Do the words of a poem lose their poignancy once its author departs this world?
Can the limp of ‘progress’ outshine the ‘grand procession’ of great accomplishment?
Can a culture, basking in the glory of its own achievement, be made mute by a faithless generation of technocrats?
Can an optical bench test inspire more than a night spent behind the eyepiece of a grand old telescope?
Let us venerate that which is deserving of veneration!
Whose crown shall we adorn with a laurel wreath?
Let us sing again of old dead men
And clear the cobwebs from their medals.
For they have no equal in the present age
No muse to light their way.
An essay dedicated to Peter Huesmann.
“Many astronomical observations are useful for only a few years before they are completely superseded by new ones. This is especially true in fields that rely on instruments developed with rapidly changing technology. In marked contrast, the best visual measurements of moderately close double stars made 100 to 150 years ago are still good by modern standards. The measurements by Wilhelm Struve, E. Dembowski and W.R. Dawes are valued highly by today’s calculators of binary star orbits”
Thus wrote Dr. Joseph Ashbrook, former editor in chief of Sky & Telescope magazine in a short essay on the life of William Rutter Dawes (1799-1868). With small classical refractors, made with just a token nod to optical theory, this benevolent English observer greatly advanced our knowledge of the heavens, showing that, above all, acute and highly trained eyes could see things other amateurs could not, even using larger instruments. Born on March 19, 1799, Dawes could be said to have had astronomical blood in his veins. His father, William Dawes the Elder (1762-1836) served as the astronomer on Governor Philip’s first expedition to Botany Bay, Australia, on board HMS Sirius back in 1787. By the time young William was born, his father had taken up a post as a mathematics teacher in a school for boys. The Elder Dawes had a strong moral compass though. Amongst many other things, he was an outspoken abolitionist. Perhaps this is why the Elder Dawes entertained very different aspirations for his son, whom he wished to enter the Clergy of the Church of England. Young William was unmoved at first by his father’s wishes, deciding instead to train as a physician at St. Bartholomew’s Hospital in London.
It was only after Dawes moved to Liverpool, in or around 1827, do we see a blossoming of his interest in astronomical topics, especially double stars. Writing many years later to his friend, Sir John Herschel, Dawes recounted how he got the double star bug;
“Having obtained the loan of a volume of Ree’s Encyclopaedia, I had copied out the list of Sir Wm. Herschel’s Catalogue’s of Double Stars, arranged in classes and constellations; and [using] a capital little refractor of 1.6 inches aperture, and a copy of the French edition of Flamsteed’s Atlas, which was presented by Dr. Maskelyne to my father…..I worked away on almost every fine night, when uncertain health would permit, and found and distinctly made out ……Castor, Rigel, Epsilon 1 and 2 Lyrae, Sigma Orionis, Zeta Aquarii and many others, of which I made correct diagrams in a book now lying before me….The difficulty was often to get to bed in summer before the Sun extinguished the sight of the game.”
While in Liverpool, Dawes came under the influence of the writer and theologian, Reverend Thomas Ruffles, who provided pastoral care for the George Street Congregational Church for nearly half a century. Seeing no conflict between his faith and his inspirational work as a double star astronomer, Dawes retrained as a minister and upon qualifying, he took charge of a small congregation of the same denomination in the town of Ormskirk, situated about 15 miles north of Liverpool.
The Dollond o’ Dawes
Dawes was fully 30 years old when he found the time to resume his passion for astronomy, acquiring a fine equatorially mounted 3.8 inch Dollond achromat of 5 foot focus to which a wire micrometer had been affixed. As luck would have it, the instrument and a measure of its achievement is described in good detail in George Bishop’s (about whom I shall have more to say later) book, Astronomical Observations Taken at the Observatory South Villa, Inner Circle, Regent’s Park, London, During the Years 1839-1851;
“The instrument appears to have been one of first rate excellence, as shown by the work performed with it, such objects as Zeta Cancri, Zeta Bootis and Mu Bootis, being well separated. In observing the angle of position, Mr. Dawes placed the stars between the parallel threads – a method suggested by Sir John Herschel – and since very generally adopted. A red illumination of the field was thought to be advantageous. The magnifying power at first employed was 225, but 285 was afterwards substituted as a working power: others as high as 625 being used for very close and bright objects. The observations were almost invariably near the meridian. The most interesting objects are specified in the important contribution to double star astronomy, taken as they were, by one whom John Herschel has styled observer in this particular branch that Europe affords. The catalogue includes more than 400 sets of measures.”
Let’s look more closely at Bishop’s remarks. The telescope operated at a ‘working power’ of 285 and could be pushed as high as 625x for ‘very close and bright objects’. These are magnifications which range from 75x to 165x per inch of aperture! In my study of other small classical refractors, as well as my own personal experience with a lovely 80mm achromatic doublet of 1200mm focal length, I can personally vouch for the soundness of these amplifications in the pursuit of difficult binary and multiple star systems. These simple but highly effective instruments soak up tremendous magnifications; a sure sign of their optical excellence as well as their superior thermal properties.
