Arthur Robert Hogg 1903-1966

Written by S.C.B. Gascoigne.

Arthur Robert Hogg was born in Melbourne, Victoria, on 25 November, 1903. He became a student at the Royal Melbourne Technical College, then at the University of Melbourne, where he graduated BSc in 1923, with first-class honours in chemistry and the Dixson Scholarship, and as MSc in 1925, with the Kernot Scholarship. He went first to the Broken Hill Associated Smelters at Port Pirie, South Australia, quickly to become Assistant Superintendent of Research, a post he held until 1929. In that year he joined the Commonwealth Solar Observatory, as it then was, taking up his new position on 1 August, 1929. Dr W.G. Duffield, founder and first director of the Observatory, died on the same day; the signing of Hogg's letter of appointment had been his last official act. Hogg remained a member of the Observatory staff for the rest of his life, 37 years. By a curious coincidence the day of his death, 31 March, 1966, was also the last day of the term of office of the third director, Professor Bart J. Bok.

Hogg's scientific interests covered an unusually wide range. He began professional life as an industrial research chemist, but, on joining the Observatory, took up the study of a number of atmospheric electrical phenomena, in particular of the conductivity, ionic mobilities and ion balance in the lower atmosphere. From atmospheric ionisation to cosmic rays was a short step, and by 1935 he had set up a high-pressure ionisation chamber with which hourly measurements of cosmic ray intensity were made for the next five years. This experiment, and its subsequent analysis probably constituted his most important work. They led to the award of a DSc by the University of Melbourne in 1950, and played a large part in his election to the Academy in 1954. Hogg spent the war years at Maribyrnong, with the Munitions Supply Laboratories and the Chemical Defence Board. On his return in 1946 he found a very different Observatory. There had been a major shift in emphasis, from the solar and geophysical work of pre-war years to stellar astronomy, and Hogg became an astronomer. The field he chose was photoelectric photometry, one well suited to his background and experimental skill. There followed a long series of papers on eclipsing variables, standard magnitudes and galactic clusters; but while this work, executed with characteristic care and thoroughness, earned him a considerable, and new overseas reputation, it was probably as an administrator that he made his principal mark on the Observatory. Hogg and Woolley, who was director at that time, worked very closely together and most of the detailed administration came Hogg's way; he had a flair for this type of work, and his thoroughness and attention to detail were legendary. Besides this he played a leading part in the setting up and testing of the new 74-inch telescope, and, most congenially, in the provision of associated laboratory facilities, such as the aluminising plant.

Early in 1957 Bok succeeded Woolley as director at the same time as the Observatory was transferred from the Department of the Interior to the Australian National University. These changes led to a substantial rearrangement of Hogg's duties. For a while he was able to devote more attention to astronomical work, but pressure to conduct a survey for alternative astronomical sites in other parts of the country had begun to mount. Conditions at Mt. Stromlo were clearly threatened by the growth of Canberra, and in any case it seemed probable that there were better sites further inland. Also an increasing number of enquiries were coming from overseas observatories interested in establishing observing stations on the Australian continent. The survey proper was initiated by Bok. Hogg, however, was prominent in it from the beginning, and before long had taken over the major responsibility. The results of this survey, which came to occupy the greater part of his later years, have already proved most important for the development of astronomy in Australia. Thus the choice of Siding Spring Mountain was essentially Hogg's. Intended originally as the Mount Stromlo out-station, it is now to be the site also for the Anglo-Australian 150-inch telescope. It is a matter for regret that Hogg did not live to see so satisfying a culmination to his efforts. As it is, astronomers the world over will be permanently in his debt for the care and thoroughness with which he carried out this arduous task.

A gregarious man, Hogg was well known in both Australian and international scientific circles. He was a frequent attender at ANZAAS; had been convenor of the National Committee on Astronomy since 1947; was President of the Royal Society of Canberra in 1954, and was a Fellow of the Australian Institute of Physics, serving as Chairman of the Australian Capital Territory branch in 1964. He had been elected a Fellow of the Institute of Physics in 1938. He served on a number of Commissions of the International Astronomical Union (IAU) notably that on Astronomical Sites, and from 1961 to 1964 was President of Commission 6 on Astronomical Telegrams.

