The ‘English Fraunhofer’: Resolving a Paradox?

By Neil English

What has been is what will be,
and what has been done is what will be done;
there is nothing new under the sun.
Is there a thing of which it is said,
‘See, this is new’?
It has already been, in the ages before us.

Ecclesiastes 1:9-10.

http://www.youtube.com/watch?v=5L8-FTvSVxs

The ancient English city of York has enjoyed a long and illustrious history spanning two millennia. Founded by the Roman Governor of Britain, Quintus Petillius Cerialis in 71AD, it lies at the junction of two great rivers – the Ouse and Foss – and quickly grew from a garrison town into a major northern city of the Roman Empire. The 2nd Century Spanish emperors, Trajan and Hadrian, knew the place. The 3^rd Century African Princeps, Septimius Severus, died there, and in the 4th Century, Constantine the Great was proclaimed the Western Augustus by his troops within its walls.

Fortified and expanded by the Vikings and Normans who followed them, York also basked in the noon day brightness of the Industrial Revolution, attracting all manner of skilled artisans to its bustling streets. And it is here that our story begins, when and where a young man named Thomas Cooke founded a telescope making dynasty that restored Britain’s talent for scientific innovation throughout the Victorian Era and beyond.

Cooke was born on March 8 1807 in Allerthorpe, Yorkshire. The son of a shoemaker, he received only the briefest of formal education, when after two years at an elementary school, he was put to his father’s trade. But it soon became evident that such an occupation was not for the dreamy boy who pined for maritime adventures. Bright and curious, Cooke soon resumed his learning, teaching himself mathematics, navigation and astronomy. Fortunately for us hopeless telescope junkies, Cooke never did set sail on the high seas, his mother having persuaded him (insisted?) to seek local employment instead. From 1829 to 1836, he pursued a teaching career as an assistant schoolmaster and private tutor. And it was during this time that he met his future wife, Hannah Milner.

Cooke’s interest in practical optics impelled him to begin work on his first telescope, one of the lenses of which he ground from the bottom of a whiskey tumbler and mounted the objective inside a tin tube which he soldered together from s**** metal. That same telescope was bought by a one John Philips, the then Curator of the Yorkshire Museum, but who later became an active member of the British Association for the Advancement of Science. Philips was to prove a powerful ally in the advancement of Cooke’s subsequent career.

His marriage to Hannah was bountiful too. She bore him seven children in all. Two of his sons, Charles (1836 - 98) and Thomas (1839 - 1919) subsequently joined him in the business he founded in 1837, at 50 Stonegate, York, with a loan of £100 from his wife’s uncle. From this unassuming, rented premises, Cooke began work repairing and making instruments to order.

Throughout the 18^th century, Britain had established a solid lead in optical glass manufacture, attributed no doubt, to the extraordinary success of the Dollond dynasty and the many artisans who grew up round them. Yet, by the second decade of the 19^th century, England’s optical glass industry was crippled. In a penetrating modern analysis, historian Myles Jackson referred to the affair as “the British Crisis,” in which the government maintained a stranglehold on the glass furnaces by enforcing heavy taxes on the manufacture of crown and flint glasses, while domestic types were exempt from duty. The motivations of Her Majesty’s Government lay with the large quantities of wood and (later) coal, consumed to produce the melts. Those raw materials – the energy resources of Empire – were to be prioritised for other purposes. Indeed, although at the beginning of the eighteenth century 13 optical glass works were in operation across the country, only three remained by 1833 and with them a drastic loss of skilled artisans. It was to be another 12 years before Parliament repealed these heavy tax levies.

Meanwhile, in Bavaria, Germany, optical glass working was undergoing a bit of a Renaissance. A Swiss bell maker turned glass worker Pierre Guinand, under the aegis of Joseph Fraunhofer, hit upon a way of making larger blanks of both flint and crown using a new and improved stirring process. The German furnaces, unlike those used in England, were still fuelled by wood but didn’t generate the same kind of heat as coal, rendering the homogenisation of the melt more problematic unless a more effective way of stirring it were achieved. Guinand succeeded in that goal where many others failed. Fraunhofer however, was a paranoid soul and, as a result, the secret was closely guarded.

Some of the greatest British scientists of the age tried everything to get their hands on the details of the new German technology. Traditional diplomacy quickly descended into bribery (Fraunhofer was offered £25,000 for information), but without success. Circumstances changed though, as they invariably do. In the summer of 1826, the Mozart of practical optics died prematurely, opening the way for glass makers to sell their secrets to the highest bidder. It marked the end of Fraunhoferian hegemony and the beginning of a new age in British optical glass making.

