For any southern observer the establishment of pairs that were discovered or observed by Dunlop and Rümker marks an important stepping stone in the emergence southern astronomy. Both share the honour of producing the first true survey of the prominent southern double stars.
In the early 18th Century, double star astronomy was still in its infancy that had initially arrived in the 1780’s with the visual observations of Sir William Herschel. Herschel then quickly realised the particular importance of double stars by immediately realising the apparent discordance between the statistical probabilities of two stars lying close together against the apparent large numbers of observed pairs. In turn, it was certain that some of these pairs had to be gravitational associated, being true binary stars. From the careful observation of several of the bright pairs he soon detected the first evidence of such gravitational attachment via the components moving in curved arc by their mutual orbital motions. Herschel then deduced that if we could understood these motions, then over time we could obtain some ideas of enormity of true stellar distances and even perhaps the overall scale of the Milky Way. Another secondary advantage of observing visual binaries - which was to eventually became the ultimate investigating goal - was for the direct determination stellar mass. After this, Herschel embarked on the systematic survey of the heavens whose aim was to find as many close pairs as possible. Realising the component’s motions were often on a much larger scale than any human lifetime, the archetypal undertaking of Herschel works were to prove his enduring legacy, This endeavour of study now continues today and shall likely persist in the coming centuries. The very exciting discovery of the existence of real gravitationally attached binaries first gave the means of achieving some of the important goals towards astronomy has striven, that is, a complete understand of the stellar realm. This strong motivational force in early observational astronomy was to give modern astrophysics the drive for many 19th and early 20th Century astronomers, and enhamce today’s understanding of our Universe.
When Dunlop’s doubles observations were being made in 1827, much work on the observations of pairs had been achieved. Comparitively, in the northern skies observational standards of similar observers had improving significantly, making the observations of Dunlop almost seem incompetent and quite ameteurish. You have only to compare the work of likely the greatest double star observer, Frederick Struve, who at the same time was undeertaking the methodical discovery and measures of many northern pairs. Furthermore, Struve had discovered ten to fifteen times as many doubles as Dunlop - measuring each of them accurately with a very high-quality filar micrometer.
During Dunlop’s time the southern skies below about -30o declination were literally the ‘virgin frontier’. In the annals of history Dunlop’s and the Rümker’s, works marks only the initial survey. Historically among the astronomical community we have berated Dunlop and have persistently criticised his first catalogues. Many still claim, as also placed towards in the 623 objects in his deep-sky catalogue, that the majority of these were imagined or were optical mirages from his poorly made equipment. Anoother common complaint is that the positioning of all his objects was often poorly obtained and contained too many avoidable errors. In regards his double star measures, many still deem these observations of little consequence mainly because they are generally wide and likely only optical pairs.
Although some of this maybe true, the reasons behind his presumed weaknesses was Dunlop’ isolation from his peers and the rest of the astronomical community in both England and Europe. With the new colony of New South Wales some months, away by ship, communication was very difficult. A simple question could be three to five months to answer. At first, the new discoveries were hailed with many accolades, but when the observations were later reobserved by others, the elation soon turn to disappointment and honest criticism. The crux of the problem is that Dunlop had never claimed that his observations were perfect nor very exact. His new pairs were only really made in the least favourable conditions such as poor seeing or when the Moon was full - when deep-sky observing or astrometric measuring of star positions was impossible. Double star observing was only done so no opportunity of clear skies would be wasted.
Rümker on the other hand has a better reputation because many of his pairs are closer and thus thought more significantly important. Today his reputation regarding double stars remains much higher than Dunlop. However, Rümker found only twenty-six pairs. It is interesting why people have this impression. For me, after I looking at the latest data in the Washington Double Star Catalogue (WDS05), we have observed many of his pairs very little - at least when compared other more ‘valued pairs’.
Perhaps we have wrongly interpreted our perspective on the achievement of these two double star catalogues. Observationally, and in the light of what was to come, Dunlop and Rümker reputations were somewhat tarnished. This degradation began when the eminent John Herschel’s started his southern observations that aimed to join his father’s earlier northern work into the first all-sky survey. Between 1834 and 1838, some ten years later, John Herschel found thousands of new pairs and measured many of them. It was John Herschel who found many of the errors in Dunlop’s catalogue. Once Herschel produced his own catalogue criticism started to mount against Dunlop - inferring that he was essentially a poor observer. This reputation stuck.
