Transcript
What Makes a Telescope "Great"?: Observations on the restoration of a once great reflector
Matthew Churchward, Barry Adcock & Matilda Vaughan
14 June 2017
[CHAIR & INTRODUCTIONS: FIONA KINSEY]
Welcome everyone to the June presentation in the 2017 History, Culture, and Collections lecture series. It's hosted by the Humanities Department here at Museums Victoria. I'd like to acknowledge the traditional owners of the land we are on today, the Bunwurrung and Woiwurrung people of the Kulin Nation, and pay my respect to elders both past and present.
So without further ado, our first speaker is Matthew Churchward, Senior Curator of Engineering and Transport here at the museum.
He's been with the museum for 25 years, and he's the lead curator on the Great Melbourne Telescope restoration project. Please join me in welcoming him to the stage.
[APPLAUSE]
Thank you, Fiona. Well, good afternoon, everyone, and thanks for coming along. In my part of the talk, which is the first third of the talk, I'm going to address the rhetorical question, what makes a telescope great? I'll explain how considering this question is central to applying the Burra Charter to our restoration process. After I've finished, Barry Adcock, one of our key volunteers on the project, will talk about the work that he's been undertaking over the past 12 months or so with a small sub team, examining in detail the optical characteristics of the original Huygenian eye pieces. And this will ultimately enable us to create precise working replicas of those eye pieces.
Then finally to finish off, Matilda, my fellow curator, will talk about the work she's been doing to restore-- oversee the restoration of the original components of the optical tube and prepare for the reconstruction of missing sections of the latticework.
So what makes the telescope great? The origin of the word "great" in the Melbourne telescope name goes right back to before the telescope was even beginning to be constructed, the early days of the project. It was really an idea that was developed by Reverend John Thomas Romney Robinson, who was director of the Armagh observatory in Northern Ireland. In 1849, he became the president of the British Association of Advancement of Sciences, and he began to promote an idea that a large, reflective telescope should be built with funding from the British government, to be placed in a British colony in the southern hemisphere.
To backtrack for a moment, the development of the reflective telescope goes back to the early part of the 17th century. There's a little bit of dispute as to exactly who was the original, or first, proposer of a reflective type design. A number of different opticians, and mathematicians, and so on, proposed designs, before Isaac Newton finally became the first person to demonstrate a practical, working reflective telescope in 1868. And at the top right there is a reproduction of his second model of telescope, which was actually presented to the Royal Society in London in 1871.
Over the subsequent 100 years, the reflective telescope was developed enormously as a research tool to penetrate the darker parts of the universe, the parts that can't be seen with the naked eye. William Herschel, in particular, was a particular pioneer in this work. Over a period of about 40 or 50 years, he built a series of progressively larger reflective telescopes and undertook a systematic survey of the northern skies, discovering almost 1,000 previously unidentified objects.
These objects were not stars. That were faint, little smudges, almost, of light. Originally referred to as nebulous objects, for many decades, there was a lot of conjecture about what they actually were, whether there were clusters of stars or, in fact, as we subsequently discovered, galaxies of millions and billions of stars, clouds of gas, and so on. There was a number of different theories that were in circulation in the early decades. And part of the work with early reflective telescopes was trying to really get to the bottom of what this was.
William's son John Herschel followed on from his work in the 1820s and 1830s. He rebuilt one of his father's more medium sized telescopes and took it to South Africa, where he set it up near Cape Town in 1834. And over the next few years, he subsequently discovered almost as many objects as his father had found in the northern skies in the southern skies.
So by the late 1840s, there had been thousands of these nebulous objects discovered. There was a lot of interest in the potential of actually whether increasing the size of aperture of telescopes could actually enable people to find further objects or to study the ones that have already been discovered in more detail.
And so we get to the point, in 1852, where the Council of the Royal Society passed a resolution-- and I've got it up there-- that "the President and Society agree with the British Association in considering it desirable to proceed, without delay, in a [INAUDIBLE] establishment of a telescope of very great optical power, for the observation of nebulae in a convenient locality in the southern hemisphere, and that a committee be appointed to take steps, as they seem most desirable, to carry out this resolution."
Subsequently, a committee of about 15 of the most eminent astronomers in Britain was established. And those key members were instrumental in shaping the design. Originally, it was known as the southern telescope, formally sometimes referred to as the Great Southern Telescope. Within 12 months, they'd more or less knotted out a suitable design.
Central parts of that design were that they felt it should have a primary mirror of at least 48 inches in diameter. It should be equatorial mounted, so it could track objects in the sky with a clockwork drive. It should have an open latticework, or skeleton form, tube, which was regarded as essential to good optical seeing. Some of the other detail took a little while to establish. But Reverend Robinson was a key advocate of the potential for it being a Cassegrain telescope. And he eventually held sway and managed to convince the other committee members.
They presented the proposal to the British government in 1853. Unfortunately, the Crimean War had just broken out, and there was not sufficient funds, although the parliament responded and said they thought the idea had merit. They didn't feel there was funding to actually invest in such a project at the time.
