History of the Telescopes
 The
72 inch telescope after restoration
© Birr Castle Archives |
| "Visitors to Birr have sometimes commented on
the absence of any biography of my great-grandfather and grandfather,
remarking that the lack is curious in view of their contributions
to scientific knowledge. I believe that two main reasons have accounted
for this till the present time. First, they never kept the results
of their observations to themselves; all discoveries were reported
immediately to the relevant scientific publications of the day, and
therefore became common knowledge. Secondly, some of their major deductions
were so revolutionary that, in default of any outstanding proof, they
were not generally accepted by other scientists of the time. It is
only in quite recent years that the general correctness of my great-grandfather's
conclusions, reached by his studies of the galaxies, has been proved.
Similarly, my grandfather's estimate of the heat of the moon is now
known to have been absolutely right."
The opening words by Laurence
Michael, Sixth Earl of Rosse from the forward of "The Astronomy
of Birr Castle" by Patrick Moore", originally published in 1971.
During the 1840's and starting
from virtually first principles, the third Earl of Rosse designed
and had built the mirrors, tube and mountings for a 72 inch reflecting
telescope which was the largest in the world at that time and remained
so for three quarters of a century. With this instrument, situated
near the middle of Ireland, Lord Rosse was able to study and record
details of immensely distant stellar objects and to provide evidence
that many of these mysterious nebulae were actually galaxies located
far outside our own. |
 Illustration
of the 72 inch Telescope
© Birr Castle Archives |
|
The future third Earl of Rosse, William, was given the title of Lord Oxmantown
from his birth on the 17th of June, 1800. He was the eldest of three
sons, all of whom were educated at home in Birr by private tutors
rather than being sent to public schools in England. This type of
education may have greatly helped William's natural instinct for
engineering, practical aspects of which surrounded him on the Birr
Castle demesne. Private tuition gave way to a university education
when William was 18 years old. He then went to Oxford College, and
graduated with a first class honours Mathematics degree in 1822.
William joined the Royal Astronomical Society in 1824 and represented
Co. Offaly (at that time known as King's County) from 1823 to 1834
in the House of Lords, supporting both Catholic Emancipation and
the Reform Bill. He retired from politics to pursue his scientific
and engineering interests and in 1836 he married Mary Wilmer-Field,
a wealthy heiress from Yorkshire. The financial security from this
marriage, as well as the ownership of Birr Castle (which his parents
granted him before they left Ireland to live in Brighton, England)
allowed him to realize his scientific ambitions and plans. In 1828
he published his first experimental results regarding the grinding
and publishing of telescope mirrors in the Edinburgh
Journal of Science. At the outset of his career William decided
to publish all his results, in marked contrast to many other telescope
makers. In 1831, he joined the British Royal Society (and was its
president from 1848 to 1854).
The first major telescope built by Lord Rosse was based around
a 36 inch (91 cm) mirror. After experimenting with copper and tin
alloys for the metal reflecting surface and developing his own steam-driven
mirror grinding machine, this telescope was completed in 1839. It
was supported in a frame which was a modification (and improvement)
of a design by William Herschel. A circular track allowed this telescope
to reach most areas of the sky. Despite this telescope being "home-built"
(the mirror was cast in the grounds of Birr Castle) two contemporary
expert observers confirmed it's quality and performance, which allowed
significantly increased resolution of stellar objects, one of them
(Dr. Thomas Robinson of the Armagh Observatory) calling it the most
powerful instrument of it's time. Lord Rosse used the 36 inch telescope
to study the Moon in greater detail than was previously possible,
under a magnification of 900 times. He also studied star clusters
and nebulae (fuzzy patches of gas) which earlier telescope were
incapable of resolving to any great degree. |
 Lady
Rosse, The Fourth Earl and Dr. Boeddicker
© Birr Castle Archives |
Lord Rosse spent over three years building his next
large telescope, "The Leviathan of Parsonstown" based around a 72
inch (183 cm) mirror, with which he hoped to confirm that some of
the nebulae contain stars, hints of this being given by the 36 inch
telescope. This telescope was basically a 17 m tube, suspended between
two 15 m high walls, with a network of supports which enabled the
tube to be moved freely in a vertical direction, but restricted
horizontal movement to the extent that the total viewing time on
any particular object varied from about 50 minutes for an object
at the equator to about 2 hours for an object at the highest level.
