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 γ2 Andromedæ, and μ2 Boötis. An acute eye will master these stars with even a less aperture on very favourable occasions, γ2 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.
TESTS FOR SPECULA.