During the early 1830’s, Dawes conducted over 600 measures of doubles with his little Dollond and in so doing introduced some refinements to the techniques that were commonly employed at the time and which made his work more accurate than his mentor, Sir John Herschel. A notable advancement in this regard was the introduction of a Barlow lens to double the magnification without increasing the apparent thickness of the micrometer threads. Another innovation introduced by Dawes involved the employment of a prism diagonal to render the apparent orientation of the line of intersection of two wires either horizontal or vertical, which helped reduce errors in measuring the stars position (a bearing like 360-degree scale, measured anti-clockwise from north).
His exemplary work as an amateur came to a rather abrupt ending towards the end of the 1830’s, when his first wife, Mary Scott, the widow of his former tutor, being considerably older than him, fell ill. He nursed her night and day and could not attend his flock, causing him to relinquish his pastoral duties at Ormskirk. After Mary died in 1838, Dawes was in need of employment and to that end, accepted a post as assistant astronomer at the residence of the tycoon, George Bishop, who erected a lavishly equipped observatory (as was his euergetic duty in the Age of Empire) at his home in Regent’s Park, London, after his retirement at age 50. There Dawes would put his world-class skills as an observer to good use. And it was here also that Dawes met his second wife, another widow of Mr. John Welsby, whom he married in 1842.
A successful international wine merchant, Bishop amassed considerable wealth and as was his euergetic duty, founded an observatory with which he might put some of his great wealth to some practical use. Back in 1836, Bishop had erected a fine 7-inch refractor (by Dollond) near his residence at South Villa. He was only too delighted to have the by-now famous double star astronomer join his establishment. Like Dawes’ smaller instrument, the 7-inch Dollond is vividly described in Bishop’s aforementioned book;
“The telescope had a clear aperture of 7 inches and a focal length of 10.75 feet. The tube is of brass, and has been covered in a coating of paint for the sake of preservation. The mounting, generally, is similar to that of Captain Smyth’s equatorial now at Hartwell House, and has been found to answer extremely well. The polar axis is formed of four slabs of mahogany, braced together by strong iron screws; its length is 13 feet, and its breadth in the broadest part (which is near the middle of its length is 9 inches……..The clock-driven movement of the equatorial, an indispensible appendage, also by Mr. Dollond, is firmly attached to a stone pillar……The eyepieces attached to the telescope magnify respectively 45, 70,108, 200, 320, 460,700 and 800 times. There is a polycratic wheel containing six magnifiers, the highest of which is 1200. One of the greatest conveniences in the measurement of double stars has been found to be the equatorial reclining chair. It was constructed by an ingenious mechanic…….The chair is of mahogany, with brass wheels and fittings. It is attached to the floor of the room, and revolving on an iron pivot, can be brought into any position that may be required.”
Over the next quarter of a century, the post of assistant astronomer was to be filled by other observers of notoriety including J.R Hind, Eduard Vogel, C.G. Talmage and Norman Pogson. Things went swimmingly for Dawes under the aegis of Bishop between 1839 and 1844, during which time he pursued double star mensuration with singular gusto. After that time, the relationship between Dawes and Bishop began to deteriorate, as his employer (a non-astronomer) attempted to pass off many of the observations made by Dawes as his own. But in marrying a well-to-do widow, Dawes found a way out of the situation without causing further consternation between himself and Bishop.
In 1844, Dawes was able to leave the employ of Bishop and take up residence in a grand country house near Cranbrook in Kent, a site very near where his old friend, Sir John Herschel, had long since put down roots. Here, he erected his own observatory, equipping it at first with a 6.3 inch Merz equatorial refractor in 1846. With this fine instrument, Dawes put his back into planetary work.
By 1850, the great observer had independently discovered the Crepe ring of Saturn, before news of its sighting crossed the Atlantic, that W.C Bond, using the 15-inch refractor at Harvard Observatory had spotted it on the nights of November 11 and 15, 1850. William Lassell, with his large an unwieldy 24 inch (speculum), had failed to see it under conditions that he himself described as an ‘excellent night of November 21′. But Dawes had success with the much smaller Merz on the faithful evening of November 25.
“I detected for the first time a light within the ansa of the ring at both ends while examining the planet with my Munich refractor of 6.3 inches aperture.”
On December 3, 1850, Dawes showed the Crepe ring to Lassell in person and while he did concede that it was visible through Dawes’ long focus achromatic, he was not quite prepared to admit that it escaped his notice in his grotesque speculum. In time, Lassell would see the Saturnian novelty many times with his large telescope. Genius really is the art of seeing the obvious!