Cooke learned of these new techniques and applied them to build his first ‘serious’ telescope, a 4.5” equatorial, for a well-to-do lawyer, William Gray. McConnell describes the process of preparing the glass in her book, Instrument Makers to the World:

“Glass for optical purposes was not blown or rolled, but allowed to cool slowly in its pot, then removed and broken up. Flawed pieces were discarded and the remaining blocks cut into disks. These were sent to the optician who ground them into the shape of a lens with perfectly spherical surfaces. The convex shape was ground in a saucer shaped iron plate covered with pitch, hatched by cross grooves to take away the waste material. The pitch was warmed and covered with rouge - a fine abrasive. The lens was then rubbed and turned by hand or machine to achieve the desired curve.”

Beyond that, little else is known about the precise techniques Cooke employed to figure his object glasses. But what we do know is that the prescription of the early Cooke objectives differed a little from the standard Fraunhofer template, with the latter usually (although there are some variations I understand) having a more strongly curving outer crown element than its Fraunhoferian counterpart. What’s more, the fourth (innermost) element of the Cooke object glass was flat and so didn’t require figuring.

The 4.5” equatorial was apparently a great success, for news of its quality spread far and wide. Cooke made more instruments and built his reputation. His second large commission was for a 7.5” equatorial for a Mr. Hugh Pattinson. When his friend, the noted astronomer, Isaac Fletcher, had a chance to evaluate its optical and mechanical performance, he was so duly impressed that he wrote to Sir George Bidell Airy, the then Astronomer Royal (the man who, quite literally, divided the world in two, by establishing the new Prime Meridian at Greenwich) suggesting that Cooke be commissioned to make the lens for the Cape Observatory in South Africa. And although that job had already been contracted out to another party, it certainly helped Cooke climb that ladder of notoriety and finally enter the conversations of the inner sanctum of the astronomical elite.

The Victorian Era was no Antonine Age. Indeed the Imperium Britannicum had not seen a year free of the ravages of war in all the days of Victoria’s reign. And while the construction of large equatorially mounted refractors were statements of scientific prestige, Cooke & Sons played their dutiful part in helping to sustain the machinery of Empire; theodolites for surveying, spyglasses for naval officers, range sights for more accurate killing machines, and magnetometers for mining geologists in their ever more efficient plunder of colonial resources. With the railway coming to York in 1839, many new opportunities afforded themselves across the country and Cooke’s goods found their way into every major optician’s premises from Sutherland in the north to Cornwall in the south. Cooke also displayed a penchant for practical horology and set his considerable mechanical abilities to good use, manufacturing turret clocks for church towers.

In 1855, Cooke moved to bigger premises, the Buckingham Works, at Bishophill in York, where factory methods of production were first applied to optical instruments. Employing six workmen and one apprentice, everything, with the exception of the glass blanks, were made in situ; workshops for glass, brass and wood, and a foundry where all but the largest castings were made. In the same year, Cooke decided to bring his new wares to the continent, exhibiting his instruments at the Universal Exhibition in Paris- the NEAF of its day. His gamble paid off, for he came away from the event with a First Class Medal for his clock-driven equatorial mounting and a ringing endorsement from the chattering classes.

Figure 1. An instrument for a well-heeled amateur c. 1899. Image credit Doug Daniels

Cooke received commissions for several more large refractors ( from 5-10” aperture), but perhaps the most prestigious of all came from an order by Prince Albert, who, in 1860, summoned Cooke to Osborne House, on the picturesque Isle of Wight, to discuss the construction of a telescope for the viewing pleasure of the Royal Family. They chose a fine equatorial refractor of 5.25” aperture.

Arguably one of Thomas Cooke’s greatest achievements was the construction of the 25 inch 'Newall' refractor. Built for Robert Stirling Newall, the story behind the completion of this great telescope is a particularly sombre one. After accepting the commission for the giant lens, Cooke greatly underestimated the length of time it would take to complete it. Indeed, he surmised that it would take no more than a year. What is more, in an attempt to undercut a quote from his rival, Sir Howard Grubb, he charged too little for the work.

These realities conspired in such a way that Cooke failed to meet several new deadlines he agreed with Newall. What is more, Newall accused Cooke of ‘taking his eye off the ball’ as it were, and even threatened to withhold further down payments on the project. The probable reality on the ground, as McConnell convincingly argues, is that Cooke was under enormous pressure to complete other sizable commissions on time. Thomas spent his twilight years a sickly man, finally giving up the ghost on October 19, 1868, aged 61.

