New Life for the Great Melbourne Telescope: Australian groups are restoring what was once the world's largest equatorially mounted telescope.
However, the GMT eventually fell on hard times. Challenges in printing delicate nebulae sketches plus a worldwide financial crisis in the 1890s that also took its toll on Australia prevented the publication of two decades of observations, making it seem that the telescope had been unproductive. In part as a result, telescope optician George W. Ritchey (who later helped figure Mount Wilson Observatory's 100-inch mirror) famously passed off the GMT as a "failure" in 1904.
In 1945, the GMT was sold to Mount Stromlo Observatory near Canberra. There, it was reconstructed three times (1950-55, 1960-61, and 1989-91), including being fitted with two different 50-inch borosilicate glass mirrors. During each rebuild, more of the GMT's original parts were removed. Fortunately, Museums Victoria retrieved them for safekeeping in off-site collection storage in the 1980s. The telescope's last incarnation, as an automated instrument that discovered a form of dark matter in 1993, incorporated just five assemblies of the GMT's original mount. In January 2003, the telescope was engulfed in raging wildfires that swept Mount Stromlo.
The upshot of this story: Many today are left thinking that the GMT was originally flawed and then was taken out of commission. However, neither of these "facts" is true. For the past decade, several organizations in Melbourne have been documenting, cleaning, and reassembling the GMT from its original parts--70% of which survive at Museums Victoria. Their goal is to reconstruct the instrument with as much historical authenticity as possible, but with modern optics so the public may look through it and amateurs can use it for research. In so doing, they also seek to restore the GMT's reputation and its rightful place in history.
Chronology of the GMT
Beginning with William Herschel's large reflecting telescopes in the 1780s, astronomers used a half dozen major instruments to reveal thousands of faint nebulae amidst the stars in the northern heavens; in the 1830s, William's son, John Herschel, discovered countless more in the southern heavens. Were these nebulous clouds of gases relatively nearby, lying within our own "sidereal system"? Or were they independent "island universes" at such vast distances that their stars could not be resolved?
Those fundamental questions might be answered by stationing a capable observer at the eyepiece of a gigantic telescope in the Southern Hemisphere, to compare later observations with John Herschel's meticulous drawings of decades earlier to see whether any nebulae had changed appearance. In 1852, the British Association for the Advancement of Science and the Royal Society of London jointly appointed a Southern Telescope Committee of the British Empire's top astronomers, to determine what kind of instrument should be built, who should build it, and where it should be located.
By 1853, the committee had settled on a design and appointed a subcommittee to oversee a proposed contract with Dublin telescope engineer Thomas Grubb--but then the Crimean War (1853-1856) halted everything. It seemed the project was dead.
A decade passed. In 1863, the British colony of Victoria--wealthy from a gold rush since 1851 and hankering to establish itself as a cultural center of learning--authorized 5,000 [pounds sterling] for a large telescope for Victoria's capital city of Melbourne. The Southern Telescope Committee dusted off its plans. After extensive discussion about technical advances over the intervening decade, it reaffirmed the major concepts of the original plan. The telescope's final specs were thus a combination of conservative and innovative design choices.
Something Old, Something New
The proposed mirror would be 48 inches in diameter, made of polished speculum metal--a white bronze alloy of copper and tin whose casting, figuring, and maintenance were well understood. Despite the adaptation of a process for depositing a thin coat of silver on glass to telescope mirrors by physicists Leon Foucault and Carl August von Steinheil in 1857, casting and silvering such a large glass mirror was as yet untried; the long-term stability of glass as a material and silver as a coating were unknown; and silver, although more reflective than speculum metal, tarnished more quickly. Moreover, the use of speculum metal would minimize differences in reflectance as a factor in comparing observations made decades apart.
The telescope was to have a Cassegrain focus rather than Herschelian or Newtonian, a pioneering choice that let the astronomer observe more safely from the ground or low steps instead of perched two or three stories up in the air in the dark. Nebulae were to be drawn by hand by a visual observer using a precision bifilar micrometer (photographing them was impossible with the very slow emulsions of the 1860s), but the secondary mirror cell could be swapped out for a camera to allow photography of bright objects at prime focus.