So fast forward a further decade, and the young colony of Victoria decided that what we needed was a large, reflective telescope in order to make our mark in the scientific world. And a number of different people, including Professor Wilson, the first professor of mathematics at Melbourne University, had a lot of contact with the early committee members in the Royal Society and the British Academy. And they worked together to restart, relaunch this idea that there should be a large telescope built. This would eventually emerge into the proposal to build what was originally known as the Melbourne telescope. By the time the telescope was actually installed, it had officially been renamed the Great Melbourne Telescope.
Going back to the original quote, the original intention of that name was to indicate that it was a large telescope. It would be larger than anything that had previously been used in the southern hemisphere. That was their intention of using the word "great." However, as Richard Gillespie has pointed out in his wonderful history of the telescope, it was not uncommon in those days for the adjective "great" to be used to a whole variety of engineering achievements, like the Great Western Railway, the Great Eastern Steamship, and so on. Even the nation itself that produced these wonderful feats was named, itself, Great Britain.
[LAUGHTER]
So finally, after some persuasion, the Victorian government voted funding to construct the telescope at an estimated cost of 5,000 pounds, quite an investment in those days. And a contract for manufacture was signed on the 19th of January, 1866, with Thomas Grubb of Dublin.
Construction started almost immediately, with the completion of design and manufacture of components for the two principal backbones of the telescope, the polar and declination axles. It appears from evidence that has emerged during the restoration process that there may never have been a full set of working drawings of the telescope, certainly not when they started work. Many of the details of smaller components were evidently resolved as the work progressed, and parts were fitted to the emerging joint.
After eight months, they cast the first of the great specular mirrors. The first attempt was a failure. However, the second and third attempts were successful.
By early 1868, the telescope was ready for testing, and a subcommittee of three members of the original committee were given the task of actually testing the telescope and signing it off for approval. It was then dismantled, packed into shipping crates, and shipped out to Melbourne, where its erection on the Melbourne Observatory sight began in the early part of 1869.
By July 1869, it was ready for operation. However, it would take a further two years of bedding down a number of teething problems and working out some of the fine detail of operating the telescope before it settled down to some good, solid work. And then over the next two decades, it undertook quite a volume of detailed research work, unfortunately, a lot of which never got published. So it's actual achievements are somewhat under-recognised by subsequent generations.
Melbourne Observatory closed in 1944. And the Melbourne telescope, which had essentially been mothballed for about 40 or 50 years, since the late 1890s, when the first director, Robert Ellery, retired, was sold off to Mount Stromlo Observatory for scrap. It was shipped up to Mount Stromlo and re-erected in 1954, refitted with a 50-inch spherical Gregorian mirror. Wasn't terribly successful in that configuration. but took some early photometrical studies.
Then in 1961, it was refitted again with a more modern, 50-inch borosilicate, or Pyrex, mirror. The focal length of the telescope was shortened to make it more effective.
And then there was a third stage of its rebirthing. In 1992, when the telescope was rebuilt for a third time at Mount Stromlo, to form part of a major international research project known as MACHO Project, or the Massive-- sorry, I've just forgotten the name of the MACHO. But the MACHO Project was basically aimed at trying to discover examples of a particular phenomena that had been predicted by Einstein, the microlensing of light waves as they passed from distant galaxies around larger, more massive objects in the foresight. And that was a demonstration of the existence of dark matter in the universe that couldn't be directly observed. It was highly successful in this work and made international headlines for its achievements.
Then in January 2003, a massive bush fire came over the mountain at Canberra, and the telescope was completely burnt out. It remained a rusting relic for the next few years. Then in 2008, a project was launched at the initiative, largely, of the Astronomical Society of Victoria, and a three way partnership was formed between the Royal Botanical Gardens in Melbourne, Melbourne Museum, and the ASV, to jointly undertake the restoration of the telescope and return it to its original home in the Melbourne Observatory.
Now, as part of this project, we're being guarded by the Burra Charter. The Burra Charter, established by ICOMOS Australia in 1979, is a key document that guides us in the conservation of heritage places in Australia. It's also applicable to movable objects, such as telescopes. One of the key parts of the charter is it sets out that it's important to establish the cultural significance of an object or a sight first of all, before you actually begin the process.
It also identifies three different types of repair and conservation of heritage places. Preservation, where existing fabrics are just maintained and preserved. Restoration, where existing fabric is returned to a previous state. And reconstruction, where new fabric is introduced to replace fabric that's been lost. And in actual fact, although we refer to it as a restoration, the Great Melbourne Telescope restoration is somewhere between a restoration and a reconstruction. It involves elements of both.
Some of the key principles that we're following in the process, conservation is based on the respective existing fabric. Wherever possible, we try and use traditional techniques of manufacture, where we have to introduce new components or replacement components. We try and keep change to a minimum possible. And so on.
