In the _English mounting_ the telescope, like a transit instrument, has
on each side a pivot, and these pivots rest on a frame somewhat larger
than the telescope, pointing to the pole and supported by two pivots,
one at the bottom resting on bearings near the ground, and the other
carried by a higher pillar clear of the observer’s chair. The motions of
the telescope are similar to those given by the German mounting in all
essentials; the Greenwich equatorial is mounted in this manner. It is
carried in a large cylindrical frame, supported at both ends by two
pillars—above by a strong iron pillar, while the other end rests on a
firm stone pillar, going right to the earth, independently of the
flooring. This mounting, though preferred for the large instrument at
Greenwich, has been discarded generally, as the long polar axis is
necessarily a serious element of weakness; the telescope is supported on
its weakest part, and it is liable to great changes from contraction and
expansion of the frame.
_The Forked Mounting._
It is now getting more usual to mount Newtonians of large dimensions
equatorially, in spite of the immense weight to be carried. One of the
first methods was to use a polar axis in the same manner as for a
refractor, only that it bifurcated at the top, forming there a fork, and
between this fork the telescope is swung, after the same manner as a
transit. This method of mounting was adopted by M. Foucault in the case
of his first large silvered-glass reflector. The height of the
bifurcation is dependent on the distance between the centre of gravity
of the tube and the speculum, and if we use an extremely light tube, or
if,—as it is the fashion to abolish them now altogether for
reflectors,—we use a skeleton tube of iron lattice work, this
bifurcation of the polar axis need not be of any great length. The polar
axis being entirely below the telescope and being driven by the clock,
we have a perfect method of mounting a speculum of any weight we please.
This arrangement was first suggested and carried into effect by Mr.
Lassell for his four-foot Newtonian, which was mounted at Malta. The
polar axis was a heavy cone-shaped casting resting on its point below,
and moving on its largest diameter just below the base of the fork. Lord
Rosse has recently much improved upon the original idea.
As the observer must be at the mouth of the tube, he is in a very bad
position as far as comfort goes, especially as the eye-end changes its
position rapidly in consequence of the great length of the tube from its
centre of gravity outwards. The platform on which he stands is raised on
supports, extending from the floor and going up to the opening through
which the telescope points to the heavens, and the whole platform is
sometimes fixed to the dome of the observatory, so that it travels round
with it.
Public-domain text, read in full here on John Shaqi.
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