The mounting would be an equatorial made of cast iron, rather than an alt-azimuth built largely of wood (like John Herschel's) or masonry (like Lord Rosse's). Despite the telescope's moving mass topping 18,000 pounds, Grubb devised a pioneering system of counterweights to relieve frictional loads on bearings. Thus, one person could slew the telescope with ease, and a gravity-powered clock drive acting on the polar axis could compensate for Earth's rotation. The telescope would be sited at low elevation near a thickly settled area (a practical cost-saving choice, then the norm).
The order was signed with Thomas Grubb in early 1866, and the entire 48-inch telescope was completed by February 1868. Construction was supervised primarily by Grubb's 21-year-old engineering student son Howard, who was in the process of launching his own illustrious telescope-making career. The Grubbs cast and figured two speculum primary mirrors, so that (following the method used by John Herschel in South Africa) one could always be used for observing while the other was being repolished to remove tarnish. They deliberately designed the English cross-axis equatorial mount to be strong and stiff enough to carry the mass of a 60-inch reflector, should a larger telescope someday be desired. And to support the 2,200-pound 48-inch mirror without noticeable distortion at all viewing angles, Howard refined his father's 48-point floating suspension system of counterbalanced triangular levers--a design still often used today.
After meticulously inspecting the assembled GMT in the Grubb factory yard and testing the mirrors and photographic system on various celestial objects, including the Moon, double stars, and nebulae, the Southern Telescope Committee was delighted with both its optics and mechanics. The GMT was disassembled, crated, and shipped to Australia, arriving at Melbourne in November 1868. It was reassembled on bluestone (basalt) foundations and piers. A rectangular telescope house with a novel roll-off roof--likely the world's largest at the time (although not the world's first) and now the oldest surviving--was built around it so observing could be done in the open air (a choice both conservative and prophetic). At last, in mid-1869, the GMT turned its giant eye up toward the spectacular southern heavens.
Almost immediately, complaints arose about poor imaging. Finger-pointing letters sailed between the Northern and Southern Hemispheres, some printed in British newspapers. The Southern Telescope Committee and the Grubbs had made what in retrospect was a problematic call. Instead of sending Howard south with the telescope to supervise its installation, the intended primary observer - a Cambridge mathematician named Albert Le Sueur - was hired by the Melbourne Observatory. Le Sueur was sent to the Grubb factory to witness the casting of the mirrors and other key milestones, and to learn how to reassemble and operate the telescope, as well as the mirror grinding and polishing machine. But once en route to Australia, Le Sueur was on his own and had to make judgment calls. Grubb had specified that the mirrors' protective coating of shellac be removed with pure ethyl or methyl alcohol. But pure alcohol was in short supply in Victoria. Instead, Le Sueur started the job using methylated spirits and finished with pure alcohol. Although the smaller mirrors retained their high polish, either the shellac applied to one of the primary mirrors in Dublin or parts of its surface may have been contaminated. A cloudy residue yielded diffuse reflections, and accusations flew. Making matters worse, an irate Melbourne amateur astronomer wrote angry letters both to British and Australian scientific societies, claiming (inaccurately) that "[t]he instrument has cost us colonials 14,000 [pounds sterling]" and asserting, "It is a grievous failure."
Over the next year, Le Sueur hand-polished both primaries with washed chalk to near perfection, but in August 1870 he resigned in frustration and returned to England. Although he and Melbourne Observatory director Robert Ellery were staunchly backed by the observatory's capable Board of Visitors, the bad-PR mud stuck.
In the meantime, no one in the Southern Hemisphere had experience in working with a telescope of such a large aperture, so on nights of ordinary seeing, a person accustomed to observing with smaller apertures could find the GMT's performance on a planet or double star actually "disappointing," noted Ellery. But "on really good nights it is quite different ... indicating an optical perfection which under other conditions was not apparent."