When Hogg joined the Observatory in 1929 the main lines of work were the measurement of the solar constant and other solar phenomena, the luminosity of the night sky, atmospheric ozone, and the atmospheric potential gradient. Interest in atmospheric electricity has waned in recent years, or at least moved into the adjacent field of cloud physics, but in the 1930's it was an active subject, important enough to attract men of the calibre of Hess, Schonland and Simpson. Hogg arrived with at least one problem ready made, and within two months had begun regular observations of atmospheric conductivity. From the present point of view the atmosphere is a leaky dielectric separating the positively charged stratosphere from the negatively charged earth. It leaks, or conducts, because it is ionized (to a very small degree) by radioactive elements in the ground and in the air itself, and, to a less extent, by cosmic rays. Hogg's researches were concerned with the nature of the ions themselves, and with the processes which govern the ion balance in the lower troposphere. He counted ions of various types, measured their mobilities, estimated their sizes, studied their rates of formation and recombination, and, the subject being very statistical in nature, spent much time analysing his data for the effects of meteorological factors, and of annual and diurnal terms.

Much of his equipment he designed and built himself. Hogg was a skilful and ingenious experimenter, a facility which stood both him and the Observatory in good stead throughout his life. He had a meticulous eye for detail, and took the greatest pains to eliminate systematic effects from his measurements. His early work, on the diurnal variation of conductivity, and on the average lives, rates of production and mobility of small ions, attracted the attention of Whittle, who invited him to work at Kew Observatory. Hogg went there in 1937-38, taking with him a beautiful piece of equipment he had built at Canberra, with which he made some of the first measurements of the mobilities of the intermediate atmospheric ions, relatively frequent in the industrial atmosphere of London. In this important piece of work he was able to show conclusively that these ions existed in discrete groups, composed of one or more droplets of sulphuric acid, each containing about 2200 molecules. While at Kew he also carried out some experiments which led to a reconciliation between two apparently conflicting methods of measuring the air-earth current; the difficulty was traced to the local effect of radioactive matter in the soil. His final paper on the subject also dates from this time, although it was not published until 1950. It was a statistical examination of the air-earth current at a number of stations widely distributed over the earth. Hogg was able to derive an annual term for the variation, and to show that it lent substantial support to the thunderstorm theory, advanced by Whittle, for the maintenance of the earth's electric charge.

Hogg was clearly very much attracted by atmospheric electricity. When Woolley assumed the directorship late in 1939 one of his early actions was to present a paper to the Advisory Board in which he stated his views on the future research programmes of the Observatory. These were heavily weighted towards stellar astronomy, but he included also a programme of Hogg's for further work in atmospheric electricity, which would have taken several years to carry out, remarking that 'his (Hogg's) dissociation from the astronomical work of the Observatory would have proved complete'. it was atmospheric electricity, and not cosmic rays, which Hogg preferred at that time.

Hogg's habit of producing his own reliable and accurate version of fairly standard equipment, and of using it for long methodical series of observations rather than for shorter experiments aimed at solving particular problems, is nowhere better illustrated than in his work on cosmic rays. This had its origins partly in his investigations into atmospheric ionisation, and partly in a recommendation made by the 1930 meeting of ANZAAS. Hogg began preliminary work in 1932, with both Geiger counters and ionization chambers. Late in 1963 the Geiger counters were abandoned, as being too unstable for the type of long-term investigation he had in mind. The ionization chamber was redesigned, built in the Observatory workshops, and went into regular operation in September 1935, continuing until stopped by the war in August 1940. Altogether four chambers were used, two for an absolute calibration, and two for routine observations. The main chamber was a thin-walled steel vessel filled with C0₂ at 10 atmospheres, and shielded by 10 cm of lead, so that it measured predominantly the hard or meson component. A continuous photographic record of the current was made, from which readings were taken every hour. The whole was enclosed in a thermostatted hut.