So let's get back to what makes a telescope great. The Oxford dictionary of the word "great" suggests that "great" has a wider meaning of "an extent, amount, or intensity considered above average. Impressive or grand. Of ability, quality, and eminence considered above average.
A key principle of the Burra Charter is to establish a statement of significance, first of all. And that is a document that then can then guide the appropriate process of restoration, and preservation, and so on. So we've set about and produced a statement of significance for the Great Melbourne Telescope a few years ago. And some of the key things we have identified in this process help us to understand, really, what makes the telescope significant, and therefore, what makes it great.
First of all, the Great Melbourne Telescope stands as a colossus astride a key watershed in the history of astronomy. So it can be seen as partly the loss of the great speculum, metal mirrored reflectors, ending a pioneering era of research into the nebulous, deep space objects. It's also, in many ways, the first of the modern research telescopes. It was equipped from new with a micrometre, photographic camera, and spectroscope, which became the three basic tools of research astronomy over the following decade.
It was the first large Cassegrain telescope built. And Barry will explain a little bit more in detail about what Cassegrain telescopes are. It was the world's largest, equatorially mounted, and trackable telescope for over 40 years; the largest aperture telescope in the southern hemisphere until 1940; and incorporates some very unique design features that some of our later speakers will touch on.
So in the course of considering our statement of significance, we also examined what actions were appropriate-- what aspects of the telescope significance were fundamental to its significance, and therefore, should not be altered during the restoration process. And the first block of dot points there are some of the things we've come up with. The basic Cassegrain layout, because it was so pioneering, needs to be retained. The size of the primary mirror, that was a signature for all large telescopes. They became known by the size of their aperture. So we think it's important. Although, it was modified to a 50 inch telescope in Canberra, known as the inch, to actually reflect the fact that it's a 48 inch telescope.
Replicating the original clockwork mechanism. Ability to use the telescope in its original form, particularly with its original eye pieces. And the two images on the left there show the importance of those massive eye pieces and the actual way observers interacted with the telescope. And then there's a few aspects below that that we feel could possibly not be modified without significantly compromising the significance of the telescope.
One of the key things about the Great Melbourne Telescope is it had a very long focal length by modern standards. A key to understanding this is that there's two aspects to the focal length of a Cassegrain telescope. One is the primary focal length. Now this graph here shows about 50 of the key reflective telescopes built over a 200 year period from 1870 to 1970. You see the GMT in the middle. At it's prime focus, in other words, the primary mirror was pretty much bang on the norm line for the trend of telescope development. However, once it was converted to a Cassegrain telescope with a secondary mirror, it sits way up above the curve. And it was largely because it was one of the first Cassegrain telescopes that-- there was no sort of precedent. That did cause problems with it in use. And one of the aims of the restoration is to try and rectify some of those issues.
So one of the key things we're going to do is we will slightly reduce the overall focal length of the telescope. The primary focal length will be retained about the same. These are two optical layouts, the original layout on the top, and what we're proposing for the revised layout on the bottom.
So one of the other key modifications is the introduction of baffles, the conical shaped devices there, which stop stray light coming around the secondary mirror and will significantly improve performance.
I think my time, almost up. So I'll just very quickly-- I just wanted to move on very quickly now to just touch on some of the work we have to do. So when the project started, we brought together at our mall and annex all the remaining surviving elements of the telescope. The remaining spine of the instrument that was still in use at Mount Stromlo until the bush fires was dismantled, and brought down to Melbourne, and painstakingly dismantled, and cleaned, and so on, all the rust removed.
Fortunately, the museum had actually collected a large number of components of the telescope removed during those different phases of upgrading of the telescope over previous decades. And they had been brought back to Melbourne for safekeeping. So today, we have around about two-thirds of the original telescope components. This composite image was done by one of our volunteers, Steven Bentley. And it creates a really nice illustration of-- the items shown in green are the bits of the telescope that are missing. So all the other bits, we've got.
There's around about 300 or 400 components in total that we're actually missing. And there's little in the way of original documentation to actually identify what those components are like. The key reference guide is a number of historical photographs.
So I want to just highlight, very briefly, one particular part. This part circled here. It's known as the-- the full name is the declination axis fine positioning control worm shaft mounting bracket, that we just commonly refer to as the H casting, because it looks somewhat like an H shape.
And I mentioned, there's little in the way of surviving drawings of the telescope. There is a paper that was published by Robinson and Grubb, and Thomas Grubb, in 1869, the year the telescope was finished. And it includes about 50 technical diagrams. They're not really engineering drawings, but they do provide a lot of detail.
However, these drawings were obviously prepared before the telescope was finished. Because you'll see key differences. So you can see the H casting highlighted in that view on the right. But it's in a different position and a different orientation. It's a mirror image of the actual casting that was built. So that is not a great deal of help. That provides some guidance. The historical photographs are a better guide.