Moreover, the Southern Telescope Committee's intended observing program--visual observations of nebulae to detect changes--was highly subjective, depending on individual observers' visual acuity and artistic skill. Indeed, for those nebulae that were distant galaxies, detecting any change (short of an extragalactic supernova) over mere decades was downright impossible--although, of course, absence of change was a crucial test.
Finally, after the financial crash of the 1890s nearly halved the observatory's budget at a time when it had committed heavy resources to the international Astrographic Catalogue, the GMT was not adequately maintained and most of the astronomers' meticulous drawings were never published.
Against this background of woes, in 1904 Ritchey parenthetically commented in a treatise on making and testing telescope mirrors, "I consider the failure of the Melbourne reflector to have been one of the greatest calamities in the history of instrumental astronomy; for by destroying confidence in the usefulness of great reflecting telescopes, it has hindered the development of this type of instrument ..." Ritchey never looked through the GMT, much less tested it. Nonetheless, his offhand remark was the last straw for the GMT's reputation.
Phoenix from the Ashes
Fast forward a century to the year after the devastating Mount Stromlo bushfire. Members of the Astronomical Society of Victoria (now 1,000+ members strong) began brainstorming with managers at Museums Victoria. If the major castings of the equatorial mount were retrieved from Mount Stromlo, could the GMT be reconstructed from surviving original parts using modern optics? In November 2008, the GMT's cube, polar axis and bearing mounts, declination axis and bell-shaped housing, north bearing quadrant, modern mirror cell, and fragmented 50-inch mirror were retrieved from the incinerated building and trucked to Melbourne.
Extensive talks led to a formal memorandum of understanding among Museums Victoria (owner of the GMT parts and eventually of the completed GMT), the Royal Botanic Gardens Victoria (manager of the Melbourne Observatory site where the original GMT house still stands), and the Astronomical Society of Victoria (ASV, source of volunteer labor with optical and engineering skills). The projected cost of the restoration project was estimated at A$3,000,000, a sum the groups hoped to raise via grants and donations. In 2013, the Bureau of Meteorology also joined as Melbourne Observatory had operated the Victorian Weather Office from 1863 to 1907.
Since 2009, a dozen or so ASV volunteers--mostly retired engineers and optical specialists--have spent Wednesdays working under the leadership of museum curators and conservation staff in Museums Victoria's cavernous warehouse, complete with 10-ton overhead crane. Because the telescope is part of Victoria's State Heritage Collections, decisions are guided by the Australia International Council on Monuments and Sites (ICOMOS) Burra Charter for the conservation of historic structures.
Volunteers began by photographing, weighing, and measuring every component as it existed (including any Mount Stromlo modifications) and ascertaining their positions and functions in the GMT. Then came the writing of a proposal for the work needed to restore the telescope. Every part and fastener was numbered and tracked; by the end of 2017, the inventory identified some 1,200 components and included some 700 drawings.
Because no original engineering records survived the 2003 Mount Stromlo bushfire (or an earlier one in 1952), CAD drawings had to be reconstructed for the entire GMT--including the shapes and dimensions of missing components (40 to 50 major parts plus several hundred knobs, gears, and other small items). Although schematic drawings for some larger assemblies were published in 1868, for other parts all that remained were several 19th-century photographs. Finally, a gigantic flow chart was drawn up to identify critical paths: which jobs needed to be done in what order, including contracting for custom-made missing pieces.
Then came the heavy physical labor: unbolting and disassembling every subassembly and cleaning every part and fastener by hand down to bare metal. To support the project, both individuals and philanthropic organizations donated half a million dollars plus crucial in-kind design and engineering work.
In late 2014 came a trial reassembly of the cleaned parts. The cube and southern cone of the polar axis were bolted together, the upper and lower bearings placed on a giant frame, and the tube suspended from the overhead crane for a staged photo shoot giving a feel for the GMT's mass, size, and eventual appearance.