Concurrent records were kept of atmospheric pressure B and temperature T. These were necessary because fluctuations in the cosmic ray intensity were known to be associated with variations in T and B. The mechanism, elucidated and confirmed by Hogg, depends on the fact that the hard component is composed of unstable mesons, with life-times of a few micro-seconds, formed from the primary cosmic radiation high in the atmosphere. If the level of formation is raised, by an increase in B or T, the mesons will have further to travel and a greater chance to decay before reaching ground level, so that the cosmic ray intensity will fall. Conversely, from a knowledge of the relation between the intensity and the variations in B and T one may deduce the mean rest life and absorption coefficient of the mesons. Hogg's careful statistical analysis of this relation is probably the best of its kind which has been made, and led him to lifetimes and absorptions in good agreement with those determined by other methods.

He now removed the P and T terms from his intensities and tested them for the presence of solar and sidereal periodicities. He found, as other observers had, a small diurnal term with a maximum shortly after local noon; this term exhibits a seasonal change in amplitude, the cause of which remains uncertain, but which may be due to a seasonal variation in the height of the meson-producing layers. The reality of the sidereal term was important, because of its far-reaching implications as to the mode of origin of cosmic rays. Hogg found a real term, with an amplitude of 0.08 per cent; but his term differed in phase by about twelve hours from a similar term observed in Europe, so that its origin must have been terrestrial rather than galactic. This discovery closed a long-standing and vexed problem, one on which an enormous amount of work had been expended. It is interesting to note that a sidereal term has recently been found by Jacklyn in Hobart, with a narrow-beam meson telescope, but at a level far below the ability of Hogg's equipment to detect. Hogg looked also for correlations with solar phenomena such as sunspots and flares, and for a 27-day periodicity, but found nothing which could not be attributed to variations in the earth's magnetic field (which is itself subject to influence by solar events). He studied bursts, isolated two types, and estimated cross-sections for the particles producing them, tentatively identified as electrons and mesons. We may leave the final word with the examiners of his DSc thesis – 'This work represents an extremely thorough, careful and extensive study of the variation of cosmic ray intensity with time...the experimental technique is obviously sound and the results are analysed statistically with great care'.

In November 1940 Hogg went to Maribyrnong, to the Chemical Defence Section of the Munitions Supply Laboratories (now the Defence Research Laboratories). I am indebted to Mr W.G. Jowett for the following remarks:

'He was engaged on research and developmental work on physical aspects of protection against chemical agents. Perhaps the most notable of a number of contributions he made during his service here was the development of an ionization penetrometer to measure the penetration of charged particles through respirator filters. He also worked on the development of wool-resin filter materials and took an active part in the early work which led to the development of the DSL Dust Respirator which was used extensively by the Services. He was transferred in mid-1944 to the newly formed Secretariat of the Chemical Defence Board and acted for about 18 months as secretary of the Physical Sub-committee.'

A man of his background and temperament could hardly have helped being valuable at Maribyrnong; Hogg clearly enjoyed his time there, and for a while I think toyed with the idea of remaining permanently.

By the time he returned in 1946 the Observatory had tripled in size, had acquired five years war-time experience in the manufacture of optical instruments, and was firmly set on its new course of stellar astronomy. Hogg's decision to cast in his lot with the astronomers must have been a difficult one. At the age of 43, he had to leave the two fields in which he had acquired real authority, for a new subject with new methods, new concepts, and almost a new philosophy. In the event he never moved as easily in astronomy as he had in atmospheric electricity or cosmic rays. He could not have been helped by the heavy administrative load he carried under Woolley – not, it must be added, that he found this uncongenial; he enjoyed responsibility, and a formal and orderly man himself, was well at home within the formal and orderly framework of the Public Service. All in all it is surprising that he published as much and achieved as high a reputation as he did; but he was too good a physicist not to seek out significant problems and to make important contributions to them.

