A history of the university of London observatory 1928–1962

The original observatory building (the 'Wilson Building') contained, in addition to the dome, a spectrographic laboratory and darkroom, computing room and a workshop. A second building was erected in 1931 to house the Fry refractor, and a second-hand dome obtained, formerly having been used at a house in West Wickham, Kent.
The third building, erected to house the Radcliffe refractor in 1938, also provided a room for an astronomical library. In 1951 the library building was demolished, and replaced in 1953 by a two-storey structure. The ground floor contained an office for a secretary, a measuring room and a room to house the library. The first floor contained a lecture room, an office for the Director and an office for the scientific staff.

A further building was erected in 1954 to house electric generators obtained from the Admiralty. The increasing number of students soon rendered this accommodation inadequate, and yet another building was completed in 1961, comprising a second spectrographic laboratory and darkroom, an office for staff, an office for research students, and a room for the use of undergraduate students and temporary visitors.

W.E. Wilson established an observatory at Daramona, Street, County Westmeath, in 1871 and equipped it with a 12-inch equatorial reflector by Grubb. Wilson (born in 1851, elected FRS in 1896, an original member of the BAA, awarded an honorary DSc by the University of Dublin in 1901, High Sheriff of Co. Westmeath in 1894), observed the transit of Venus in 1882 and solar eclipses at Oran in 1870 and Spain in 1900, published many papers in Proc. R. Soc, Proc. R. Dublin Soc., Proc. R. Irish Acad., etc., and died in 1908) enlarged his observatory in 1881 and installed a 24-inch reflector by Grubb on the mounting previously used for the 12-inch reflector. Ten years later a new mounting was constructed. It is this mounting which was moved to Mill Hill in 1928. Dr. Wilson used his telescope to make some of the best photographs of his time of star clusters and nebulae, and he worked extensively on problems of solar physics and the solar constant. The telescope may be used in Newtonian and Cassegrain forms; the focal length of the mirror is 10 feet and the equivalent focal length at the Cassegrain focus is approximately 42 feet. The telescope was moved in 1928 from Ireland to University College, where minor modifications were made to the focussing arrangements and plate-holder, and an electric motor was added to rewind the driving clock automatically. A 6-inch objective by Cooke, originally purchased by University College for the 1927 eclipse expedition, was mounted as a guide telescope; its focal length is 12 feet. The telescope was erected in 1929. Special precautions were taken to minimise the effect of traffic vibrations from the Watford By-pass Road, by excavating the soil around the base of the pier to a suitable depth, revetting the sides of the excavation and filling in with coke breeze to a depth of 4 feet below the surface of the ground. The 18-foot dome was constructed by Messrs. Cooke, Troughton and Simms, Ltd., and was equipped for rotation by an electric motor.

The coordinates of the centre of the axes of the Wilson reflector were determined to be

• Longitude 0° 14' 26.050" W (0h 0m 57s.77)
• Latitude 51° 36' 46.28" N.
• Altitude 266.5 feet.

[A Bench Mark on N.E. Angle is at height 255.50 feet, Newlyn datum].
The mirror was originally silvered, but was aluminized from 1935 onwards. In 1935 a 4-prism spectrograph by Tremblot was installed at the Cassegrain focus.
Since its erection in Mill Hill the telescope has been used for proper motion measurements, for stellar spectroscopy, for occasional observations of comets, supernovae, Pluto, and other objects, and for experimental work-on photoelectric photometry and the recording of artificial satellite transits. It has also been used by students.

On July 24, 1934 Sir Frank Dyson wrote to the Secretary of the Committee of Management of the Observatory stating that the Radcliffe Trustees were willing to present to the University of London the Radcliffe Twin Refractor, then in Oxford, and no longer required by the Trust upon the latter's impending removal of the Radcliffe Observatory to South Africa. The offer was accepted by the Senate of the University in 1935, and the telescope was dismantled and removed to London in March and April 1935, and stored. The dome and rising floor were also dismantled. The collection of plates taken with the Radcliffe telescope were brought to Mill Hill with sundry other equipment. The mounting in the new building during 1937–38 was made difficult by the massive nature of the telescope, the declination axis and cross-head weighing about 3.5 tons.
The telescope was made by Grubb, Dublin in 1901. It consists of a 24-inch photographic objective of focal length 22 feet 9 inches and on the same mounting an 18-inch visual objective of the same focal length. With the initial fittings, the telescope was available for astrometric photography and visual observing. Later attachments have enabled two colour photography and photo-electric work to be carried out. The telescope was. dismantled upon the outbreak of War in 1939, the objectives being stored for safety. It was re-commissioned in 1946.

