mirror, which returns the rays towards the centre of the large mirror and causes them to converge less rapidly. They then meet a small plane mirror supported at the point of intersection of the polar and declination axes, whence they are reflected down through the hollow polar axis as shown in fig. 2, and come to focus on the slit of the powerful spectroscope that is mounted on a pier in the chamber of constant temperature as shown in fig. 20. In this case the equivalent focal length is 150 ft.
(3) As a Cassegrain reflector, for photographing the moon, planets or very bright nebulae on a large scale, as shown in fig. 21 (c), with an equivalent focal length of 100 ft.
(4) As a Cassegrain reflector, for use with a spectroscope mounted in place of the photographic plate, fig. 21 (d); in this case a convex mirror of different curvature is employed, the equivalent focus of the combination being 80 ft.
Fig. 20.
From Professor Hale’s The Study of Stellar Evolution, by permission of the University of Chicago Press.
Type E.—In the Comptes Rendus for the year 1883, vol. 96,
pp. 735–741, Loewy gives an account of an instrument which he
Fig. 22.—Loewy’s Coudé Equatorial.
calls an “equatorial coudé,” designed (1) to attain
greater stability and so to measure larger angles than is
generally possible with the ordinary equatorial; (2) to
enable a single astronomer to point the telescope and
Lowey’s equatorial coudé.
make observations in any part of the sky without changing his
position; (3) to abolish the usual expensive dome, and to substitute
a covered shed on wheels (which can be run back at pleasure),
leaving the telescope in the open air, the observer alone being
sheltered. These conditions are fulfilled in the manner shown in
fig. 22. E P is the polar axis, rotating on bearings at E and P.
The object-glass is at O, the eye-piece at E. There is a plane
mirror at M, which reflects rays converging from the object-glass
to the eye-piece at E. A second mirror N, placed at 45° to the
optical axis of the object-glass, reflects rays from a star at the pole;
but by rotating the box which contains this mirror on the axis of
its supporting tube T a star of any declination can be observed,
and by combining this motion with rotation of the polar axis the
astronomer seated at E is able to view any object whatever in the
visible heavens, except circumpolar stars near lower transit. An
hour circle attached to E P and a declination circle attached to the
box containing the mirror N, both of which can be read or set
from E, complete the essentials of the instrument.
There must be a certain loss of light from two
additional reflections; but that could be tolerated
for the sake of other advantages, provided that
the mirrors could be made sufficiently perfect
optical planes. By making the mirrors of
silvered glass, one-fourth of their diameter
in thickness, the Henrys have not only
succeeded in mounting them with all
necessary rigidity free from flexure
but have given them optically
true plane surfaces, notwithstanding
their large diameters,
viz., 11 and 15·7 in.
Sir David Gill tested the
equatorial coudé on double
stars at the Paris Observatory
in 1884, and his last
doubts as to the practical
value of the instrument
were dispelled. He has
never seen more perfect
optical definition in any
of the many telescopes he has employed, and certainly never
measured a celestial object in such favourable conditions of physical
comfort. The easy position of the observer, the convenient position
of the handles for quick and slow motion, and the absolute rigidity