We have always existing components. So we start by looking at the existing components. We can actually examine the areas where the part that's missing was attached. And here we see evidence of the shape, or the footprint, of the way the part was mounted, and the bolt holes, and their spacings, and so on.
We then take the historical photographs. And again, this is a couple of drawings prepared by Steven Bentley. And we can go through a detailed process of scaling off those historical photographs. The dimensions with actual beside them, they are parts that exist. So we can use those as a basis to estimate the dimensions of the missing component. We end up with a CAD drawing, something like this, which can then go off to a pattern maker to be cast, to turn into a wooden pattern, which then goes to a founder to be cast to make the basic metal part that is then machined for the final part.
Just recently, Patrick Millers joined our team as a recent engineering graduate, and he's brought some fantastic new skills with 3D CAD modelling, which has given us another tool to actually help sort of analyse these parts before we make them. And this is his 3D model he produced over the weekend, forming of this casting. Can you just launch the video there. And he's even created a little animation, where you can actually see the thing rotate.
This is a fantastic tool, because it enables us to actually look at assemblages of parts and identify issues with clearances, and alignment, and so on, before we finalise design and begin manufacture.
So I'll finish there, and we'll hand over to Barry. Thank you very much.
[APPLAUSE]
Thank you, Matthew. And good afternoon, everyone.
I'd like to address a specific part of the problem of the Great Melbourne Telescope and that of eye pieces. The next slide shows a familiar picture of the Great Melbourne Telescope, and eyepieces are down here at the bottom. A lot of you will be familiar with the eye pieces associated with binoculars and small telescopes. They are about 2 and 1/2 centimetres in diameter, and they give magnifications of perhaps 5, 10, 20 magnifications.
Well, the eyepieces associated with the Great Melbourne Telescope were very much bigger. And I'd like to talk just for a minute about this word magnification, because it has a lot of ramifications for the Great Melbourne Telescope.
There's a picture of a country scene, and we'll put the moon in the sky. And the question is, how big is it? We can't use a ruler or a pair of callipers to measure the diameter of the moon. The only thing we can do is measure its angular size. And if we look at the bottom of the moon our eye, then we raise our eye to look at the top of the moon, that's the-- the angle that your eye turns through is the angular size of the moon, and that's round about half a degree.
If we put a telescope in the scene, and we look at the moon magnified, then calling that the object, and that magnified version is the image, people's concept of magnification is simply the angular size of the image divided by the angular size of the object. And if we look at the moon through a pair of seven by 50 binoculars, the magnification is, in fact, seven.
Just having a look further at the magnification problem. That shows a box of optical components that we can call a general telescope. And the light entering the telescope, its diameter is defined somewhere in there by a lens, or a mirror, or a diaphragm, and that's called the entrance pupil.
Our eye likes to see a parallel light. That's like looking at something at infinity, or way, way distant in the horizon. And the light will come out here somewhere as a parallel beam, and that's defined as the exit pupil. And that's where we look through the telescope.
Another concept of magnification. I've drawn there a very, very simple telescope with a lens and an eye piece. The focal length of that lens is defined as that distance there. The focal length of the eye piece is defined as that. And we often describe the magnification as the focal length of the lens, divided by the focal length of the eye piece. The only trouble with that is that often we don't know what the focal length of the eye piece is. It's not always obvious.
A second way of looking at this magnification problem is to define those terms I talked about before. That's the entrance pupil. That's the exit pupil. And the magnification is the entrance pupil over the exit pupil. Now, that's an important concept. Because your eye has got to accept all of the light coming up from the exit pupil. And if the exit pupil is bigger than your eye can accept, then the telescope won't, well, will not work to its full aperture, full diameter. And that particular magnification is called the richest field magnification.
Now, the diameter of the human eye, as I've mentioned, is not absolutely fixed. It varies with people's age. It varies simply with people. But we commonly accept the eye as having a diameter around about six millimetres under dark conditions. That's the dark adapted size.
And so we can work out the minimum magnification of a telescope as the diameter of the entrance pupil, the mirror, or the lens, divided by 6. And in the case of the Great Melbourne Telescope, the telescope had a 48 inch, or a 1,219 millimetre diameter mirror. And that gives rise to a maximum magnification-- sorry, a minimum magnification of about 190.
And I can assure you that from a point of view of looking through a telescope, that's a very high magnification. If we go below that magnification, down to perhaps 150 or 100, that's possible, but the telescope will no longer be working as a 48 inch telescope. It will go down to perhaps only a 20 inch telescope. And that's something we don't want.
Matthew mentioned before the Cassegrain design of the telescope. There's a concave primary mirror and a convex hyperbolic secondary mirror. And the light enters from the left in that manner. And the focal point is arrived at-- down at that point there. And if we pull in an eye piece and then look through the eye piece, that's the way that Great Melbourne Telescope operated.