In his Will, Cooke bequeathed ‘everything’ to his wife, who immediately instructed her sons to see the Newall project to completion and the telescope transferred to his estate in Ferndean, Northumberland. It arrived in 1870 and became fully operational a year later. The Newall telescope enjoyed the distinction of being the largest refractor in the world for only a year, when Alvan Clark’s latest monster refractor, erected at the US Naval Observatory, Washington D.C., literally inched it out of first place in 1872. After Newall's death, the instrument was donated to Cambridge University and lauded for its fine optics. By the 1950s however, the Golden Age of Astrophysics having come and gone, the telescope fell into disuse. Finally, in November 1958, the decision was made to sell it to the Greek National Observatory, which housed it in a magnificent dome atop Penteli Mountain, just a day’s walk north of Athens.

Messrs. Cooke continued to secure enough orders to keep the business (which now had grown to a workforce in excess of 100) ticking over, and in the decades ahead, continued to provide both private individuals and public observatories scattered across the world with large instruments, including an 18” refractor for the Brazilian National Observatory.

Several optical firms in the United States and in Germany were employing new and more sophisticated lens making techniques that rivalled or exceeded the quality produced by the Cooke brothers. To add insult to injury, many other, smaller firms were beginning to compete with the British optical giant, undercutting their powerful rival.

T Cooke & Sons evolved and adapted to their changing circumstances as best it could. For example, the company was known to supply telescopes for re-branding, such as the fine 4” F/15 refractor supplied to Ross of London (shown below), or the elegant astronomical instruments of Negretti & Zambra (active 1850 – 1999) which often employed high quality Cooke objectives.

Figure 2. A 4” F/15 Cooke branded A. Ross. C. 1860. Image credit: Richard Day

It is difficult to assess how well T. Cooke & Sons penetrated the European market. Interestingly, a study of the origin of the wealthy Hungarian astronomer Miklós Konkoly-Thege’s (1842-1916) instrumentation from 1870 to 1910, conducted at his private Budapest Observatory, shows that 66% of the instruments came from German manufacturers and 17% from other foreign countries, mostly England. An interesting trend emerges if one looks more closely at the individual decades. Throughout the 1870s, there were apparently more English than German instruments, due, no doubt, to Konkoly’s earlier trip to the English workshops. In the 1880s, the number of German instruments increased, but also the number of instruments made in Hungary. That said, by the turn of the century, German instrumentation dominated.

The decline in British optical ingenuity was well underway by the end of the 19^th century. But that wasn’t the end of the story, for the Company was about to be restored to an even greater level of prestige, when Messrs. Cooke took on an extraordinary young man under their wing. His name was Harold Dennis Taylor (1862-1943) and his ingenuity became the brain and glory of T. Cooke & Sons in the post Victorian era.

Beginning his professional career as a trainee architect, Taylor soon became bored and disillusioned with it. He was offered an apprenticeship – which he enthusiastically accepted - with the Cookes at the Buckingham Works. McConnell describes the culture of his new work setting upon the young man’s arrival.

“At the time of his arrival, optical design was, as it had been since the time of Thomas Cooke senior, a matter of trial and error based on experience and practice, with only a token nod to theoretical formulae.”

The elder Cooke, like all other opticians of his time, probably relied heavily on visual inspection of images through his objectives in the assessment of optical quality. Dr. Jackson describes the ‘litmus test for achromaticity’ as was then employed by telescope makers:

“The examination of a bright object on a dark background, as a card by daylight, or Jupiter by night, which high magnification powers affords as is well known, the severest test of the perfect achromaticity of a telescope, by the production of green and purple borders about their borders in the contrary case.”

Judging by their many happy customers, the Cookes must have done extremely well in their task. That said, Taylor was a different kettle of fish to the Elder Cooke. He quickly absorbed the work on diffraction set out by G.B Airy and established new and higher standards of optical testing and evaluation. Within a year, he had designed a novel kind of photographic exposure meter. Several other patents followed – mainly camera lenses – some of which he sold to Messrs. Cooke outright, and others he received a royalty from. By 1893, aged just 30, he was placed in charge of all optical projects, followed two years later by a seat on the Board of Directors. For the next two decades, Taylor dedicated himself to the advancement of optical knowledge. In 1891, he published a new treatise on refractor optics, The Adjustment and Testing of Telescope Objectives, followed in 1906 by a System of Applied Optics, which still serve as invaluable resources today. Arguably Taylor’s crowning technical achievement was the design and construction of a new kind of refractor objective; an instrument that could be used both photographically as well as visually. Enter the remarkable photo-visual triplet.