For me it all depends on how you look at his double star catalogue. If you look at it from the perspective of an initial survey then what he has produced really just lays down the groundwork for the other surveys. John Herschel found much promise in the observations of Dunlop, and began to advocated for Dunlop’s isolated position. While John Herschel was still in England, and up to just before his southern expedition, Herschel read Dunlop's deep-sky and double star work. The latter was read on the 09th May 1828 to the Royal Society, where Dunlop himself places the assessment on the merits of his work;
“In presenting this list of double stars, it may be necessary for me to make some apology for its imperfect state, as regards the true and apparent distance and position of a great many double stars, the situation of which points out in the heavens.”
Dunlop is not really being just humble here. It is likely Dunlop honestly concerned with the observations and clearly states the nature of his own programme. For Dunlop "nebulae being a prime object to me" was one of his main goals towards his endeavours, which he "devoted the whole of the favourable weather." It seems the collection of double stars were only of secondary importance and many of then were likely found while doing his deep-sky surveys. It is likely that he observed the pairs again when the next opportunity came along under these poorer conditions. Ie. Observing in moonlight or during poor seeing. (Note: The latter condition is the worst time to observe pairs.) Incidentally observing during bad seeing might also accounts for many close pairs that he missed.
In the scheme of things Dunlop and Rümker’s observations maybe of little consequence but they did lay the foundations of later surveys by other astronomical “heavyweights”. To me we should perceive Dunlop, and to a lesser extent Rümker, as the ones who opened the celestial door into the southern skies. It was everyone else who later found the nature of the treasures contained within. As for opening this part of the celestial vault - we should be eternally grateful.
James Dunlop observed Encke’s Comet some thirty (30) times between Oct 26 and Dec 26 1828, measuring and reducing the positions. This was done at Sir Thomas Brisbane’s Observatory, Makerstown, Roxburghshire. (MNRAS, 1, 120 (1829))
Other than double stars, nebulae and clusters in the southern skies, Dunlop also produced a paper himself on stars entitled “IV. Observations of the Magnitude, Colour, and Brightness of Stars in the Southern Hemisphere.”; MNRAS, 2, 190 (1830) This little examined document contains information on some 400 southern stars under headings of magnitude, lustre and colour. The magnitude of the stars was measured by means of a ‘double eyepiece’, where the relative magnitude and colour in the telescope being determined by the sizes of the artificial disks. The original submitted paper is currently available in the R.A.S. Library in London.
Also closely examined were the comparisons between “neighbouring stars”, in the hope, in Dunlop’s words, of “...enabling observers to discover any changes that may hereafter occur.” This project was an additional project to the observations for the positional catalogue of southern stars in the ‘Paramatta Catalogue of Stars’.
The Paramatta Catalogue was in fact the second specific positional star catalogue that was made in the southern hemisphere. First was the star atalogue made by the French astronomer, (Abbe) Nicolas-Lois de a Lacaillé (1713-1762), who made his southern observations from the Cape of Good Hope between 1751 and 1753. Results of this work was later published in its partial form in 1763 and had its full reduction completed in 1847. The initial catalogue version was named the ‘Caelum Australe Stelliferum’ or ‘Coe Australe’ and appears in ‘Histoire Céleste Française’. It contains in total some 9 776 stars made within the short time of eleven (11) months using only a small equatorially mounted 1.3cm (0.5-inch) refractor.
This star catalogue was mainly of the naked-eye stars and many 7th and 8th but the positions certainly were not sufficiently accurate for astronomical purpose such as proper motions.
Dunlop used this catalogue as his basis of the Paramatta Catalogue, which has many of his stars listed through its pages. He also partial examined the errors of Lacaillé. Whilst doing this star catalogue, Lacaillé discovered and catalogued forty-two (42) nebulae and clusters that was first published as “Sur les étoiles nébuleuses du ciel austral”; Mémoires de l’Académie Royale des Sciences (1755). Of these some thirty-four (34) are actual known objects, some twenty-six (26) being Lacaillé’s own discoveries. [My Shortened Notes about the Para. Cat.]
According to the Paramatta Star Catalogue observations began in November 1821 and finished on 2nd May 1822. After this in June 1823 only a few further observations were made by Brisbane himself, but the majority of visual observations and measurments were obtained by Dunlop, and to a lesser extent by Rümker. Other separate positional observations were made specifically for the Admiralty between the 2nd May 1822 and 2nd March 1826. These results were consequently sent back to England during 1829 and in 1830. The catalogue results were soon followed where Mr. William Richardson was duly employed by the “Lord Commissions of the Admiralty”, who then was directed to re-reduce all the observations.