Moving on to a third definition, which I alluded to before. Again, that's a simple view of a telescope, perhaps looking at the moon. If the telescope was pointed, or looking at the bottom of the moon, the light goes through that configuration. Then looking at the top of the moon, it would be in that configuration. And putting those together, we can define the magnification, as we did in the earlier slide. That's the actual field of view. The apparent field of view. And the magnification is that quotient of those two terms.
I said before that the Great Melbourne Telescope had some very big eyepieces Well, there they are. It's hard to see just how big they are there. But the set provides magnifications of 234 up to 1,000. Unfortunately, that eyepiece marked 420 times has a missing field lens. And I'll mention just what that means in a few minutes.
The magnifications are marked on the eyepieces. It's hard to see, some of that's 330, 420, and so on. But just pressing on. That shows the lowest power eyepiece, the one with the longest focal length. There's it's magnification marked at the top, 234 times. There it is. And just comparing it with objects that we know, like a pen and a six inch ruler. That's how big it is. It's bigger than most people's normal telescopes.
[LAUGHTER]
The optical configuration of the eye pieces, they're all the same. They're Huygenian eye pieces, invented in the 17th century by Christiaan Huygens. He was a Dutch astronomer. It has what we call a field lens and an eye lens. And if we open up one of the eye pieces, there's the bits. That part there is the field lens, and that's the eye lens. And the other parts are components to hold them all together. That's the main body of the eye piece. It screws into the telescope there. And then there are particular cells, and eye caps, et cetera, to put it together.
I mentioned before, the field of view of the telescope. Over all, the Cassegrain telescope has a focal length of 50 metres. And to me, that's 2 and 1/2 cricket pitches. It's a very, very long focal length for a telescope. And using the formulae that I've just talked about, the largest field of view that we can see is about 14 arcminutes, or roughly a quarter of a degree.
And if we look at the moon, which I've already said is half a degree in diameter, through the Great Melbourne Telescope, at lowest power, that's what the moon would look like. If I can just go back, we can only see just part of the moon. That's how small the field of view is for the Great Melbourne Telescope.
In our work, we have no knowledge about who made the eyepieces, or who made the glass, or the glass type. We don't know who scribed the magnification onto the eyepiece bodies. And we don't know how they determined the magnifications in the first place. But we do know that one of the scribes is blatantly wrong.
In our work at [INAUDIBLE], we have measured the radii of the lenses with a spherometer, which measures the thickness of the lenses with a jig. We've measured the focal lengths with optical tests. And we've determined that the glass-- which may not mean much to some people-- but the glass has a refractive index of around about 1.53 and a dispersion of about-- which is otherwise known as the Abbe number-- of about 48. And that has led us to believe that the closest thing in a modern catalogue is a glass called LLF6.
The original Great Melbourne Telescope had a primary mirror, diameter 1,219 millimetres. Effect focal length that I've already talked about, 50,650 millimetres. And if you divide one of those by the other, you get the f/ratio called f/41.5.
We have proposed a change to the Great Melbourne Telescope. We're going to make the primary mirror not from specular metal, which is not a good idea, but we'll make out of either borosilicate material or Zerodur, which is a very modern, zero expansion material. We're going to shorten the focal length down to 38,064 millimetres, and that means it's going to have an f ratio of 31.2.
Why are we doing this? The shorter focal length will enable a lower magnification to be used, and that means it will provide a wider field of view. We'll see more of the moon through the lowest power eye piece. We intend to make a duplicate set of the eye pieces so as to protect the originals. We'll put them in a bulletproof glass case for people to look at. But in doing that, we can make a new eye piece of even longer focal length that will give a magnification of roughly 190, that I talked about before. If we go below that, then we won't be using the primary mirror to its full aperture.
So that's the story of the eye pieces. We-- it's ongoing. We've now got to set about making our duplicate set and the new eye pieces. And they will be ready for use with the telescope.
[APPLAUSE]
[OFF MIC] Do you want that?
[OFF MIC] Uh, no. [INAUDIBLE]
[SIDE CONVERSATION]
Thank you, Barry. Good afternoon, everyone. So in this part of the presentation, I'm going to be speaking about the most immediately visible aspect of this telescope, and that's it's a very long telescope tube, and touching on what we know about its design and construction, and what we are learning to help with its restoration.
So the telescope tube has a very simple task to do, and that's to hold the secondary mirror steady at the required distance from the primary mirror. The uniqueness in this telescope is that it achieves this with a structure that hadn't quite been executed in the same way as what we see here.
The open tube structure, that sought to prevent localised atmospheric conditions in the air that could cause a distortion between the reflected image down to eye piece. And-- down to the astronomer's eyepiece. And so to give you an idea of how the structure evolved over time, have a look at some of the historical telescopes.
Famous astronomical siblings William and Caroline Herschel built a telescope with a 40 foot, or 12.2 metre, long tube. It was closed iron tube, with a 48 inch mirror. And although it was dismantled in 1840, there is still a section of it that you can see at the Greenwich Observatory. And it does show a riveted iron structure.