By the end of the 19^th century, the overwhelming majority of public observatories were equipped with large equatorial refractors. The larger instruments, of course, produced a noticeable colour fringe around bright objects. Experienced astronomers just learned to ignore it, but the secondary spectrum proved disastrous in long exposure photographic applications. Taylor’s new triplet, first produced in 1892, consisted of an outer light baryta flint lens, a middle borosilicate flint element and a light silicate crown comprising the innermost element. An air space was placed between the second and third element. Designed to be used in an F/18 format, the lens produced a wonderfully flat, aberration-free image with colour correction an order of magnitude lower than anything seen before. Needless to say, these telescopes proved hugely popular as the new bulwarks of astrophotography, finding their way into observatories on every continent.

Like all dynastic businesses, the end came slowly and unpredictably for Cooke & Sons. In the 20^th century, the firm amalgamated with Troughton & Simms (London) in 1922 to become Cooke, Troughton & Simms. By 1915 however, Vickers had acquired a 70 per cent stake in the business and by 1924, it became a wholly owned subsidiary of the same company.

In the aftermath of the Second World War, Vickers continued to thrive, selling microscopes, surveying equipment and a variety of high precision scientific instruments. Finally, in 1989, the business was purchased by the California based company Bio-Rad Micromeasurements. Vickers decided to deposit the firm’s archives and collection of scientific instruments with the University of York. The instruments are now on display in the Department of Physics and the archives are cared for by the Borthwick Institute for Archives. The Collection also includes a number of printed books, which embody a special collection in York University Library.

Figure 3. The author’s 1960’s Vickers binocular microscope.

Voices from the Grave

Is it possible to divine information regarding the general optical quality of the Cooke refractors that found their way into the private observatories and homes of Victorian gentlemen scattered across the world? One way forward is to explore the comments of historical observers who had used Cooke refractors during the course of their careers.

We shall begin with William Rutter Dawes (1799-1868), revered among double star observers for bringing us his empirical (though, as yet, unsurpassed by any pseudo-theory) formula used to work out the minimum aperture needed to resolve double stars of a given angular separation. What is less well known is that the Reverend was also a first rate planetary observer, apparently possessing extraordinary visual acuity (despite his extreme myopia) at the eyepiece. And he had an interesting purchasing history, having used refractors crafted by Dollond, Merz & Mahler, Cooke and even the shining light of American optics, a portrait painter turned telescope maker, Alvan Clark.

Dawes took an interest in Clark’s meteoric rise from early on in his career. Naturally, being an unknown in the industry, Clark at first found it hard to sell his instruments. What he needed was someone with great astronomical gravitas to champion his cause. If the astronomers didn’t come to his telescopes then he’d have to bring his telescopes to the astronomers.

And so it was in 1851, Clark wrote to Dawes, describing to him the close double stars he had observed with his newly-crafted 7.5” refractor. Impressed, the Reverend sent Clark a more extensive list of close binary stars for him to split, together with an order for the same objective!

Yet, in the autumn of his life, old ‘Eagle Eyes’ returned to a Cooke refractor. Dawes had already made some drawings of Mars in 1862 and at earlier oppositions. In 1864, he used an 8” Cooke (which later became known as the Thorrowgood), usually with a magnifying power of 258x. His drawings, wrote Richard Anthony Proctor, "are far better than any others… The views by Beer and Mädler are good, as are some of Secchi's (though they appear badly drawn). Nasmyth's and Phillips', De La Rue's two views are also admirable; and Lockyer has given a better set of views than any of the others. But there is an amount of detail in Mr. Dawes' views which renders them superior to any yet taken." Camille Flammarion concurred: "The drawings by… Dawes brought a new precision to studies of Mars."

And across the Irish Sea, to a beautiful, windswept rural estate near Milltown, County Galway, my compatriot John Birmingham (1814-1884) used a 4.5” Cooke refractor to embark on a special study of red stars, in which he set out to undertake a revision and extension of the best resource of its day on such objects, Schjellerup's Catalogue of Red Stars. In all, he included 658 such objects. This work was presented to the Royal Irish Academy in 1876 and its merit was acknowledged by the award of the Cunningham Medal. In 1881 Birmingham discovered a deep red star in Cygnus, which is called after him. He published articles on the transit of Venus and sunspot morphology made with the same telescope, corresponding regularly with the leading astronomers of his day. A lunar crater is named in his honour too.

Moving next to the Far East of Empire, at Bankura, India, Chandrasekhar Venkata Raman (subsequently knighted) the recipient of the 1930 Nobel Prize in Physics for his contributions to optical science, was fond of using a 5″ Cooke refractor. I came across one curious account Raman made whilst using this telescope to observe Saturn;

‘… not only was the Crepe ring an easy object,” he says, “ but for nearly one hour while the definition was perfect, I made out Encke’s marking in the A ring and held it steadily for practically the whole period.’