Richardson’s computations were based of all the observations made by the Mural Circle upon visual stars seen from the latitude of -33o 48' 50.685". Mr. Rümker had in “Philosophical Transactions” Hamburg (1832) determined for 1829 the longitude of the observatory as 10h 04m 06.25s E of the meridian of Greenwich, with the height estimated as 60-feet.
Richardson says that the Paramatta Catalogue is a “Catalogue of Southern Stars, as far as 8th magnitude.”. He goes on to say;
“During the greater part of the time embraced by the Paramatta observations, Mr. Dunlop was the only observer; and with the point of view the complete observations for the formation of an extensive catalogue, he abandoned the transit instrument, and fixed the Mural Circle as nearly to the meridian as he could, he commenced observing every star that circumstances permitted as it passed the central wire, regarding the time of transit, and read off as many microscopes as the interval before another object came to the wire would allow.”
Richardson goes on to state;
“In a period of about two years and a quarter... he observed 7,000 stars, and made nearly 40,000 observations, besides as extensive series of observations upon double stars and nebulae.”
James Dunlop’s main problems was having a variety of astronomical equipment for use which was only in reasonable condition. It seems probable that both the main measurement devices; the Transit Telescope and Mural Circle may have been permanently damaged during long transport by ship from England to Australia, while being delivered to the Paramatta site or when being set-up for use. We know this from the problems faced when determining observations and the general poorness of the data seen in Paramatta Star Catalogue - something that cannot be solely be blamed on the observers! Both Dunlop and Rüker attempted to determined some kind of correction to the instrumental error, but we only partially succesful. Many of the corrections were never used with the catalogue data.
At Paramatta Observatory, Brisbane brought an accurate clock made by Hardy that showing local sidereal time. This was used in conjunction with the principle positional equipment being a 16-inch repeat circle, the 5½-foot Troughton transit telescope, and a 2-foot Mural Circle also by Troughton. The Transit circle caused many problems for the catalogue, so Dunlop alone made the critical decision to do star transit observations using the less precise Mural Circle. This choice was to have serious consequences on the questionable quality of the final star catalog. All these instruments were place in solid masonry at the observatory site. After the Paramatta Observatory was demolished, the equipment was placed in Government storage. During and prior this time many of the smaller part of the equipment went missing - that was the important part to make the equipment unuseable. When Sydney Observatory was finished in the late-1850’s, he equipment was exhumed and placed at Sydney Observatory and it remained there until the 1990's. In all this time no serious observations were made using it except for the clocks. Much of this was only used for visual display. Eventually the equipment was cleaned by the Sydney Powerhouse Museum and returned to the Sydney Observatory as part of the exhibits. Most of it is now kept under glass displays for inspection, but some of it is often hidden or rotated over the years.
In the years 1823-1827, James Dunlop (1795-1848) observed the southern skies from the Brisbane observatory at Paramatta, New South Wales, Australia. He compiled several catalogs, among them the Brisbane Catalog of over 7000 southern stars, and A Catalogue of Nebulae and Clusters of Stars in the Southern Hemisphere observed in New South Wales of 629 entries for deepsky objects [Dunlop (1828)]. However, many of these objects are badly described, so that John Herschel could only verify 211 of them. New research, led by Glen Cozens, brought up a number of further ‘real’ objects, so that actually roughly over 300 (or about 50 %) seem to belong to "real" deepsky objects. The other half is asterisms and multiple stars which Dunlop's comparatively small instruments (a 9-feet FL, 9-inch aperture reflector with speculum metal mirror, perhaps equivalent to about a modern 6-inch reflector) didn’t resolve.
The most spectacular original discovery of Dunlop is perhaps that of peculiar radio galaxy NGC 5128 in Centaurus (also called Centaurus A), his Dunlop 482. Also included in his original discoveries are Sculptor Group galaxies NGC 55 (Dun 507), 300 (Dun 530), and 7793 (Dun 608), and a considerable number of further southern galaxies, open and globular clusters, diffuse nebulae, and 4 planetary nebulae (NGC 2818=Dun 564, NGC 5189=Dun 252, NGC 5882=Dun 447, and NGC 6563=Dun 606). Dunlop has included 6 Messier objects in his list: M54=Dun 624, M55=Dun 620, M62=Dun 627, M69=Dun 613, M70=Dun 614, and M83=Dun 628. The Dunlop catalog is arranged in the order of increasing declination or south polar distance, from south to north; objects which are nearest to the South Celestial Pole come first. This explains the late numbering of the comparatively northern Messier objects; M83 is (almost) the northernmost Dunlop object. This endeavour was the second major effort of a deep-sky object survey of the southern skies, after Lacaillé’s list of 42, of 1751-1752.