The giant telescope on the right there was known as the Leviathan, built by Lord Ross on his estate. And it had a six foot mirror. And it had a closed, wooden telescope tube that was 56 feet long, or 17 metres long.
It was during this time in the 1850s, or so, that several of the most prominent engineers and astronomers started to weigh in on commentary on what-- on using the-- or adopting a new, open skeleton tube. And with that view in mind, William [INAUDIBLE] built his in 1859. And it was 37 feet long, 11.3 metres. And it had long, iron slats with reinforcing rings. And at one stage, this telescope was actually being considered to be gifted to Melbourne, in lieu of what would be known-- or become to be known as the Great Melbourne Telescope.
So the GMT, built in the late 1860s, was the first telescope to use an open structure in this spiralled lattice configuration. And as to why a spiral. Well, there was some correspondence by John Herschel, son of William, in the 1850s that suggested the structure should be, ideally, made to secure a continuance of that slow, spiral movement of the internal air by the inter change of air, within and without. Perhaps that's where the idea germinated.
The Grubb firm adopted the same-- the image in the middle, on the top there-- the same spiral lattice design for the telescope that was ordered in 1912 for the Crimea but wasn't delivered until 1924 because of the interruption of World War I. And there's nothing left of that telescope. It was destroyed during World War II. But the photograph survives. And it does show the same lattice style, but seemingly wider strips at the base, and still tapering to the top.
The GMT's life at Mount Stromlo still retained part of the lattice section. There it is, being hoisted into position. And it was later covered, and finally removed and replaced with an open truss section. And eventually, spiral lattice designs, such as the GMT, were superseded by those lightweight, open truss structures that you see mainly in telescopes today.
However, the design has reappeared. Most recently not on telescopes, but what is colloquially known as the Gerken in London, and also a pedestrian bridge in France. See, the same structure.
So the two sections of this telescope that I'll be talking about are the boiler plate tube on the left hand side, spiral tube on the right hand side. And these components were not on the telescope when it burned down in 2003. They'd already been retrieved from the paddocks around Mount Stromlo in 1984 and had been in the museum storage ever since, and so remained largely untouched until it was-- the project began in 2008.
So apart from having a unique spiral tube, another great part of this telescope is it's riveted construction. There are very familiar riveted constructions and structures, such as the Eiffel Tower, 2.5 million rivets. The Titanic, 3 million rivets. And our very own Sydney Harbour Bridge, constructed at a time when rivets were really on their way out. They were eventually replaced by electrically welded structures. But it used six million rivets, made in Melbourne. The GMT has 1,200. Not as many. Not as big. But they are enough to keep us occupied.
Before rivets, the most common metal fasteners-- or the only metal fasteners-- were nails for wooden structures and bolts for wood or iron. The emergence of iron and steel as the predominant structural material of the 19th century really herald in the era of rivets. Bolts could be used for largely static structures or ones that required dismantling. But structures that were subjected to vibration, or movement, or internal stressors, like locomotive boilers, ships, bridges, telescope tube, were using rivets to provide a more secure fastening. And the different industries use shapes and sizes that were found, in practise, to be suitable for the use of the structure. For example, round heads on boilers, pan heads on ships.
The period where rivets really found most favour was a 100 year period from 1840 to 1940. And their first use was in boiler making, from about 1810. Then the shipbuilding industry, 1830s to 1840s. Then following on into the iron and steel structures, like bridges and buildings, from the 1850s.
Now, it's important to know about rivets, because there's still plenty of historical structures out there that are still standing that used a lot of them. And due to cost implications, and also heritage, in some cases, the structures are not going to be replaced anytime soon and will eventually require some form of intervention. Knowing about rivets is important, because with this restoration, it was becoming clearer that we were going have to remove and reinstate rivets, in order to return the telescope to looking like it did when it was brand new.
So looking closely at the exterior of the boiler plate tube in the original photographs, you clearly see that there are no rivets in this section. Whereas, now we look at our artefact, and we've got rivets that are prominently on the main body of the boiler plate tube. There was always these strips around-- down the side that had the rivets. But what this indicated was that in its time at Stromlo, what had happened was the Stromlo had actually cut a section out of the-- they had lowered the hole that's in this cradle, and cut a section out, and put it at the top, and then reinforced the whole structure with two extra mould steel rings.
And the materials analysis that we performed on this confirm that the body of the boiler plate material was wrought iron, as were two of the rings. And the other two rings were mild steel, which confirmed that were actually added-- a 20th century addition.
The way this section of boilerplate tube was constructed, was there were four actual wrought iron sheets that are shaped like that, with a longitudinal strip to hold the parts together, and riveted to hold the parts together. Which, if you're adopting that kind of construction, that probably meant that, at the time, it wasn't really possible to roll a seven foot strip of metal right around into a hollow tube. And some smaller diameters could be managed, and larger ones could have sections of riveted plate joined together.