Aided, no doubt, by some previous experience in photoelectric photometry, in the shape of a series of observations made with a cadmium cell of solar ultra-violet radiation, he very soon had a stellar photometer built and operating. He turned first to the problem of eclipsing binaries, important because they are one of the very few types of star for which masses and especially radii can be determined; an essential requirement is that they be observed intensively and very accurately. He began with V Puppis, a well-known but poorly observed system. Hogg obtained an accurate light-curve, and had the data reduced and published within a year, a quick piece of work for a man entering a new and by no means simple field. He became well known for his work on eclipsing stars, publishing seven papers on them in all. The most important concerned the system zeta Phoenicis, the binary nature of which he discovered himself. This has turned out to be an important system, one of the thirteen for which 'first-order' masses and radii can be determined, and the only one in the southern sky. Hogg's work on it is definitive.

Astronomers have traditionally measured stellar magnitudes in broad wave-bands, about 1000A wide. These have two substantial drawbacks: they do not admit of a simple physical interpretation, and in particular do not allow an unambiguous determination of temperature; and difficulties arise in intercomposing the results of different observers. Woolley was very conscious of the advantages of narrow-band magnitudes, and largely at his instigation Hogg undertook a programme of narrow-band photometry. If the idea was Woolley's, the organization and execution were Hogg's. The 50A band width was isolated by a slit in the focal plane of a spectrograph. The resulting magnitudes, for 63 stars, remain among the most accurate which have been determined. Hogg was a pioneer in this field, and had he remained in it might have anticipated many subsequent developments in observational astrophysics. Only in very recent years have narrow-band methods begun to come into general use.

Instead he went on to another important problem, the integrated magnitudes and colours of the Magellanic Clouds. The Clouds are the nearest of the external galaxies. They are faint extended objects, and have to be measured against a background which is relatively bright (it contributes about 80% of the total signal) and which varies both with time and with position in the sky. The elimination of the background is a tricky problem, calling for great care in the planning and reduction of the observations. Hogg's was the first accurate surface photometry which had been carried out on the Clouds. The programme has been repeated, with variations, on three other occasions, in all cases confirming Hogg. Hogg found also that the Small Cloud was appreciably bluer in its bright nuclear regions than in its outskirts, an important and so far unexplained observation.

Hogg's fourth and probably his main astronomical interest lay in galactic clusters. This was a very active subject in the 1950s. Stars in galactic clusters are presumed to have been born at the same time, from material of the same chemical composition. Using this, measures of magnitudes and colours of individual cluster stars can be made to give an age and distance of the cluster, an estimate of the amount of interstellar absorbing material in the line of sight, and an approximate figure for the chemical composition. Moreover, star clusters were not only the primary testing-ground of the rapidly developing theory of stellar evolution, but provided also essential clues to our understanding of the evolution of the galaxy. It is not surprising then that many astronomers work on clusters. Having published ten papers in the field, Hogg was well-known among them. He measured colour-magnitude diagrams for five galactic clusters, wrote a very competent review article for the 1964 Report of the IAU, and finally published a photographic atlas of galactic clusters south of -45°. This was a collection of 98 charts taken on the. 74-inch reflector. It was intended for reference, and as a basis for further work, and has already proved its utility in that one of the clusters, NGC 3680, which might otherwise have remained unnoticed for years, has proved to be unusually old. Not only this, but a search of the plates revealed the existence of 22 hitherto unknown clusters.

No account of this part of Hogg's life would be complete without reference to the 7⁴-inch telescope. When this telescope, at that time well up in the ranks of the world's larger instruments, was erected in 1955, the images were found to suffer from an unacceptably large amount of astigmatism. The difficult task of deciding whether this lay in the primary mirror, the mirror supports or the secondary flat, fell mainly to Hogg. There being no one with experience of this aspect of large telescopes on the staff, methods and tests had to be devised ab initio. After much tedious work the trouble was traced essentially to the main mirror, which had to be returned for refiguring, but on the way a firm foundation had been established for the basis of large telescope technology, without which no big observatory can flourish, and which can be acquired only by first-hand experience. Many of the present-day practices at Mt. Stromlo go back, at least in part, to Hogg. One field he made particularly his own was aluminizing. He designed the first plant, made it work, and had a direct hand in most of the actual mirror aluminizing. He once remarked that aluminizing gave him more satisfaction than any other aspect of astronomy. His skill with large telescopes was by now well known, and it came as no surprise when the Egyptian Government invited him for a three-months visit in 1963 to advise on the adjustment and operation of their own 74-inch.

