Flamsteed Astronomy Society

“The Other Six” — The World’s Great Refractors

  June 9, 2005

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by Mike Dryland

This is certainly not a scholarly work.  At best it’s a piece of amateur research — browsing really.   It began with a simple question, but got out of hand.   The question is this — If the Greenwich 28-inch Great Equatorial is the world’s seventh largest refractor, then what are the other six please?    Er.....

First, background.  What is a “Refractor” ? — Telescopes [ More; More] come in two principal varieties:  first, those which use lenses as their main means of collecting and focusing light; called ‘refractors’ because lenses refract (bend) light rays;  second, those which use mirrors as their main means, called ‘reflectors’.   The telescopes in this piece of browsing are the biggest refractors that were made.  They mostly date from just over 100 years ago, mainly the last 20 years of the 19th Century.  The great refractors were the products of the final years of argument about the relative merits of refractors versus reflectors.  In the final years of the 19th Century most of the leading professional astronomers favoured refractors for their precision.  Big reflectors were initially the province of gifted (and rich) amateurs like Herschel and Lord Rosse with his 72-inch ‘Leviathan’ at Parsonstown in Ireland, but from early in the 20th Century the technology for big mirrors took the lead, and the world’s greatest telescopes today are now all reflectors.  Compared to lenses, mirrors can be made with huge light-collecting power.  The largest lens used was just over 1 meter in diameter — telescope mirrors today exceed 10 meters and larger instruments are planned.

So the Great Refractors were products of their age.  At the end of the 19th Century two revolutions in astronomy were being completed:  The application of photography was being perfected for astronomical observation. Early photographic emulsions, however, reacted differently than eye-sight to light of different colours and significant re-alignment was necessary to alter a telescope to bring images into sharp focus for photo plates.  As a result these telescopes were built either as visual- or photo– refractors, but most had some (usually cumbersome) means of re-alignment to use for both modes.  Some are double refractors — two telescopes, one visual, one a photo-refractor, in the same tube.  The development of better photographic emulsions just after the period in question, made all that malarkey obsolete.  Second, the analysis of light spectra, ‘spectroscopy’ and ‘spectrography’, was revolutionising astro-physics.  Spectral analysis could show what elements made-up a celestial object, in what abundance they occurred, what their temperature was, and even their radial velocity relative to the Earth.  Almost all the telescopes here were built to support ever-better spectroscopic and spectrographic equipment.

How did these machines come about?  At the end of the 18th Century British instrument makers and opticians like Jesse Ramsden [ More; More], Edward Troughton [ More],  and the Dollands were pre-eminent, but the largest quality lenses being used were only around 4 or 5 inches diameter.  It was not possible to get high-quality glass blanks to grind bigger lenses.  All that changed in the 1810s in Munich.  The team of Pierre Guinand (a Swiss) and Joseph Fraunhofer (L) transformed lens and telescope making.  Guinand worked-out how to cast much larger high-quality glass blanks and Fraunhofer painstakingly devised the methods to turn them into precision lenses mounted in the most sensitive instruments yet made.  On the way Fraunhofer made several fundamental discoveries in optics and spectroscopy.  Many will have heard of his Fraunhofer lines—the dark lines he mapped in the solar spectrum and characteristic of the elements in the Sun.  Guinand and Fraunhofer re-invented the high-quality optics and instrument industry.  Fraunhofer’s instruments included the Great Dorpat Refractor [ More] of 1824 (just under 10-inches diameter) and the Heliometer of 1829 used by Bessel to measure the parallax of 61-Cygni, the first-ever accurate parallax determination (see more in our article about James Bradley).

Their methods spread across Europe and to the USA, kick-starting the building of bigger and bigger telescopes. By 1838 we see the Northumberland Equatorial [ More; More; More; More] of almost 12-inches (lens by Cauchoix and Guinand, mounting by George Airy) and in 1839 Georg Merz [ More] who followed Fraunhofer in the Munich business) builds a 15-inch for the Pulkovo Observatory followed in 1847 by a similar machine for Harvard College.  The Craig telescope of 1852 was something of an aberration at 24 inches and never worked well.

By 1861 we see the emergence of the Clarks in Massachusetts as the builders of the world’s biggest with the 18.5 inch Dearborn (Chicago) Refractor and then in 1873 the 26-inch USNO Refractor in Washington.  That brings us to the earliest telescope in our top-7 list: their 1885 30-inch Pulkovo Refractor.  More of the Clarks later.

So much for background.  On with the show.  The trouble is things are never simple.  As I dug into the history books and websites it was soon clear that existing lists [ More; More] of the largest telescopes are different.   The main difference turned-out to be caused by telescopes that had been built but don’t survive today.  When they’re taken into account it appears that the Greenwich 28-inch Great Equatorial is indeed the seventh largest in existence...  but the NINTH largest ever completed...

...this article had to be re-titled THE OTHER EIGHT”.


Read on...

The 28-inch Great Equatorial at the Royal Observatory Greenwich © NMM

Joseph Fraunhofer