Cloudy Skies, Clear Genius, and Lensless Telescopes

Commenting on this blog's post about M51 a while ago, and on Lord Rosse's giant (for the 1800's) 72-inch telescope, Mike Peterson said:

The next trick would be to create a formula that would evaluate, on the one hand, the optical properties of a lense crafted in 1840 compared to the potential for such a lense crafted with modern techniques ... and then compensate for the fact that the stars he was gazing at were only competing with candles and the occasional rush torch on earth.

Wow, where to start? I think I'll start with the last part:

I) Dark Skies, Short Nights

Lord Rosse's 19th century skies were certainly darker than 21st century, light-polluted views -- but he (and all others who used the Leviathan of Parsonstown) had two pretty bad things to deal with that most modern observatories don't: a far northerly latitude and Irish weather. The combination is deadly for observing.

Records from the seven decades when the Leviathan was a productive instrument (and the largest in the world) indicate that there were only about 60 to 70 nights a year when the sky was clear enough for any work to be done. (Interestingly, since the reconstructed Leviathan went into operation at the same site in 1997, the average is only about 35 to 40 -- but that may be due to a difference in perception of what constitutes "workable" rather than a real change in cloud cover.) Most of those would be in the summer -- when darkness was short.

That latter bit -- the shortness of summer nights -- came across clearly to me when reading a bit of correspondence in the Birr Castle archives room from J.L.E. Dreyer (then an observing assistant to the 4th Earl, who, like his father, was an accomplished astronomer) to his boss on June 9th, 1875. Dreyer reported to the 4th Earl (who was summering in one of his homes in England) that he had given a visiting dignitary a demonstration of the telescope the previous night, and that the visitor "... would not wait beyond 12 o'clock, as it would become light again almost at one." He goes on to explain to his boss:

Since I last had the honour to write to you, I have observed on 3 or 4 nights, but there has, of course, been very little work done, as the nights only are two hours long. I intend to leave [to visit home in Denmark] this on the 21st inst. [sic] in the morning and shall be back till [sic] the first of August. My quarter-salary will not be due untill the first of July, but under these circumstances I should be very much obliged, if your lordship would not object to hand me a check for the portion of the quarter already past. Yours very truly J. Dreyer

In other words, no work's gonna get done here until August, anyway, Earl, so I'm gonna go home. Please pay me.

(At least one person who reads this blog will recognize the place of J.L.E. Dreyer in the recent history of astronomy. For those who don't, you might want to click here.)

Here are the last two pages of the letter quoted above:

(Click on the images here to see -- it is hoped -- larger and legible images. Please e-mail me if they're not legible, and I'll upload bigger image files.)

That's my finger at the top of the right-hand image. The current Lord Rosse is not terribly enthusiastic about digitizing the Castle's archives, but he's being worked on. Meanwhile, treasures like this have to be photographed one at a time, without flash, by those of us lucky enough to stumble into the opportunity.


II) Mirrors vs. Lenses
Addressing Mike Peterson again: Mike, you implicitly questioned the quality of a telescope lens crafted in the 1840s compared to a modern one. No problem here: the only lenses in the Leviathan were little ones in the eyepieces.

The Leviathan's main light-gathering element was a 72-inch wide metal mirror, a mammoth expansion on an idea first put forward by Isaac Newton in the late 1600's: that a concave paraboloid mirror could focus light just as well as a lens, but at half the effort (because only one surface, not two or more, need be accurately shaped.) Reflecting telescopes would not become common in research astronomy until almost a century after the Leviathan was built, though, because of difficulties in maintaining the reflective surface (the current process of misting aluminum onto carefully-shaped glass wasn't developed until the 1920's) and because of simple, human inertia among the professional astronomical community. Reflectors were... well... just weird. Lord Rosse wasn't constrained by such community biases because he wasn't a member of that particular community -- at least he wasn't until his observatory became a prestigious place for members of that community to gain employment!