At the end of 1955 Woolley, having served a momentous 16-year term, left Mt. Stromlo to become thirteenth Astronomer Royal. There followed a difficult 15 months during which Hogg was temporarily in charge. One of Woolley's last acts had been to ensure the transfer of the Observatory to the University. Hogg was strongly opposed to this move, and apprehensive about the future. The simultaneous transfer to the University and change of director did in fact affect his position a good deal; he had little to do with the students, who soon came to play an important part in the research activities of the Observatory, and he did not achieve as close a personal relationship with Bok as he had with Woolley.

It did not take Bok long to decide that, astronomically speaking, the climate at Mt. Stromlo left a good deal to be desired, that with the growth of Canberra, conditions could not but deteriorate, and that a search should accordingly be made for a site where a field station could be established in the fairly near future. The search began within the year. The strategy was to gather meteorological data, with special emphasis on cloud cover, from as many potential sites as possible, then to carry out detailed testing on the more promising. Sites had to be selected (with an eye to many factors), local Boards and Councils interviewed, and arrangements made for the collection of the meteorological data. This could, of course, be done only at first hand. Much of it came Hogg's way, and in this field his easy manner, good sense, and acceptability to country people were major assets. Astronomical sites are commonly remote, and ours being no exception, we find that within a relatively short time Hogg had covered much of the back country between Meekatharra and Kalgoorlie, and had made separate extended visits to the Musgrave Ranges in northern South Australia, to the Flinders Ranges, and to the Barrier Range, north of Broken Hill. He clearly enjoyed these trips. He liked the outback, and the strenuous physical regime, and the more peaks there were to be climbed, the more effortlessly his long-striding, rather angular figure seemed to travel; among the site-testers his stamina was a by-word.

It would of course be unfair, to Bok in particular, to suggest that Hogg was alone in all this, but he did play a substantial part from the beginning, and on the completion of the initial phase took over the immediate direction of the whole enterprise. The task was indeed an immense one. The corresponding survey of the USA, covering about the same area, has taken decades, involved many institutions, and cost millions. If immense, it was no less important. Within the fairly near future between fifty and a hundred million dollars will be invested in major observatories in the south. The siting of these is critical, as witness the sending of at least a dozen site-testing expeditions to the south in recent years, from Europe and the USA Three have come to Australia, sponsored respectively by the Yale and Columbia Observatories, the University of California, and Mt. Wilson Observatory (CARSO), all of them working closely with the Mt. Stromlo survey. Hogg, with relatively slender resources, often with adapted or improvised equipment, and with a staff he had to recruit and train from scratch, nevertheless produced a body of data which can more than hold its own even in this eminent company, and which in its way is unsurpassed. This is as much a tribute to his organizing powers and human qualities as it is to his scientific ability.

By the end of the initial phase about twenty stations, distributed over most of the southern half of the continent, were sending in data. A first assessment reduced this number to four, on one of which, Mt. Bingar, near Griffith, NSW, a temporary field station centred on a 26-inch reflector was established towards the end of 1959. Meanwhile the problem had expanded, and while priority remained with finding a site for the Mt. Stromlo field station, which for accessibility and convenience had to be in NSW, interest was such, especially from what later became the Anglo-Australian Large Telescope organisation, that it was decided to continue the search on a nation-wide basis. Specifically this meant that beside the four NSW sites, testing was to continue on Mt. Singleton in Western Australia, on Mt. Woodroffe near the South Australian-Northern Territory border, Mt. Serle in the Flinders, and on Mt. Robe north of Broken Hill.

