The Craig Telescope(home page:- The Craig Telescope http://homepage.ntlworld.com/greg.smyerumsby/craig/articles.html)
GIGANTIC TELESCOPE AT WANDSWORTH-COMMON
The construction of a monster reflecting telescope by the Earl of Rosse
constituted for a considerable period a prominent topic of interest and
conversation in the scientific world. The patience and perseverance of the noble
projector under every kind of discouragement, and the unwavering faith with
which, at a large outlay to himself, he prosecuted the enterprise to a
successful conclusion, secured to him the admiration and esteem of all who took
an interest in the higher departments of science, while the discoveries that
have since been made through its instrumentality have amply borne out his
anticipations and rewarded his exertions. Its was necessary, however, that
something further should be accomplished. To those not conversant with the
subject, it may be necessary to state that a reflecting telescope on a large
scale must always be a work of exceeding difficulty, and comparatively limited
utility. The possibility of constructing an achromatic instrument of a power
equal to Lord Rosse's, and through which the object looked at could be directly
magnified (as with an opera-glass), has long been considered extremely doubtful;
in fact, beyond the reach of mechanical and optical appliances. This desideratum
is, however, now on the eve of being supplied.
In the course of a recent ramble on Wandsworth-common our
attention was attracted by a singular-looking structure, consisting of a plain
tower with a long tube slung by its side, surrounded by a wooden-hoarding to
keep off intruders. On making inquiries we learned that it was a new monster
telescope on the achromatic principle in process of construction, under the
superintendence of Mr. W. Gravatt, F.R.S., for the Rev. Mr. Craig, vicar of
Leamington. Having obtained an introduction, we inspected the instrument, and
ascertained some particulars respecting it which may not be uninteresting. The
site, consisting of two acres, has been liberally presented by Earl Spencer in
perpetuity, or so long as the telescope shall be maintained. The central tower,
consisting of brick, is 64 feet in height, 15 feet in diameter, and weighs 220
tons. Every precaution has been taken in the construction of this building to
prevent the slightest vibration; but, if any disappointment in this respect
should arise (which, however, Mr. Gravatt does not anticipate), additional
weight can be obtained by loading the several floors, and the most perfect
steadiness will be thus insured. By the side of this sustaining tower hangs the
telescope. The length of the main tube, which is shaped somewhat like a cigar,
is 76 feet, but with an eyepiece at the narrow end, and a dewcap at the other,
the total length in use will be 85 feet. The design of the dewcap is to prevent
obscuration by the condensation of moisture, which takes place during the night,
when the instrument is most in use. Its exterior is of bright metal, the
interior is painted black. The focal distance will vary from 76 to 85 feet. The
tube at its greatest circumference measures 13 feet, and this part is about 24
feet from the object glass. The determination of this point was the result of
repeated experiments and minute and careful calculations. It was essential to
the object in view that there should not be the slightest vibration in the
instrument. Mr. Gravatt, reasoning from analogy, applied the principle of
harmonic progression to the perfecting of an instrument for extending the range
of vision, and thus aiding astronomic research. By his improvements the
vibration at one end of the tube is neutralized by that at the other, and the
result is that the utmost steadiness and precision is attained. The ironwork of
the tube was manufactured by Messrs. Rennie, under the direction of Mr. Gravatt.
The object-glasses are also of English construction, and throw a curious light
upon the manner in which an enlightened commercial policy has reaction upon and
promoted the advancement of science. Up to a recent period the flint glass
for achromatic telescopes was entirely of foreign manufacture. Since the
reduction in the duty great improvements have been made in this department. The
making of the large flint glass was instructed to Mr. Chance of Birmingham, who
at first hesitated to manufacture one larger than nine inches in diameter. Upon
being urged, however, by Mr. Craig, he has succeeded in producing one 24 inches;
perfectly clear, and homogenous in structure. Besides this, there is a second of
plate-glass of the the same dimensions, cast by the Thames Plate Glass Company,
either of which the observer may use at his option. The manner in which
these object-glasses are fitted into the tube is a marvel of artistic invention.
By means of 12 screws, numbered according to the hours of the day, they can be
set in an instant to any angle the observer may require, by his merely calling
out the number of the screw to be touched. The object-glasses also move round in
grooves to wherever it may be considered that a more distinct view can be
gained. The tube rests upon a light wooden framework, with iron wheels attached,
and is fitted to a circular iron railway at a distance of 52 feet from the
centre of the tower. The chain by which it is lowered is capable of sustaining a
weight of 13 tons, though the weight of the tube is only three. Notwithstanding
the immense size of the instrument, the machinery is such that is can move
either in azimuth, or up to an altitude of 80 degrees, with as much ease and
rapidity as an ordinary telescope, and from the nature of the mechanical
arrangements, with far greater certainty as to results. . . . The
preparations for this really national work have been progressing for the last
two years under the superintendence of Mr. Gravatt as engineer and
mathematician, but it is only about three months since the superstructure at
Wandsworth-common was commenced, and it is already near completion.
Times August 23, 1852