The Leviathan's mirror was of a kind never before fabricated on such a scale, and seldom attempted since: a solid metal, speculum mirror. Facilities to forge and shape the mirror had to be first invented, then built, and then operated on the castle grounds. The Third Earl (and his wife, Mary, a pioneer in photographic image processing) did all of that, and their efforts (and the sheer spectacle of their operation) engendered the following bits of florid prose among many others on display in the Birr Castle Demesne's museum:

From T. R. Robinson, Director of the Dunsink Observatory, in 1842, accounting a nighttime operation of the speculum forge:

On this occasion, besides the engrossing importance of the operation, its singular and sublime beauty can never be forgotten by those who were so fortunate as to be present. Above, the sky, crowded with stars and illuminated by a most brilliant moon, seemed to look down auspiciously on their work. Below, the furnaces poured out huge columns of nearly monochromatic yellow flame, and the ignited crucibles during their passage through the air were fountains of red light, producing on the towers of the castle and the foliage of the trees, such accidents of colour and shade as might almost transport fancy to the planets of a contrasted double star. Nor was the perfect order and arrangement of everything less striking: each possible contingency had been forseen, each detail carefully rehearsed; and the workmen executed their orders with a silent and unerring obedience worthy of the calm and provident self-possesion in which they were given. [SH: the fuel for the forge's spectacular fires was, of course, peat. This, too, is unique in the history of astronomical technology!]

From an anonymous account in the King's County Chronical [sic] (County Offaly was called King's County from the plantation time until the establishment of the Republic), it is clear that the Earl himself was viewed in a heroic light as well:

I saw the Earl, the telescope maker himself -- not in state, with his coronet and ermine robe on, but in his shirt sleeves, with his brawny arms bare. He had just quitted the vice at which he had been working and, powdered with steel filings, was washing his hands and face in a coarse ware basin placed on the block of an anvil, while a couple of smiths sledging away on a blazing bar on another, were sending a shower of sparks about his lordship which he little regarded as though he were a "Fire King". This was in a spacious, rude, smithy which almost occupies one side of the court yard of the castle and in which not only were swing bridges and force pumps, and tackle for scientific instruments constructed, but common and everyday articles in the shape of agricultural gates, sub-soil ploughs, etc, for use on his farms... As he drew on his coat... the Earl looked an intelligent foreman..." [SH: The true heroism of the Third Earl and, especially, his wife Mary would not become evident for a couple of years -- when the famine hit, and all astronomical operations were suspended while they devoted their efforts to famine and economic relief for the village of Parsonstown, now Birr.]

The last bit of the quote about "swing bridges and force pumps" refers to the Parsons family's already well-established reputation for engineering innovation; the "swing bridge" is the Demesne's suspension footbridge across the River Camcor (an innovation of the third Earl's father, and possibly the first modern suspension bridge in the world) and "force pumps" refers to the Parsons' re-directing of the River Camcor to better configure their Castle's Demesne.



III) Restoration of the Leviathan

The giant telescope fell into disuse, then disrepair, then rot, then oblivion after the death of the Fourth Earl, and World War I, and the subsequent shift in the passions of the Parsons from astronomy and engineering to botany. The current Earl of Rosse, Brendan Parsons, is a world-renowned arborist (having worked for thirty years with the UN on various reforestation projects in third-world countries), but he also has a keen understanding of the remarkable place his family holds in the recent history of science and technology. He orchestrated the construction of a replica of the Leviathan in the 1990's, a project that came to fruition ten years ago. Here's a picture from the Castle Museum of the new tube being backed in to the main gates:

... and here's one of the reconstructed Leviathan from behind in August of 2006 (the new mirror is at the end of the tube closest to us in this picture):

Inside the black box at this end of the tube is a replica mirror -- but not a speculum metal mirror. While the effort to reconstruct the telescope was aimed at being as faithful as practicable to the Third Earl's original design, a speculum disk of that mammoth size was prohibitively expensive, so cast aluminum was used instead. That caused problems: aluminum isn't as heavy as speculum, so that caused all kinds of problems balancing the finished instrument.

Those sorts of headaches were dealt with admirably by the chief engineer of the project, Michael Tubridy, a civil engineer of great prominence in Ireland (he was chief of the project to construct Dublin's newest airport terminal, for example.) "Chief" is an appropriate word, by the way: Michael Tubridy was also the original flautist for the Chieftains.

It's all so, well, Irish, isn't it? A tale of great achievement, little-recognized in the larger world, and basically futile, anyway (the world's largest telescope built where it can't really be used?), a work that spans five generations and is revived by a musician, and a tale that can only be gotten through with the help of liberal sprinklings of Guinness. Perhaps. Truth be told, I haven't really tested that last bit.

And this is far from even a thumbnail account of the Parsons' story. That's a book, not a blog entry.