to fund the construction of a Photographic Zenith Tube based on one used for time determinations at the US Naval observatory in Washington.

At about the same time, a comprehensive investigation was commenced of the theoretical aspects behind the instrumental errors associated with reversible transit instruments. From this, it was concluded that developing a new type of instrument free of some of the inherent errors of existing designs would be a more promising way forward than trying to find better ways of compensating for them. Work on developing a Horizontal Transit Instrument free from the problems of flexure commenced in 1944/5. This included the construction of an ‘experimental’ instrument.

By 1946 however, thoughts had also turned to the possibility of building a Mirror Transit Circle. Through the remainder of the 1940s and into the 1950s, parallel development work took place on all three instruments, each of which had a different but specific purpose:

  • Photographic Zenith Tube – for time determination by measuring the transits of stars near the zenith
  • Horizontal Transit Instrument – for measuring right ascension only
  • Mirror Transit Circle – for the measurement of both declination and right ascension.

Although fully developed designs were eventually produced for the Photographic Zenith Tube (PZT) and Mirror Transit Circle, only the former made it into production. Developed by Dudley Perfect (a member of staff who originally arrived at the Observatory on secondment in 1942), it came into use in 1955.


The Horizontal Transit Instrument

As well as working up the design of the PZT, Perfect was also responsible for working up the design of the Horizontal Transit Instrument. This seems to have commenced in 1944/5 and was reported by Spencer Jones to the Board of Visitors in his annual report. 

As envisaged, the instrument consisted of two fixed (though adjustable) horizontal telescopes aligned on the same east-west axis, both pointing towards a mirror arrangement at their centre. This had two reflecting surfaces set at an angle of 90º to each another and could be set to the appropriate declination of the heavenly body to be observed. In the moments before it crossed the meridian, its light was reflected by the east side of the mirror into the eastern telescope. In the moments after, it was reflected by the west side of the mirror into the western one. Observations were to be made from each telescope in turn.


The Mirror Transit Circle

In 1946, the development of a design for a Mirror Transit Circle began. Although a similar instrument had been proposed by HH Tuner (a former Chief Assistant) as early as 1897 (click here to read more), this proposal had long since disappeared into oblivion. The idea supposedly arose anew to a later Chief Assistant, Robert Atkinson, while he was away from the Observatory on war time secondment. Returning to Greenwich on 6 June 1946, he set out his ideas in a paper which was published in the Monthly Notices of the Royal Astronomical Society the following year (see below).

Like the Horizontal Transit instrument, the Mirror Transit Circle also had two fixed (though adjustable) horizontal telescopes aligned on the same axis. In this case though, instead of being aligned east-west, they were aligned north-south in the plane of the meridian. Between the two telescopes there was a circular plane mirror that could be pivoted about an east-west axis and adjusted to the declination of the object being observed. Objects in the northern half of the sky were observed though the northern of the two telescopes as they crossed the meridian. Those in the southern half were observed with the southern one. As well as being used to make the observations, the telescopes had a secondary function as collimators.


Progressing the designs and the fate of the Horizontal Transit Instrument

The development of the Horizontal Transit Instrument and the Mirror Transit Circle continued in fits and starts over much of the next decade, with more time being devoted to the Mirror Transit Circle as Perfect focussed his energies on the PZT. The Board of Visitors was updated on progress by the Astronomer Royal in his annual report.

By April 1946, the construction of the ‘essential’ optical components of the Horizontal Transit Instrument was at an advanced stage. Soon after, work ground to a halt owing to pressure of work elsewhere and a loss of staff. It resumed in 1949/50 and by April 1953, the telescopic system of the instrument had been erected on a concrete pier in the Optical Laboratory and was undergoing testing. At around this time, Perfect became very ill and went on indefinite sick leave with no likelihood that he would return. Development work on the Horizontal Transit instrument then appears to have stopped, never to be restarted. The current whereabouts of the optical parts is unknown.


The fate of Atkinson’s Mirror Transit Circle

In 1958/9 a plane quartz mirror and a steel axis to mount it in was supplied to Atkinson’s specification. Atkinson carried out extensive tests which proved successful. By 1963, a design had been worked out in detail with the manufacturers Messrs. Grubb Parsons and by 1964, a 1:10 scale model of the Mirror Transit Circle pavilion, pier and instruments had been completed and their inter-relationship studied in detail with satisfactory results. Atkinson retired later that year. Despite the success of the trials, the telescope was not put into production. The reasons for this are unclear. It may have been an early casualty following the transfer of the funding of the Observatory from the Admiralty to the Science Research Council. Alternatively if may have foundered because it was at this point that Atkinson retired ... or perhaps, as Atkinson himself came to believe, Woolley had never intended to take the design through to production, but was merely indulging him.

Although the Royal Observatory did not get its instrument, others based on the same principle were constructed elsewhere at Ottawa, Oporto and Pulkowa. By 1973 however the first two had been abandoned for various reasons.

Atkinson’s mirror is conserved in the collections of the National Maritime Museum (Object ID: ZBA1697)


Contemporary accounts

Information about the concepts behind the two instruments was given together with updates on progress was given in the annual Reports of the Astronomer Royal to the Board of Visitors. The 1945 report was the first to include anything and the 1964 report the last. Although information about the Mirror Transit Circle was also published elsewhere, this was not the case for the Horizontal Transit Instrument. For convenience, these have been transcribed and can be read by clicking here.

A proposed “mirror transit-circle”. Atkinson, Robert D'Escourt. Monthly Notices of the Royal Astronomical Society, Vol. 107, pp.291–307 (1947)

Minutes of a meeting of the Royal Astronomical Society held on 8 April 1960. The Observatory, Vol. 80, p. 88–90 (1960)

Design and Tests of a Mirror Transit Circle Axis. Atkinson, Robert D'Escourt. Royal Observatory Bulletins, Number 34 (1961)

Oral History Transcript – Dr. Robert Atkinson (1977)


Further reading

The Royal Greenwich Observatory. Spencer Jones, Harold. Proc. R. Soc. Lond. A. Vol. 198 No. 1053 pp.141–169 (August 15, 1949). A link to this excellent paper can be found on the British Geological Survey website.

Modern Developments of the Meridian Circle. Høg, E. IAU Symposium, No. 61, pp.243–255 (1974)

Transit Circles Today. Tucker, Roy. H. Quarterly Journal of the Royal Astronomical Society, Vol. 10,  pp.223–232 (1969)

Quartz clocks of the Greenwich Time Service. Smith, H. M. Monthly Notices of the Royal Astronomical Society, Vol. 113, pp.67–80 (1953)