The vertical movement was contained between the lowest level of
15 degrees elevation and the highest level of 15 degrees beyond
the vertical, i.e. a total vertical travel of 90 degrees. Again
the mirror was cast in the grounds of the Castle, using 3 large
crucibles and resulting in a mirror that weighed over 3 tons. Construction
of the telescope and mountings took over 2 years but finally, on
15 February 1845, the weather cleared long enough for a short viewing
of the double star Castor, which confirmed the potential of the
new telescope. In April 1845, Lord Rosse was able to deduce the
spiral nature of the M51 nebulae. |
 A
drawing of the 72 inch telescope in its viewing position
© Birr Castle Archives |
The potato famine in Ireland prevented regular use
of the telescope until 1848. The telescope was initially used to
view Jupiter and the moon, although the bulk of the work done by
the 72 inch was in the area of nebulae and star clusters. At the
time it was commonly thought that these gas clouds were part of
our galaxy. Lord Rosse observed many nebulae and was able to resolve
spiral arms, indicating that at least some of the nebulae were more
than just collections of gas. In 1923 Edwin Hubble showed, through
his measurement of the distance of variable stars, that some of
the starry nebulae were in fact galaxies separate (and in many cases
larger), than our own. |
 M51,
a spiral galaxy 37 million light-years distant
© Birr Castle Archives |
Lord Rosse also realised that he was observing edge-on
spiral objects and by the end of 1850, the number of spiral objects
seen had reached fourteen. He made drawings of the objects he observed
(astronomical photography was well established by the time of the
fourth Earl, but the 72 inch telescope was too unsteady for the
long exposures required), and noted various types of spirals. Lord
Rosse studied M1 (The first object in Charles Messier's list of
over 100 star clusters and nebulae) naming it the Crab Nebula, along
with many other nebulae, including M51, M87 and the planetary nebula
M27. Use of the telescope was hampered however, by the cloudy Irish
weather and the necessity of replacing the metal mirror which tarnished
quickly, with the backup mirror, at regular intervals.
In addition to Lord Rosse, visiting astronomers from many parts
of the world came to use the telescope and it's fame was such that
it is referred to in Jules Verne's early Science Fiction novel
"From the Earth to the Moon"
"The distance which had then separated the projectile
from the satellite was estimated at about two hundred leagues. Under
these conditions, as regards the visibility of the details of the
disc, the travelers were farther from the moon than are the inhabitants
of earth with their powerful telescopes. Indeed, we know that the
instrument mounted by Lord Rosse at Parsonstown, which magnifies 6,500
times, brings the moon to within an apparent distance of sixteen leagues."
Among the astronomers who took advantage of the telescopes unequalled
light gathering power were J.L.E Dreyer, who later compiled the
New General Catalogue of clusters and nebulae. Lord Rosse's son
Laurence who became the fourth Earl on the death of his father in
1867 also used the telescope regularly and made the first accurate
estimate of the temperature of the Moon. However, the instrument
used for this work was the 36 inch reflector, which the fourth Earl
had fitted with a drive to enable tracking of the Moon's movement.
The 72 inch telescope was used less frequently and on the death
of the fourth Earl it was dismantled.
The third Earl of Rosse occupies an important position in the
history of astronomy. In the words of Professor Sir Bernard Lovell:
"He succeeded in an almost impossible task, the
measure of which can be appreciated from the fact that his telescope
remained the largest in the world for three-quarters of a century......The
Birr Telescope is a tribute to the third Earl's skill in engineering
and optics: the results he obtained with it are a remarkable tribute
to his observational skill and to his insight that such a device would
record more of the depths of the universe than man had yet conceived.
I have before me two illustrations of the nebula in Canes Venatici-
a galaxy more than ten million light years away in space. One is a
drawing made by Lord Rosse as he saw it in the Birr telescope. The
other, a photograph taken a century later by the 200 inch telescope
on Mount Palomar. The identity of the two is dramatic and the spiral
form of the galaxy is shown with far greater form in the drawing.
It is to the everlasting credit of Lord Rosse that he discovered the
spiral structure of the nebulae and thereby opened an avenue of exploration
which today has lead us into the inconceivable depths of space and
time." |
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