Having long abandoned the fickle behaviour of reflectors, the experience Dawes garnered with the 6.3 inch Merz – its sheer superiority in resolving power over the smaller Dollond – convinced him that bigger really was better. Predictably, Dawes became an enthusiastic, serial collector of ever larger and more powerful refractors. He embraced the promising new telescope maker in the United States – Alvan Clark –ordering a 7.5 inch Clark in 1854 and culminating with a 8.25 inch Clark in 1859. With these fine instruments, Dawes was able to extend his tally of double star measurements to 2,800. This vast treasure of field experience allowed him to derive an empirical relation between the aperture of a telescope and its ability to resolve tight stellar pairs. In a curious development, published in Vol. 35 of the Memoirs of the Royal Astronomical Society (1867), Dawes penned the formula that was to make him famous;
“It is a point of considerable interest to determine the separating power of any given telescope aperture. Having ascertained about five and thirty years ago, by comparisons of telescopes of very different apertures, that the diameters of star disks varied inversely with the diameter of the aperture, I examined with a great variety of apertures a vast number of double stars, whose distances seemed to be well determined, and not liable to rapid change, in order to ascertain the separating powers of those apertures, as expressed in inches of aperture and seconds of distance. I thus determined as a constant, that a one-inch aperture would just separate a double star composed of two stars of the sixth magnitude if their central distance was 4”.56; -the atmospheric circumstances being moderately favourable. Hence, the separating power of any given aperture, a, will be expressed by the fraction 4”.56/a
Dr. Ashbrook, commenting in his short essay on Dawes in his Astronomical Scrapbook, expressed this view of Dawes’ famous relation;
“While this formula is very familiar to amateurs today, its narrow range of validity is often overlooked. Determined with small refractors and Dawes’ eye, the law does not tell what another observer can see with a large reflector or a catadioptric”
In light of ongoing studies conducted by amateurs using both contemporary refractors and high quality reflecting and compound telescopes, it appears that the Dawes’ limit is still a formidable resolution limit for many double star observers. Ashbrook conveys the impression that ‘modern telescopes’ ought to reach this limit owing to advances in optical technology but intriguingly, this author has thus far obtained evidence that only achromatic refractors – both large and small – can exceed these resolution limits and that, visually at least, reflectors (and by implication compounds) have a much harder time reaching the Dawes limit in field tests.
That said, it need not necessarily imply that greater resolution feats on equally matched binary systems cannot be attained. One problem with the Dawes limit is that it is closely associated with an ‘optimised’ wavelength of 562nm. Yet, the dark adapted eye is optimised at a significantly shorter wavelength of 510nm corresponding to ~ 10 per cent increase in angular resolution. Indeed, this author has suggested that red-green colour blind individuals could conceivably resolve even finer pairs. Certainly, images conducted by noted UK imager, Damian Peach, has clearly shown how angular resolution is inversely related to wavelength by means of using colour filters. Furthermore, previous work has suggested Dawes himself was able to exceed his own limit using at least one of the instruments he used to carry out his double star measurements. In addition, there are several sources in the literature that suggest that faint pairs consisting of early spectral type components (O, B and A) can be resolved beyond the Dawes Limit. In light of these revelations, one must thread very cautiously indeed in declaring some sort of universal ‘law’ on the resolution limits of a given individual and telescope aperture.
Dawes & the Soulish Animals
Though Dawes became independently wealthy later in his life, corresponding regularly with many of the astronomical elites such as Sir John Herschel, Richard A Proctor and Admiral W.H. Smyth, he gave up much of his valuable time in the service of the local community, providing them with free medical care and regularly visiting the eldest and most vulnerable of his congregration. According to his great nephew, the Reverend Arnold Taylor, Dawes enjoyed the company of animals, particularly dogs and horses, which he kept as pets on his estate. Many tales abound of his special relationship with canines, including one that suggests he understood them in unusual ways. One day, whilst on one of his house visits, he came across a ferocious guard dog which had been chained to its kennel. And though he was duly warned to keep a safe distance from the angry beast, Dawes walked up to it, stared into its eyes and forced it to retreat. Then, the man o’ the cloth dragged the cowering dog from its kennel, patted and made friends with it!
Dawes suffered bouts of ill health throughout his life, including migraines, asthma and the chronic symptoms of ischaemic heart disease. Yet despite this malaise, he went about his astronomical work with great diligence. Sometimes he would be found fast asleep in his observatory chair, the great equatorial having swung to a completely different part of the sky. In 1857, he and his wife left Cranbrook and moved to a charming estate in Haddenham, Buckinghamshire, where he was to live out the remainder of his life.
After his second wife died in 1860, he only had the company of his retriever to tag along with him on his daily stroll to his observatory. Despite rapidly failing health in his autumn years – which was, by now, compounded by deafness also – he continued to make observations of the bright planets and double stars. In 1865, the Royal Astronomical Society awarded him its highest accolade – the Gold Medal – for his services to astronomical knowledge. On the morning of February 15, 1868, he passed away, nearing the startline of his 70th trip ’round the Sun.
The life of William Rutter Dawes remains an inspiration to those who see harmony between religious faith and scientific advancement. As a Christian, he had a duty to seek wisdom from the Book of Scripture and as an astronomer – from the Book of Nature. Those two books have and always will be in perfect accord!
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