Since its erection in Mill Hill the telescope has been used for parallax work,.proper motion studies, cluster photometry, photo-electric photometry and miscellaneous observations of supernovae, comets, asteroids and other objects. It has also been used by advanced students.

This 8-inch Cooke refractor (made in 1862) was presented by Mr. H.R. Fry of Barnet in January 1930, and he met most of the cost of its reconditioning and re-erection. It has a focal length of 126 inches. Observational work with the Fry telescope has included magnitudes of novae, double-star observations and occultations. The telescope has been extensively used for demonstrations to visitors and for student instruction.
[A Bench Mark on Fry wall is at a height of 247.50 feet, Newlyn datum].

(a) In 1932 the Trustees of the late Mr. Joynson of Liverpool presented to University College a 6-inch Cooke equatorial refractor (made in 1863) which was mounted in the Chadwick (north) dome in the Bloomsbury courtyard in place of a 4-inch refractor which had been there previously. In 1946 this 6-inch refractor, which survived the War, was moved to the Observatory, erected under the Radcliffe floor, and used for student instruction on cloudy nights.
(b) A 10-inch Calver reflector, which was housed in the Brocklebank Observatory on the roof of University College, was used for teaching up to 1939, but was lost by enemy action together with other smaller instruments.

(c) A 3.5-inch transit circle by Cooke was housed in the south dome in the courtyard at University College and used for teaching up to 1939. It survived the War, was moved to the Observatory in 1946, and used under the Radcliffe floor for student instruction on cloudy nights.

(d) A portable transit circle was also moved from University College to the Observatory in 1946 and is now used on the stand close to the Fry telescope.

(e) A Reversible Transit was purchased from Dr. R.L. Waterfield in 1949 and erected behind the Radcliffe building.

(f) A Grubb Chronograph was moved from University College to the Observatory in 1946; it was modified in 1948 to make two simultaneous recordings.

(g) A 10-foot Rowland Grating was transferred from University College to the Observatory in 1929.

(h) A spring-driven Coelostat, 7.5-inch aperture, was installed on the Observatory roof in 1929, under a cover designed by L.N.G. Filon and C.C.L. Gregory. A new mounting was designed by Gregory so that a single setting on a divided circle would set for any star, apart from setting the hour circle. A 7-inch objective of 12-foot focal length was used.

(i) The spectrograph being constructed in 1962 for the new spectroscopic laboratory will use a plane grating in a stigmatic Ebert type mounting with an 18-inch mirror of 129-inch focal length. It will be mounted vertically in the laboratory pit provided for the purpose, and will use both photographic and fast photoelectric recording. The style of this instrument will be similar to an earlier version set up by E.W. Foster in the Imperial College in 1957.

(j) A spectrograph was constructed at the Observatory in 1958, containing Hilger glass and quartz optical trains.

(k) A Hilger Recording Microphotometer was purchased in 1954.

(1) Calculating machines were purchased in 1937 (Brunsviga), 1951 (Marchant) and 1960 (Marchant).

(m) Generators were obtained on indefinite loan from the Admiralty in 1954.

(n) Two Hilger plate measuring machines are in use, one was purchased in 1951 and one on loan from the Royal Society.

(o) A Grubb plate measuring machine was presented to the Observatory by the Radcliffe Trustees.

(p) Other instruments and equipment are available for laboratory, expedition, workshop, office, and teaching purposes.

Mr. W.E. Wilson obtained with his telescope a number of photographs of various regions of the sky during 1894-1899. C.C.L. Gregory repeated some of these photographs between 1929 and 1934. These pairs of plates were measured and reduced by Violet F. White, and discussed in five published papers. Proper motions were obtained for 1645 stars to magnitude 15 in a total of 7 regions of the sky. The regions studied were near M27, 52 Cygni, M31, M37, M13, the Orion Nebula and the Ring Nebula in Lyra. Satisfactory accuracy was obtained in spite of the moderate size of the instrument, on account of the long time interval between the two sets of plates.

Although the Radcliffe Telescope and the plates of the Radcliffe proper-motion programme are both in Mill Hill the question of continuing this proper motion work was not considered until a request came from Groningen for a re-measurement of the standard areas 3, 4, 5 and 20. Repeat plates were then taken during 1958–60 and the areas 3, 4, and 5 were re-measured (mainly by vacation students).

From 1930 to 1937 a programme of observations of lunar occultations was followed, by C.C.L. Gregory and F. Robbins, using the Wilson and Fry telescopes. In all 166 successful observations were obtained, the results of which were published.