Now, the removal of those Mount Stromlo rivets was done over several sessions, starting in October last year and finishing in March. We used an angle grinder with a grinding disc to carefully rub away the Mount Stromlo rivet head on the inside of the tube, until it was flush with the surface, being very careful not to remove material from the actual rings. Then if we were lucky, we were able to punch the rivet clear, using a drift, and a hammer, and a lot of effort, as some people heard in the workshop. Otherwise, we had to carefully drill out the centre of the rivet to weaken the edges of the rivet and allow it to be punched through.
We also reopened some of the original holes in the body of the boiler plate tube that had been patched by Mount Stromlo. There is a line of rivets that are hidden, or holes, that were there-- were there and down here-- where the original rings were located. But when Mount Stromlo patched those, they just used some hexagonal type bolts. They cut to size, put in from the inside, and then braze it over with-- I mean, you could see that there were lovely, golden blobs of where they had smoothed and finished that surface over. So they didn't rivet those holes. They just filled them in. And we have knocked those out so that we can put the original rings back to where they were.
And then we prepared for the sandblasting, which is pretty much to remove all that leaded paint, layers and layers of leaded paint over the many decades. And that was done off site. And what that allowed was to see a bit more about details of the construction of the tube, including the iron maker and the size of the original rivets.
So that's a before and after shot, with the paint removed. And you can see on the right there, there's best best [INAUDIBLE] best, which [INAUDIBLE] best best best. There were three grades, [INAUDIBLE] best, [INAUDIBLE] best best, and [INAUDIBLE] best best best.
[LAUGHTER]
And yeah, very-- but that's come back to being-- that maker's mark was used by TE Wright, which was a Staffordshire iron maker. And that was what he put on three-- or that was what was found on three of the plates. The fourth plate has a big hole cut out of it. So it could have possibly have been there.
And we've also got a remnant, just on "best best" is written there, on the inside of the wrought iron angle iron. And also, the other thing we see here, there's a couple of cracks there. This is what-- this is evidence of the forge welding that was done to join the rings, the original wrought iron rings, together. So you heat it up and bash it until they're squashed together. And that also appeared.
And what you're looking at there is the outside at the top, and the inside, corresponding inside, of one of those lattice-- one of those longitudinal-- oh, sorry about that. Getting ahead of myself. I meant to use-- oop. We'll come back to that one and that one. There. Sorry about that. The--
What we're seeing here is the original Grubb rivets, and these are Stromlo rivets. And that's the inside of the Stromlo rivets, compared to the inside of the Grubb rivets. So when Grubb manufactured this, the rivet went in from the inside and was hammered home from the outside. When Stromlo did it, they put them in on the outside and, I don't know, kind of just closed them over a little bit. They don't quite look as finished as the Grubb ones.
So what we see here, on the left side there, is a close up of a original Grubb rivets. So there's just one part of this boiler plate tube where Stromlo conveniently sliced through, for whatever reason, a section. And it shows the size of the Grubb rivet and the Grubb hole. And typically around when this sort of construction time, there was a rule of thumb about size of hole versus size of the shank, or the diameter, of the rivet, which was around a one millimetre difference. And so are the careful measurement does show that there is some following of that kind of construction mode.
And also, although we're going to stick with the half inch rivet, because that's much more commercially available these days. That's also what was used for the Stromlo restoration, which means that the holes that we're trying to put rivets in are already half inch, so we should really-- I mean, it makes sense to keep them at that size for now.
OK, now onto the lattice. So when we look at the original installation photograph and compare it to what we have in the store now, it's glaringly obvious that we're missing quite a bit of it. It was originally 21 foot long, just over seven metres. And so what's coloured there in green is what we actually have in our store. And I know what happened-- no one knows what happened to the upper part of the lattice. But I imagine it might turn up in some Canberra garden, or whatever, holding up a rose, or something-- being a trellis for something important.
So it's made up of a number of parts. Down the bottom, there's a ring, a flange ring. There's also a rolled section of-- a narrow section of boiler plate tube, another ring. And then there are these 28 individual slats that are wound around. And they actually taper from three inch up to one inch the very top. Plus some reinforcing rings and a ring at the end. And it was done this way in order to reduce the weight, as well as provide some structural rigidity.
The material analysis-- this is what we've got in our store. The material analysis confirm that these three rings that are here are wrought iron, which means they were the ones that were on the telescope that were shifted down from that upper part of the telescope. And also, the steel that was used in strips was, although the materials scientist called it wrought iron, it actually has the very same amount of carbon that you would have in a modern steel. The difference being is that the 1850 steel didn't have a level of manganese that you see in modern day steel. So that's what that showed up in it as well.
So how did they deconstruct this lattice section? Hmm. Well, in the paper that described how the telescope built, Grubb notes that "the strips of steel were bent around a cylinder of the proper diameter." And that's it. So that's the only description we have of how it was constructed. So in making this missing section, rather than the fancy sort of 3D modelling, we're talking about making a forma, and using bits of foam, cardboard, et cetera, to try and think about how are we going to construct some kind of forma, and how we are going to do it.