Now the Crepe ring is quite a difficult target for a 6” scope, and for many, 8” seems to the smallest aperture they’d be happy with. The Enke division (marking) is typically regarded today as a good target for a 10” instrument (for the record, I’ve personally not seen it for sure). So, was it fine optics Raman had in his 5” Cooke, or exceptional eyes, or both? I guess we’ll never know for sure!

Figure 4. The fully restored 8” F/16 Fry Telescope, at Mill Hill Observatory, London.

We return, once again, to England, and to the fondly remembered British actor and comedian Will Hay (1888-1949). Though playing the consummate idiot on stage, behind the scenes, Hay was a gentleman of encyclopaedic knowledge, with a predilection for astronomical adventure. He set up a fine 6” Cooke refractor in a private observatory established at his home in Norbury, London, to study the planets. On the faithful night of August 3 1933, Hay used this instrument and an eyepiece delivering a power of 175x to detect a prominent white spot on Saturn. The spot, located in the planet’s equatorial zone, remained prominent for a few days before mysteriously fading away. And while similar phenomena were recorded by earlier observers (Asaph Hall in 1877 and E.E. Barnard in 1903), Hay is credited with the official discovery. Curiously, Hay’s beloved 6” Cooke, like the spot he discovered, inexplicably disappeared after his death and, despite diligent attempts to locate it, we are still none the wiser concerning its current whereabouts!

Figure 5. The second drawing of Saturn by Will Hay showing the great white spot six days after his discovery sketch. Image courtesy; Martin Mobberly.

Hay wrote a wonderful, non-technical book for the newly minted amateur astronomer, Through my Telescope, in which his great charm and insight still shines through. A timeless classic if ever I’ve read one!

Figure 6. A portable 3” F/15 Cooke refractor c. 1900. Image credit; Richard Day

Modern Perceptions

I have spoken elsewhere of my own experiences with a couple of Cooke refractors; particularly the 10” at Mills Observatory, Dundee, Scotland, which I have peeped through on many occasions during my time in graduate school, and a superlative 4” F/18 Cooke-Taylor photovisual instrument. I was duly impressed with the sharp, contrasty views they both served up. But were my opinions ‘coloured’ or even ‘representative’ of what others have experienced? How did these refractors of old settle with folk who have had the pleasure of using them over years and decades? First, I contacted Douglas Daniels, President of the Hampstead Scientific Society, England, who has had the immense good fortune of using the Observatory’s 6” F/15 Cooke since 1967. Doug spoke to me about his background and how he became acquainted with Cooke refractors.

“I have always been a keen lunar and planetary observer and telescope maker since I first became seduced by astronomy at the age of 13 in 1953,” he said. “I joined the BAA in 1956 which was the year of a very close opposition of Mars. At that time, I had built a 6-inch Newtonian reflector using a mirror made by the late Henry Wildey. I was quite impressed by the performance of this instrument, both on Mars and Jupiter, but I was soon to meet another young BAA member - Terry Pearce. Terry and I became good friends (and still are!). Terry had managed to borrow a 4.5-inch Cooke from the BAA which he had set up in his garden at Chingford in Essex. I was amazed at the sheer size of it. It was on the usual Cooke, two part cast iron column and the equatorial mount was massive for an instrument of that size. But I was even more amazed when I looked through it. The detail on both Mars and Jupiter was astounding - far more contrast than with my 6-inch reflector. That was my first taste of a Cooke.”

Figure 7. Image credit: Doug Daniels.

I asked Doug how and when he first became acquainted with the Hampstead 6” Cooke.

“In 1967, I joined the Hampstead Scientific Society, “he continued, “and was able to use the 6-inch Cooke at the Hampstead Observatory. Again, 1967 was a year with a good opposition of Mars, and the detail observed with the Cooke was so good that I began to attempt photography. I built a special planetary camera with a flip mirror system to keep the planet under close surveillance waiting for clear moments to make exposures - it was a sort of single lens reflex job but without the lens! (N.B. This was 1967!). My photographs came to the attention of an American student Ron Wells, who was doing a PhD on Martian topography at University College London. Ron was working at University of London Observatory at Mill Hill - just 15 minutes from my home. I was introduced to the Director, Professor Allen, and was allowed to use the 18-inch Grubb - I had the key to the big dome for 6 months. On the same site, there were two smaller domes. One contained the Fry Telescope - an 8-inch Cooke. Once again the Cooke was the instrument that impressed most. On most nights of average seeing, it could easily outperform the 18-inch Grubb. Only when the seeing was excellent, could the Grubb show slightly more detail.”

