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HINTS ON Silvered-Glass Reflecting Telescopes.
Of the various forms of Telescopes now in use, each has its own
peculiar advantages; but the Silvered-Glass Reflector is undoubtedly
gaining favour among our practical observers. A well-figured
speculum, being perfectly free from chromatic aberration, gives, in
a proper condition of the atmosphere, the finest possible definition
of the Moon and planets, the markings and colours of these objects
being excellently seen; while coloured stars, such as Albireo (β
Cygni), or Almaach (γ Andromedæ), are exceedingly well shown, the
beautiful contrast of the stars in the former being especially
noticeable in a reflector. The advice of “F.R.A.S.” (in the “English
Mechanic,” March 21st, 1873) as to the choice of a Telescope, may
here appropriately be quoted. After expressing a preference for
refractors when measuring close double-stars, he says, “But should
the object of your correspondent be merely to regard the wonders and
beauties of the Heavens, or notably, should he purpose to devote
himself to the study of the physical structure of the Moon and
planets, then by all means let him obtain the largest reflector he
can afford; its absolute achromatism tells most astonishingly on
these last-named objects.” This is the opinion of one who has great
practical knowledge of the different forms of Telescopes.
If Achromatic Telescopes of large aperture could be made as cheaply
as reflectors, and in as convenient a form, they would doubtless be
preferred for general star-work, although the aberrations,
especially that of colour, cannot be so perfectly corrected. A
silvered-glass reflector is, however, much cheaper than a refractor,
and, when the aperture exceeds five or six inches, is much handier
to work, and occupies less space, being only about half the length
of an achromatic of the same aperture. It is true that a reflector
will give less light than an achromatic of equal aperture—but this
is, in certain conditions of the atmosphere, a distinct advantage,
the extra aperture to give the same light adding to the definition
and penetrating power. An example of this is seen in the beautiful
definition given by an unsilvered mirror on brilliant objects, as
the Sun, Moon, and Venus. In large achromatics, the distressing
excess of light has often to be reduced by diminishing the aperture
or using a higher power than is convenient; and in such cases a
lower and more suitable power can be employed with a reflector.
When the air is unsteady, the definition of Reflectors, owing to
their tubes being open, is more liable to fluctuate than that of
refractors, although when a reflector does not give good definition
on account of the atmosphere, a refractor of the same aperture will
certainly not perform satisfactorily. Sir John Herschel has shown
that when the air in the tube of even an achromatic is disturbed in
turning from one object to another, good definition does not return
until it is brought to rest. In order to reduce the vibration of the
air in the open tube, and also that of the stand, to a minimum,
reflectors require to be very firmly and steadily mounted, and to
have iron tubes.
Many of the specula made by me are now in observatories, where they
have been compared with achromatics of first-rate quality with the
most satisfactory results. For instance, several 6-1/2 inch specula
were tried with two excellent achromatics of 4-1/2 and 6 inches
aperture, when the planetary definition was considered to be
superior with the reflectors; and the appearance of the star-discs,
with equal apertures, differed little from the beauty and hardness
of the images given by the achromatics. In viewing stars of great
altitude, the use of the refractor is extremely inconvenient to the
observer, whose position is necessarily very uncomfortable, while
with the Newtonian reflector any part of the heavens may be observed
with the same ease and comfort. In short, a good speculum, well
mounted and situated, is sure to be both pleasing and satisfactory.
It is perhaps unnecessary to remind the reader that, when the
defining powers of a telescope are put to the test, as much depends
on the acuteness of the observer’s eye and the practice he has had,
as upon the perfection of the instrument and the fineness of air. It
is a mistake to suppose that when the stars appear to be the
brightest to the unaided eye, definition will be at its best, though
this may happen; when it does so, the astronomer should make the
most possible use of the opportunity, as such nights are very
scarce. As a rule, the best nights are those when there is the
slightest possible mistiness of the atmosphere, and for the faintest
stars absence of bright moonlight. The 5 inch mirror is guaranteed
to divide stars one second apart with ease, and closer ones in very
fine air. The 8-1/2 will split such difficult pairs as γ^2
Andromedæ, and μ^2 Boötis. An acute eye will master these stars with
even a less aperture on very favourable occasions, γ^2 Andromedæ
having been seen with a 7-inch stop of a 10-inch mirror, and Mr.
Sadler, of Honiton Rectory, has split this star with his 6-1/2-inch
telescope. It sometimes happens on favourable nights that the most
difficult objects will be seen with the same telescope, which on
other occasions had failed to show them as well as a much smaller
aperture had done in very fine air. These remarks equally apply to
the observation of minute stars and planetary detail. The amateur
must follow the advice of the Rev. T. W. Webb, given in “Celestial
Objects” (pages 15-17), and must cultivate that virtue applicable to
all scientific investigation, namely, patience. The following
interesting and difficult objects may be looked for, the powers used
for their observation should vary from 150 to 300, and for the very
closest stars up to 500, or even still higher.