With respect to the tube, it is mounted like the Alt-Azimuth, but with the plane of the horizontal movement corresponding to the latitude of the place of observation, and therefore following objects with one screw movement is in reality a telescope with equatorial motions. It need not be of heavier construction than the Alt-Azimuth, and there is not the double weight of counterpoising, &c. as in the equatorial, and when circles are not desired this will be the most economical, handiest, and easily managed instrument, as it partakes of the equatorial form or motions. The telescope tube is well balanced, and the declination movement is easy and free, and fitted with a clamp screw, so that when the instrument is turned on an object that object can then be followed by an endless screw.
It can be made to suit any latitude, and by the addition of foot screws on a level floor, can be got into suitable adjustments, and if it has to be removed (for the stand is very little heavier than an ordinary Alt-Azimuth), “guides” can be provided, so that it can go very approximately into the exact place again when brought out for use. They are made, when desired, with a revolving body and fine screw-motion in declination.
THE “POPULAR REFLECTOR.”
With Angle-Block Stand.—(Fig. 4.)
The speculum is 5-1/4 inch diameter, and carefully figured, and is recommended as a very useful instrument. The size and power of this telescope has been adopted as that most likely to meet the means and requirements of a large number of amateurs. Many prefer to commence astronomical observations with “something inexpensive,” and are led to begin with the popular and well advertised 3 inch refractor at £5. These, except in a few chance cases, are sure to prove disappointing, and are perhaps the cause of their giving up any further attempts to follow up the subject of astronomy, which may otherwise be so pleasantly and profitably pursued with a reliable instrument.
It is well known that a less aperture than 4-1/2 inches is insufficient to give the observer a satisfactory idea of the varied and most interesting details of the planets, and the ever-changing outline and tone of the belts and markings of Jupiter, Saturn, and Mars. Large apertures bring out details when the smaller ones can only show a general outline. The former also delineates more with a lower power, providing the focus is of proper length.
The planets and nebulæ cannot be seen to advantage without aperture and focal length. The field of view is then flat, the object is properly illuminated, with sharp and crisp definition, and is also much less subject to annoyance from tremour, through the necessity for constant adjustments in the fields, &c., as is the case with small apertures, for the object is magnified more in proportion. A certain magnifying power is necessary in order that the object may be sufficiently large to scrutinize; and this, whether the aperture is large or small, must be from about 150 to about 300 times for the planets. But 150 on a 3 inch is a high power for the quantity of light obtained, to say nothing of the separate consideration and advantage of long focus. Aperture is a quality or function of the telescope considered separately from light or focus. For instance, suppose a 5 inch speculum is so thinly silvered that it gives the exact degree of light as a 2-1/2 inch refractor, the defining and separating power of the 5 inch aperture would be very superior to the 2-1/2 inch.
The “Popular Reflector,” with 5-1/4 inch speculum of 5, 5-1/2, to 6 feet focus, will be found very suitable, and if its illuminating power is not greater than a 4-1/2 inch refractor, its defining and dividing powers are superior. By choosing the above focal lengths, according to circumstances, the observer can reach the eye-tube, while standing erect for objects in the zenith, and it is a great mistake to suppose that the shorter the tube the more handy it becomes. These foci will not require a “Barlow” lens to flatten the field. The “Barlow” is very useful for short foci when the aperture is considerable, as it improves the imperfect correction for spherical error, but this is much better corrected in the mirror itself than by a “Barlow” lens, which cannot be used without more than one disadvantage. There must, by its insertion, be some loss of light, which can ill be spared with small instruments, and when used to obtain magnifying power there is some disturbance of colour, and this subtracts from the beauty and purity of the definition of a reflector. There is nothing equal to a good eye-piece to obtain power, and flatness of field by the curve of the speculum.
With the “Popular Reflector” and an outlay of a few shillings on some popular books, such as the excellent work “Celestial Objects,” by the Rev. T. W. Webb, and some first-class publications by Mr. R. A. Proctor, especially his smaller star atlas, &c., the amateur can compare the work he is then capable of doing with a large and expensive refractor which might be beyond his reach.
THE ADJUSTMENTS OF THE EQUATORIAL.