Figure 8. Image credit: Doug Daniels

Figure 9. Some of Doug Daniel’s recorded detail of the Martian opposition of 1967. Image credit: Doug Daniels

Doug was more than happy to recount the telescope’s long history.

“The Cooke was once owned by a member- George Avenell,” Doug said, “and we know that it was in use at the observatory in 1923. It was finally presented to the Society in 1928. Prior to this we have no information. The optical tube appears to have been manufactured around 1900, but we have no hard evidence for this date. When I began using it in 1967, it was mounted on an old Cooke equatorial from a 4.5-inch instrument that was too small. It had the old Cooke falling weight drive and a worm sector - not a complete wheel that was always getting jammed. In the end we built our own heavy duty mount in 1976, driven by a stepper motor. A couple of years ago, I was in correspondence with Martin Mobberly, who was researching the 6-inch Cooke once owned by Will Hay. I was able to confirm that the Hampstead Cooke was not Hay's instrument.”

What about the telescope’s maintenance? Is it, in any sense, fastidious in its requirements?

“No,” Doug said, “not at all. The objective is best left well alone. It gets an annual wipe over with meths and a lint free cloth and every few years is checked for squaring on, which hardly needs any adjustment for long time periods other than that I discourage anyone from touching it. That's another nice aspect of refractors, they are virtually maintenance free, unlike reflectors which are constantly going out of square and need re-coating every few years.”

Doug is no stranger either to the current proliferation of telescope types, each having their advantages and disadvantages. I asked him how he thought the old Cooke faired in the scheme of things.

“As a long standing lunar and planetary observer,” he told me, “I have over the last half century, been able to compare the performance of many different instruments. Given average seeing conditions, I have found that the images produced by the 6-inch Cooke f/15 refractor at the Hampstead Observatory will surpass most if not all other instruments of equal aperture. It will outperform Schmidt-Cassegrains and Maksutov-Cassegrains of 8-inches aperture, and on occasions, it has provided better images of Mars obtained with my own 16.5-inch Dall-Kirkham Cassegrain. It is only on the rare apparitions of excellent seeing that large aperture reflectors can outperform it. I put this down to the absence of a central obstruction that reduces image contrast in all compact reflectors. The current popularity of short focal ratio apochromats is no doubt due to their portability and the need to travel to dark observing sites. But they take longer to acquire thermal equilibrium and require expensive highly corrected short focal length eyepieces to produce sufficiently high magnifications.”

Next, I canvassed the opinion of Dr. Richard McKim, Director of the Mars Section of the BAA, who has used some of the Cooke refractors in his extensive studies of the Red Planet over the last few decades. “I have used many refractors on a regular basis since the 1970s,” Dr. McKim told me, “4 cm, 7cm, 7.5 cm, 15 cm (Cooke, my own), 20 cm (Cooke, Cambridge), 30 cm (Cooke apochromat, Cambridge) and 83 cm (Meudon Observatory). The problem is, I have no basis of comparison with other makes. Until 1988, the Northumberland telescope at Cambridge had a 30 cm Cooke apochromat, as the old lens from the c.19th had worn so much it was too small to safely fit in the original cell. After the devitrification of one component, Jim Hysom made a new lens for that year, 1988. Equally sharp in definition to my eye, but of course an unfair comparison with an apochromat. Both gave marvellous, sharp images. All I can say is that Cooke achromats and apochromats give marvellous results.”

Privately owned Cooke Photovisual refractors, as you might expect, are as rare as hen’s teeth. I have oggled a 4” F/18 Photovisual and I’ve looked through one too (but only for an evening….sigh!!) but I can tell you the images of Mars it served up in a recent opposition were nothing short of breathtaking! I wanted a second opinion though and contacted Colin Shepherd, an amateur astronomer based in Jervis Bay, Australia. Colin is the lucky owner of a 5” F/17 Cooke Photovisual (c. 1902) but also enjoys observing through his modern ultra-premium 5” Astro Physics refractor. So how did he reconcile the old with the new?

“My recollection of its performance is that it delivers images on a par with my 5" Astro Physics Starfire (AP130-EFS). I discussed it with my friend Steven Lee of the Anglo Australian Observatory and we both agreed that the performance was similar other than maybe a slight light loss which is probably due to the lack of coatings on the Cooke PV objective. I have an adapter to permit use of a 2" diagonal in place of the original prism so I can use the Cooke with modern eyepieces. The biggest difficulty with using the scope other than its weight is the long tube.”

Figure 10. A pristine Cooke lens that saw first light over 150 years ago. Image courtesy; Richard Day.