But we're definitely going to make sure our rivets don't look like-- will look more 19th century than 20th century. The top part of rivets that are on the outside of the lattice, the bottom of the corresponding ones on the inside. The ones on the far right are the ones that Stromlo added. So they looked like they've been kind of bashed. The ones on the left side is what we want. A more, cleaner finish that looks more like the original.
And so now onto riveting. So here is an image showing a riveting gang of four for the Eiffel Tower, and the type of equipment that was likely to have been used for installing the Grubb-- installing rivets in the Grubb workshop. There may have also been a fifth person here acting as a catcher for the rivet, which is being heated in the forge, thrown over to the back, put into the hole, held in place behind the beam. Another person puts the shape of the finished field head on there. And then you've got a person that's hammering it home.
So I have a short, 20-second video, shot by one of our volunteers, Bob. See you there. We tested our method of making rivets on the bench, to try and determine how much we needed to protrude the rivet in order to bash it and make the shape we wanted, so that it looked like all the other rivets. We're trying to match the Grubb rivets. And as a result of that, we moved onto the actual artefact. And what you'll see here is the rivet being heated up in the furnace, transferred to the boiler plate tube, inserted, held up by the man on the inside, and then hammered home by the man on the outside.
Our approach does need a little bit of a modification, though, in terms of minimising the time getting the rivet to the hole, as well as the alignment between the holder up on the inside and the person on the outside. It's a bit like this. It should be a bit more like this. But this was just our first go. Yeah, thanks, Tim. You can play that.
[VIDEO PLAYBACK]
[INDUSTRIAL SOUNDS]
[END PLAYBACK]
Yeah, viola! Hoorah for rivets. Hoorah for modern machinery that helps. They don't need to swing a hammer.
[INTERPOSING VOICES]
And hearing protection, yes. So and finally, this restoration project would not be possible without the great commitment of the many volunteers-- some are in this room-- who have collectively contributed many thousand of hours, and actively at the weekly workshops, or behind the scenes, ever since the project began. And so we thank them for their dedication and their support.
And to keep up to date with our project, you can follow us on Twitter. You don't have to be a Twitter user. You can go to the website and click on the Twitter link there, and we'll just show you some posts that-- I try to post something weekly that's happening in the workshop. So, yeah, thank you very much.
[APPLAUSE]
Thank you very much to our speakers. That was really wonderful. And such enthusiasm from all of the team here. Really appreciated that insight into the project. We're going to have a questions now. So if you would like to ask a question, please put your hand up, and Jen will bring the microphone to you.
You are saying with that H section that the you had had it mirrored imaged. About that time, they used to mirror image things on drawings like that to stop other engineers copying their work. Would that be the cause of it?
No. Actually, what we found by looking at those diagrams in the Robinson Grubb paper is that there's a number of instances where the detail shown is not the way they actually built it. Which leads us to believe that, in fact, they built it as they went along. And that in-- well, they actually made it up as they went along. It was a one off instrument. And their initial ideas were somewhat modified and changed. I think that's the most likely conclusion you can draw from differences from the historical photographs.
And in fact, even when they first erected the telescope for testing in Dublin, there was further changes between its appearance there and its subsequent appearance in Melbourne. So it was sort of a work in process, I think, in progress, in those early years.
Interesting there's an Irish connection, because the first metal truss bridges were Irish. And the first [? papers ?] on metal truss bridges had 60-degree cross members, which looks like the-- can you tell me what the angle on the cross members are in the truss on--
I can't tell you off the top of my head. I haven't memorised that.
[INAUDIBLE] 27 degrees.
Is there anyone here--
Patrick says 27 degrees.
Patrick knows for sure, because he's drawn it. So 27 degrees, he says.
There were riveting machines available in the 1850s. It was used by the Monroe on the Chapel Street Bridge and other bridges. Do you know whether hand riveting or machine riveting were used for the tube. And if you think hand riveting was used, in line with Burra Charter, should you be swinging a hammer rather than a compressed air machine?
My comment on whether to use traditional tools-- I think we're beyond that. We've got elderly volunteers. And it's just-- we wouldn't do it that way now. So-- it will give a better finish for us to use the machines. In terms of the starting of the machine-- with the rivet machine, starting with using steam or hydraulics, in a sort of a squeezing motion, I like to think that maybe-- being an engineering workshop, maybe they rigged up something to squeeze.
But from-- to squeeze instead of heating. But it's really hard to tell from looking at the rivets whether they were machined or hand driven. We can only assume that they were hand driven, based on that period. The machines were being used in shipyards and on long, regular sections, and pipes and tubes but in a sort of a claw or a horseshoe kind of arrangement, but perhaps not in this smaller, although well equipped workshop.
We might need to draw all the questions to a close so that Patrick can come and do his demonstration. So please join me in thanking our speakers and also to welcome Patrick up to demonstrate his bottle.
[APPLAUSE]