A parallax programme was started with the Radcliffe telescope in 1939, suspended at the outbreak of War, resumed in 1946 and continued until 1955. A total of 57 stellar parallaxes were determined and published. Many members of the staff took part in this work. In 1955 T. Kiang investigated the use of the Hilger microphotometer for measuring parallax plates.

In 1952–54 a number of photographs were obtained in red and blue light of star clusters, and colour-magnitude arrays were obtained. The observations were directed towards those objects that are not clearly of open or globular type. S.K. Wang showed that NGC 6811 has a typical open cluster array. The 18-inch visual was equipped for photographing in red region simultaneously with use of 24-inch for blue plates. V.C. Reddish studied M5, NGC 7789, Tombaugh 5, NGC 1528 and M67 (NGC 2682). The colour-magnitude array of M67 was found to be intermediate between those of globular and open clusters.

V.C. Reddish also investigated some problems in stellar evolution and determined the masses of the dwarfs and giants of Population II and of variable stars.

In 1930 Miss E. Williamson and C.C.L. Gregory constructed an electric furnace and produced a number of molecular spectra, including the second positive group of nitrogen and the calcium and strontium fluorides bands. A large number of wavelengths were measured and the spectra were compared with the solar spectrum but no identifications were made.

1952 saw the commencement of a drive towards the measurement of atomic oscillator strengths needed for astrophysical purposes. E.W. Foster constructed a fluid vortex arc with controlled conditions and temperatures ranging from 11,000K to 13000K. The spectra were recorded photo-electrically at high speed. Using the fluids H2O, C6H6, CHC13, CS2 and SO2Cl2 measurements of absolute oscillator strengths have been made in O I, C I, Cl I, and Cl II. Recordings for S I are available. Mr. A.S. Asaad used diluted copper alloys made by Johnson, Matthey and Co., to obtain arc spectra which could make a comparison of the oscillator strengths in the neutral atoms Al, Si, Cr, Mn, Fe, Co, Ni, Cu, Ga, Ag, Sn, Pb and Bi. Later C.W. Allen extended the same procedure to Mg and Ca, and more recently similar progress is being made with Sc, Ti, and V. In an endeavour to use the fluid vortex technique for iron group atoms J.B. Tatum produced an arc in TiCl4 and from the spectrum was able to make extensive measurements in both Ti I and Ti II. The most recent adaptation of this procedure has been the production of a rotatory walled arc in Mn02 by B.E. Woodgate. Further measurements are planned.

In 1955 it was noticed by C.W. Allen and A.S. Asaad that all extensive measurements of oscillator strengths showed an unexplained rapid increase with excitation potential. The effect has been studied in all subsequent measurements, including our own and a fairly complete, although empirical, explanation has been found. With this point settled it is possible to use the f-sum rule to determine absolute oscillator strengths in special cases. For atoms of the iron group an extensive comparison of measured and calculated values has been made.

In 1954 R.H. Garstang started an extensive series of calculations on the transition probabilities of forbidden lines in complex atoms. General methods were developed for electric quadrupole calculations and were applied to the spectra [Fe II], [Fe IV], [Fe V], [Ni II] and [Ni III], and in 1962 further unpublished results were available for [Fe II], [Ni I], [Cu II] and [Mn VI]. The relative intensities of the strongest lines in various multiplets were in good agreement with line intensities in peculiar stellar spectra. Among the by-products of this work was a prediction of the positions of the energy levels of Fe IV. This programme was completed in 1962.

In an investigation in 1955 R.K. Garstang showed that if observed energy levels are used instead of theoretical energies certain forbidden line strengths may be calculated. In 1961 he investigated transition probabilities of permitted and forbidden lines in Fe XIV and made calculations also for Fe X and Si X. Other investigations by Garstang on theoretical spectrum line intensities included a survey (1955) of the transition probabilities of auroral lines, calculations (1959–62) of the oscillator strengths in O I, Si I, Ge I, Sn I and Pb I, work jointly with Dr. A.S. Douglas of Cambridge on Ca II and Si IV, oscillator strengths in O I (1960), and work (1961) on hyperfine structure and intercombination line intensities in the spectra of Mg I, Zn I, Cd I and Hg I. This work has shown the feasibility of making successful calculations of the intensities of many different types of spectrum lines, including many forbidden by the usual selection rules.

The main variations on the sun affecting the earth are:

• the change in the emission of far ultraviolet solar radiation which has a dominating influence on the earth's ionosphere, and
• the change in solar particle emission which influences the geomagnetic field and several allied phenomena.