Optics and Observatory Plans
Only one of the two original 48-inch speculum mirrors is extant; the other broke at Mount Stromlo c. 1948. The GMT restoration team intends to put the remaining mirror on static display with its 48-point flotation system. For actual use, the team first sought donation of a secondhand mirror (including, through this author in 2012, the 48-inch mirror from the former Table Mountain Observatory). All attempts ran afoul of either legalities over an international transfer or lack of a central hole for a Cassegrain focus. So in October 2017, the restoration project issued a formal Expression of Interest document to the optical industry for a custom mirror of low-expansion borosilicate glass, fused silica, or glassy ceramic.
The original GMT primary mirrors had a focal length of 366 inches (f/7.625); at the f/41.5 Cassegrain focus, the effective focal length (EFL) was 1,994 inches. That gave an actual field of view of just 13 arcminutes with the lowest-power eyepiece, somewhat narrow for public viewing. Although current plans still call for reconstructing the GMT's iconic open lattice tube to full length, actual optical focal lengths will be somewhat shorter to allow a usable field of 17 arcminutes while keeping the secondary obstruction under 14 inches. The new perforated Cassegrain primary is to be f/7.0 (336 inches); with a new secondary, the final focal ratio will be f/31.2 (EFL 1,498 inches).
By June 2018, it was evident that the quoted prices for the new optical train were higher than the budget could afford, so the ASV is looking for more cost-effective alternatives. One possibility is for a team of ASV optical specialists to grind, polish, and figure the hyperboloidal secondary themselves, using a secondhand 310-mm Schott Zerodur blank.
Meanwhile, in February 2018, the Melbourne Observatory was permanently added to Australia's Federal Heritage List, opening up both greater protection and a new source of funds for the GMT restoration. In May, the Australian federal government awarded a grant of A$250,000 (about U.S.$185,000) to restore the roll-off roof of the GMT's telescope house to working order. The bluestone piers must be rebuilt as well (the original stone blocks went to Mount Stromlo but were discarded when the mount was modified). In 2013, an inspection revealed that the piers' remaining bluestone foundations were still in excellent condition. Restoration of the brickwork is expected to be completed by 2019.
The team is also focusing on rebuilding lost telescope components. The biggest missing part--the upper lattice section of the telescope tube--is now being fabricated in the workshop of Scienceworks, another Museums Victoria institution. And the Australian Antiquarian Horological Society has generously offered to make replacement telescope clockwork. The original clock drive was gravity-powered by descending weights that could run for up to two hours before requiring winding; its speed (sidereal, diurnal, or lunar) was regulated by a centrifugal governor and differential gearbox. The current plan for most observing is to drive the polar axis with an unobtrusive electric motor. However, in accordance with the Burra Charter, which advocates a cautious approach to change but allows for the introduction of new material during reconstruction as long as it doesn't damage a site's historical significance, a working reproduction of the missing clock drive will be installed.
The current goal is to fit the telescope with its new, full-length tube and put it on display at Scienceworks by June 2019, the 150th anniversary of the GMT's first light from Australia. There, its magnificence will be seen by the public as well as by the media, and by federal and state politicians. That, it's hoped, will result in more funding to support the original ambitious goal: reconstructing the GMT to full working order inside its original observatory building to make it the largest telescope in the Southern Hemisphere for astronomy public outreach.
* Contributing Editor TRUDY E. BELL, formerly an editor for Scientific American and IEEE Spectrum magazines and senior writer for the University of California High-Performance AstroComputing Center, is the author of a dozen books and over 500 articles. Her journalism prizes include the 2006 David N. Schramm Award of the American Astronomical Society. She can be reached at firstname.lastname@example.org.
She gratefully acknowledges information and images from the following sources: Roger Ceragioli, Matthew Churchward, Barry Clark, Philip Edwards, Richard Gillespie, Frederick Orthlieb, Jim Pollock, Steve Roberts, John Robinson, and Matilda Vaughan. Some information they and primary source documents provided corrects errors in earlier secondary literature.
FURTHER READING: The 19th-century debates over the design and viability of the GMT are detailed in Richard Gillespie's book, The Great Melbourne Telescope (Melbourne: Museum Victoria Publishing, 2011).