By this stage methods had stabilised, and the routine at each station was to keep records of wind, temperature and cloud, while special observations were made of seeing and transparency. Seeing is the critical quantity; it refers to the degradation in the stellar image produced by inhomogeneities in the atmosphere through which the image-forming beam has passed, and remains a little understood phenomenon, which can vary widely from site to site, and also from time to time. A seeing disk less than one second of arc is very good, one greater than three seconds barely acceptable. The difficulty of the problem is enhanced by the fact that one has to estimate, with equipment which of necessity must work under field conditions and near ground level, how the seeing will appear in a large telescope working at a height of fifty or eighty feet. Hogg trained his observers in the then standard Danjon method, essentially visual in character, then spent a good deal of time experimenting with photographic image trails, and with converted military range-finders in which one compared the quality and separation of the images formed by the two halves. These methods, though promising, did not go into routine use, and the final stages of the survey were carried out with the Babcock automatic seeing monitor, a photoelectric device which measured image movement directly. With D.G. Thomas, Hogg developed a very successful transistorised photoelectric photometer, light enough to be attached to a small telescope. It was used extensively for measuring atmospheric absorption. And seeing observations being highly vulnerable to wind vibration, he devised a neat, transportable dome which sheltered the instruments very effectively. Problems like these, together with arranging access, and power and water supplies at his remote sites occupied much of his time. He had also, of course, to supervise the reduction of very large amounts of data, the results appearing in a long series of internal and interim reports. He did not live to collect them into the definitive memoir which would surely one day have appeared.

On the organizational side Hogg had been appointed Australian member of the IAU Committee on Astronomical Sites, in which capacity he attended the IAU Symposium on Site Testing held in Rome in 1962. In 1963 he was appointed chairman of a Sub-Committee for Site Selection for the Anglo-Australian Telescope, set up by the Academy. By 1962 it had become necessary to decide on the location of the Mt. Stromlo Field Station. The choice lay between Mt. Bingar, the existing site, and Siding Spring Mountain. The issue became a contentious one within the Observatory. Bingar had been a successful site, and some observers, notably Bok, were understandably reluctant to leave it. Hogg, on the other hand, was a strong advocate of Siding Spring. Siding Spring won the day, chiefly on the grounds of better seeing. Subsequent experience has shown that the seeing there can indeed be extremely good. It must be recorded also that the issue decided, Bok threw himself wholeheartedly into the development of Siding Spring, which in consequence went ahead with great rapidity. As far as the large telescope site was concerned, Mt. Singleton was eliminated in 1967, after extensive tests, and Mt. Woodroffe because of its remoteness. Testing continued of Mt. Serle in the Flinders in cooperation with the University of California group who had occupied nearby Mt. McKinley. The Flinders sites were good, but Siding Spring had fewer logistic problems, and was already being developed, and it was no doubt for these reasons that it was specified for the Anglo-Australian telescope. The announcement was made at the same time as that to proceed with the telescope itself, only a few weeks after Hogg's death; as we have said, it was a pity that he did not live to see it.

Scientifically Hogg's virtues were of the solid kind. He was prepared to wait for his answers, and would not publish until he was certain; when he did, his results carried real conviction. Not a brilliant man himself, he inclined perhaps to an undue respect for brilliance in others. This led to a certain diffidence, which reinforced a habit of self-sufficiency: especially in his earlier days, he liked to work on his own problems in his own way. Woolley once said: 'Hogg is a better physicist than he gives himself credit for', a perceptive remark from one who knew him well.

He had an easy social manner, and was known and liked by a very wide circle, both in Australia and overseas. Some of this comes through in the wit and neatness of phrase which marked his occasional writings. A characteristically felicitous touch was his suggestion that one of the shaped pieces from the support of the Great Melbourne Telescope be adopted as the foundation stone of the Academy. He was, too, a man of character, who knew how to stand his ground. Older members of the Observatory staff will remember how level-headed and steady he was on the day of the fire which swept Mt. Stromlo and destroyed the Observatory workshops. About a year before his death he suffered a heart attack, followed some time later by a serious relapse: so warned, he bore himself in his last months with a quite remarkable gaiety and lightness of spirit. He died as one feels he would have wished, working normally almost until his last hour.

In 1933 he married, most happily, Irene Doris Yandell, and is survived by her, two sons and a daughter. His second son, Garth, is lecturer in Physics at the University of Glasgow.