Both of these relations have been studied statistically.
The ultra-violet relation is complicated by the anomalous nature of the ionospheric F2 region which has therefore been studied on a world-wide basis. The anomalies have been segregated and evaluated in a systematic although empirical manner by C.W. Allen. He also determined the numerical constants describing the precise relation of solar activity data to decimetre radio wave emission.

A renewed attempt to locate and identify solar M-regions has been made by Th. Saemundsson who has correlated all available data on sporadic and recurrent geomagnetic storms. The inter-relations between existing data are now as clear as they can be. More information will be needed before firmer deductions can be made. A real avoidance of active areas by M regions is confirmed.

As might be expected the theoretical researches have been rather diverse since they are related to the changing interests of various staff members. P.A. Sweet made a detailed study of the interaction of magnetic fields with matter in motion and was able to apply his conclusions to the general atmosphere of the sun, to sunspots, and to interstellar gas clouds. He worked out the neutral point theory which was thought applicable in the production of solar flares and was therefore most effective in directing solar studies to magnetic field details in the neighbourhood of sunspots. His 1952 calculations of the structure of rotating stars have been reconsidered and extended by J.S. Griffith who followed Sweet to Glasgow to further his investigations.

The topological rules established by Sweet were used as a guide in enabling C.W. Allen to propose a model to explain the complex features of the solar cycle.

An investigation on the spectrum of alpha Andromedae was performed in 1937-38 by C.M. Chao on 79 plates made available by Prof. F.J.M. Stratton of Cambridge. Line identifications were made and spectroscopic binary orbits obtained. From 1935 onwards spectra were frequently obtained with the Wilson telescope and Tremblot spectrograph of a number of stars, particularly of Gamma Cassiopeiae. A detailed study was made (by Miss E.M. Peachey) of changes in the spectrum of Gamma Cassiopeiae which took place in 1940. The spectrum of Nova T Coronae Borealis 1946 was also observed.
During a visit to Haute Provence Observatory in 1949 E.M. and G.R. Burbidge obtained many spectrograms of Be stars. The intensities of hydrogen and helium lines were measured, and a further study made of rapid changes of line intensities in the spectrum of Gamma Cassiopeiae. The shell spectrum of HD 217050 was also examined.

In 1951–52 K.R.W. Brewer made a study of the peculiar A star HD50169 from 8 spectra obtained at Mount Wilson. Wavelengths were measured, intensities estimated and line identifications made. In 1953 R.H. Garstang measured many equivalent widths in the spectrum of 9 Pegasi, and later he and B. Warner extended wavelength and intensity measurements to a number of other stars and determined atomic abundances in these stars.

In 1960 C.W. Allen compared the intensities of far ultra-violet lines recently derived from rocket spectra with calculations from coronal and chromospheric models. He found a much thicker transition zone than his earlier calculations had indicated. In 1961 R.H. Garstang calculated oscillator strengths for two solar ultra-violet lines of C I, and predicted the equivalent widths of these lines in the solar spectrum.

In 1960 B. Warner found evidence of neutral and ionized oxygen on the night side of Venus.

From 1960 D. McNally worked on problems concerning the condensation of interstellar clouds. In particular he considered the production of the interstellar molecules CH and CH+ and was able to show that if CH can be produced by surface reactions then the observed densities of these molecules will only be obtained provided the interstellar radiation field is sufficiently dilute in the far ultra-violet.

The aim of lunar studies introduced in 1960 by G. Fielder was to coordinate our understanding of the varied surface features of the moon. In particular the system of global rock joints known as the grid system was found to be intimately associated with the form and statistics of many other surface structures. Such associations point to interpretation of the lunar surface in terms of widespread mechanical stresses. G. Fielder, B. Warner, and Carole Jordan have contributed varied measurements and analyses to such problems.

The absence of any systematic method of expressing the space density of various objects in the sky led Allen in 1954 to analyse the question of whole-sky statistics. The methods proposed have not yet been fully applied. The starting point of further statistical work was a two-fold recognition. 1. Frequency distributions of astronomical objects are greatly distorted by observational selection. 2. The practice of assuming normal curves for these distributions is a mere habit formed at a time when the science of statistics was synonymous with biometric studies. It is in this spirit of caution and doubt that T. Kiang examined the luminosity function of galaxies, and resolved the discrepancy between Hubble's and Zwicky's points of view. The scrutiny is being continued on asteroids. The total number of accessible asteroids has been estimated anew.