Caption: THE MOON REVEALED Joseph Turner, a local photographer and amateur astronomer, was appointed lead astronomer at the GMT after Albert Le Sueur's resignation in May 1870. Though Turner completed many fine drawings and spectroscopic studies, he also mastered the telescope's photographic equipment. Turner captured this view of the Moon through the GMT in 1875.
Caption: COLOSSAL INSTRUMENT The London Illustrated News dedicated a full-page review to the Great Melbourne Telescope in November 1868. The writer enthusiastically described the mechanics of the Cassegrain focus, which gave the telescope "some important advantages, foremost among which is the fact the observer is never more than about 4 ft. off the ground." The benefits of the lattice-work optical tube were listed as "great stiffness and freedom from tremor, combined with lightness and freedom from currents of air ..." The scope was also praised for its "improved" equatorial mounting and "efficient photographic apparatus.
Caption: SOUTHERN SUCCESS Following improvements of developing and printing processes, astronomers at Melbourne Observatory began photographing nebulae with the GMT in 1882. Taken on February 26, 1883, Turner's image of the Great Orion Nebula was one of the earliest photos of nebulae taken in the Southern Hemisphere.
Caption: OBSERVATORY OVERVIEW This image offers a view of the Melbourne Observatory as it appeared in the 1880s. The main observatory building is at center. The GMT, workshop, and engine for the mirror-grinding apparatus are at the far right.
Caption: PHOTO ASSIST Since no original drawings for the GMT survive, photographs such as this one, which shows the telescope as it was in 1875, have been crucial in helping the restoration team reverse-engineer the telescope's assembly. The weight hanging next to the man (who is believed to be Joseph Turner) and the chain going over the top of the larger pier were part of Thomas Grubb's design to relieve frictional load on the main bearing of both axes so the telescope could be clock-driven. The clock drive is next to the weight.
Caption: AFTER THE FIRE In 1945, the GMT was moved to Mount Stromlo Observatory, where it was modified three times. The only original parts engulfed by a wildfire in 2003 were the cube (center), the polar axis south or upper cone (above the cube), the declination axis with its bell-shaped housing (left of the cube), and the north bearing quadrant (unseen below the wall). Fortunately, they survived. This photo was taken in 2008 before those pieces were retrieved for restoration.
Caption: THE GREAT GRINDER While most ASV volunteers focused on the telescope itself, the oldest ASV team member, David Linke, devoted his attention exclusively to Thomas Grubb's two-ton mirror-grinding and polishing machine. The apparatus was used several times in the 19th century to remove tarnish that gradually accumulated on the GMT's speculum mirrors.
Caption: EXEMPLARY EFFORT Some 8 to 15 members of the Astronomical Society of Victoria most of them retired engineers, machinists, and other technical professionals--meet at the Museums Victoria warehouse every Wednesday to work on the GMT restoration. Much of the work has been done by hand or using original techniques. Here Graeme Bannister was photographed in 2011 removing rust from the central cube.
Caption: DAD'S ARMY ASV volunteers at the Museums Victoria warehouse stand in front of a wall-sized photograph of the GMT as it appeared in 1875--an image essential to the task of recreating detailed engineering designs. Nicknaming itself "Dad's Army" after a 1970s BBC TV sitcom about the work of a detachment of Home Guard older than those sent into combat during World War II, some two dozen ASV members have dedicated well over 20,000 hours of volunteer labor over 400+ weekly sessions since 2009.
Caption: REFLECTED GLORY This image, taken in 2014 during a photo shoot to assist with fundraising, shows the polar axis of the GMT's mount--consisting of the cube and upper (south) cone--joined with the bell-shaped housing of the declination axis. The entire assembly is anchored to a custom stand whose design was donated by an engineering firm. An overhead crane holds the surviving section of the original optical tube near its correct position. The scene is reflected in the partially cleaned front surface of the surviving original 48-inch speculum-metal mirror. The scene was shot in the storehouse of Museums Victoria, where the 150-year-old instrument is being brought back to life.
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|Title Annotation:||RESTORATION DOWN UNDER|
|Author:||Bell, Trudy E.|
|Publication:||Sky & Telescope|
|Date:||Oct 1, 2018|
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