These telescopes, of course, have long been considered choice instruments for measuring double stars. Curious to see what a contemporary double star observer thought of a big Cooke, I contacted Bob Argyle, based at the Institute of Astronomy, in Cambridge, England. Bob is a highly skilled binary star astronomer and author of an influential book on the subject. He has used the 8” Thorrowgood (also at Cambridge) refractor for measuring the orbital elements of hundreds of pairs.

“I'm happy to confirm that this lens is a good as you can expect for the aperture,” Bob told me, “It was specified for double star work by its original owner, Dawes, and needless to say it fulfils the Dawes limit admirably, separating pairs as close as 0".55 or possibly a little less. On the best nights here, which would not necessarily regarded as such elsewhere, the disks are perfectly round. In 2004, when Gamma Virginis passed through apastron at 0".37, I was still able to measure the position angle of the elongated image. I gather that J. C. Adams once tried to acquire the telescope from Dawes and he was of the opinion that it was of better optical quality than the 9.6” Dorpat refractor of Struve.”

Figure 11. The 8” Thorrowgood refractor at Cambridge. Image courtesy: Martin Mobberly.

Animum debes mutare, non coelum!

What are we to make of all these opinions- both contemporary and historical? For one thing, these are not the words of egregious rogues with hidden agendas! The Cooke refractors clearly delivered and continue to deliver quality views. But, in this age of the Roddier test and interferometer, how well do the ‘fine’ achromatic Victorian Cooke lenses really stack up? Alas, hard data is not available to answer this question. I dare say, prizing one away for laboratory analysis would be rather like trying to acquire a piece of the Shroud of Turin! That said, there has been a tendency in contemporary amateur culture to assess optical quality using the Strehl ratio interpretation, based on laboratory bench tests.

My guess, based on the sample of testaments I have garnered, is that many of the larger Cooke refractor object glasses would most probably not be figured to an accuracy much beyond a smooth ¼ wave level at the eyepiece, corresponding to a Strehl ratio not much greater than 0.8 (their peak Strehl at green wavelengths being higher). Contrast that to contemporary, top-of-the-range apochromatic triplets, which can exhibit Strehl ratios higher than 0.95. Yet, as we have seen, many seasoned observers are, and continue to be, more than satisfied with the views these classical refractors of yore deliver. How can these significantly different quantitative assessments of optical quality be reconciled?

One explanation is that we are looking at this question far too simplistically. Doubtless, it is not how well the scope performs in laboratory tests that is at issue here. What counts is how well those quality images are attained and maintained under typical conditions IN THE FIELD. In my previous correspondences with optics authority Vladimir Sacek (www.telescopeoptics.net), I am reminded that, at least in terms of perceived image quality, under field conditions, there's not much difference between a true 1/8 wave and 1/4 wave p-v level optic. Indeed, all the optical greats – Rayleigh, Conrady and Marechal - to name but a few - concluded much the same. What is paramount however, is the added error, from a number of other sources, and as I’ve described elsewhere, many of these sources are close to home and may have more to do with the design of the telescope and its thermal management than has previously been acknowledged.

I asked Es Reid, a highly experienced optical engineer based in Cambridge, England, for his take on this matter. “I reckon that, although glass prescriptions and anti-reflection coatings have improved over the years, the methods of final polishing and figuring have not changed to any great extent. A long, heavily mounted refractor should outperform a short one on a lighter, modern mounting. Eyepieces can always be simpler for the same power and glass will behave thermally as glass always has. Eyes plus brain can select the best wavelength to some extent to give highest acuity in coloured images, so I think all of these factors let the old instruments compare very well indeed with modern equipment. It is also interesting that glass companies nowadays have to knock out toxic chemicals, for safety reasons, that can restrict colour correction. The inherent and continual smoothness of a refractor wave-front and the high entrance both help a lot in keeping an image sweet!”

Sweet indeed! For me, the quality views delivered by these antiquated telescopes are the result of many things coming together in one package; their unobstructed optics, their simplicity of design; a doublet objective made from thin lenses which acclimate rapidly and completely, their greater elevation off the ground, well away from any sources of ground turbulence and anthropogenic heat and their great depth of focus making accurate focussing easier and, of course, their generous image scale (1/f). Couple that to the ruthless genius of the human eye to ‘filter’ the ‘signal’ from the ‘noise’ introduced by the purplish unfocused light of the standard achromatic object glasses, and you can easily understand why they would delight a patient or experienced observer.

Such attributes, of course, make them ideal as measuring instruments and that’s precisely why, in my opinion, they were built with such enthusiasm. Indeed, J.B Sidgwick, writing in his influential book from 1971, reminds us once yet again of the advantages of these long focus refractors. They enjoy, he says, “greater independence of temperature variations, with steadier images and higher possible magnifications than with a reflector of the same aperture,” (pp 420-1). Seen in this light, these telescopes are not the ‘extinct dinosaurs’ contemporary astronomical culture would have us believe. As we have seen, there is every indication that these instruments are ‘highly adapted specialists’, supremely useful in the noble art of astronomical mensuration.

In retrospect, it seems daft (it really does!) that these instruments should ever fall out of favour with amateur astronomers. But that, sadly, is the reality on the ground. Doug Daniels thinks he knows why.

“At the end of the day,” he said, “I think the reason that the long focal length refractor has 'fallen out of favour' is simply due to the 'long focal length'. It was (is) an instrument ideally suited for a permanent observatory and to be used for visual observation. Both of these applications seem to be out of favour today. Because of the proliferation of light pollution, users want portable equipment to drag to dark locations, some going as far afield as Bermuda or the Canary Isles. Because of the comparative ease afforded by digital imaging, users demand faster focal ratios. Because of the 'competition' amongst enthusiasts to produce ever more astounding images of 'deep sky' subjects and the relatively small size of CCD chips, users demand wider fields of view. None of these requirements can be met with an f/15 refractor. BUT, if you are interested in observing and drawing planetary detail, or observing and measuring double stars and you have the space to build a 10 foot diameter dome in an unobstructed pollution free location, then the 'old fashioned' achromatic refractor is hard to beat. If my astronomical 'fairy godmother' could wave her wand and grant my wish, it would be for an 8-inch f/15 Cooke refractor made at the end of the 19th century at Bishophill in Yorkshire.”

Figure 12. In Amatam Memoriam: The author’s fine 4” F/15 refractor, inspired by the workshops of T. Cooke & Sons.

While the sun has long since set on both the Roman and British Empires, it is all too clear that the legacy of Thomas Cooke lives on in the people who have had the pleasure of using these fine telescopes – whether a gentleman’s 3” glass or a large observatory class instrument. Small wonder that Cooke’s erudite obituarist was compelled to write in 1868, ’[To] our English Fraunhofer…. Whose science and skill had restored to England the pre-eminent position she held a century ago in the time of Dollond”.

In bringing this essay to an end, I am curiously reminded of the ancient Greek philosopher, Socrates, who once remarked that all learning consists of that which is pre-existing in memory. Having recounted the allegory of Thomas Cooke & Sons to you and extolled the virtues of some of their optical wares, it is my fondest hope that this will all seem familiar to you too!

You can read a great deal more about historic telescopes (antique and modern) like these in my up-and-coming book, “Choosing and Using a Classic Telescope” which will be published by Springer in 2012.

I would like to extend my thanks to the many people who were kind enough to share their opinions and experience with Cooke refractors including, Martin Mobberly, Douglas Daniels, Richard McKim, Bob Argyle, Es Reid, Col Shepherd and Richard Day.

Digging Deeper

• Argyle, B., (2007), The Observatory, 127, pp 392-400.
• King, H. C. (1955), The History of the Telescope, Dover.
• McConnell, A (1992). “Instrument Makers to the World: History of Cook, Troughton and Simms from 1750" Ebor Press.
• Jackson, M. W., (2000), Spectrum of Belief -Joseph Fraunhofer and the Craft of Precision Optics, MIT Press.
• Sheehan, W. & O’ Meara, J., (2001), Mars; the Lure of the red Planet, Prometheus Books.
• Sidgwick, J.B, (1971), Amateur Astronomer’s Handbook, Faber and Faber.

Some technical background on the achromatic refractor to be found on Vladimir Sacek’s excellent online resource;
http://www.telescope-optics.net/polychromatic_psf.htm

More on the Fry Telescope, ULO, and its restoration;
http://www.ulo.ucl.ac.uk/telescopes/fry/

Martian observations with the Fry Refractor, ULO;
http://www.ulo.ucl.ac.uk/images/mars2003/

A 5” Cooke meets a 5” D&G
http://www.cloudynights.com/item.php?item_id=742

An inconvenient truth about small classical refractors: English, N. 2010, “Stranger than Fiction,”
http://www.cloudynights.com/item.php?item_id=2529

English, N. (2010), Choosing and Using a Refracting Telescope, Springer.

More revelations concerning the stability of long focus refractor images:

• English, N., A Glass Act, 2011, Astronomy Now, Vol 25, No. 2, pp 38-41.
• Mobberly, M. and Goward, K., Will Hay and his Telescopes
  http://martinmobberley.co.uk/WillHay.html

The astronomical adventures of C.V. Raman
http://www.ias.ac.in/currsci/25oct2010/1127.pdf

More on the Hampstead Scientific Society;
http://www.hampsteadscience.ac.uk/