Telescopic Work for Starlight Evenings. William F. Denning
turn of the adjusting-screw should make a sensible difference. On the other hand, an inferior lens will permit a slight alteration of focusing without affecting the distinctness, because the rays from the image are not accurately thrown to a point. Jupiter is also a good test. The limbs of the planet, if shown clean and hard, and the belts, if they are pictured like the finely cut details of an engraving, will at once stamp a telescope as one of superior quality. Saturn can also be examined though not, perhaps, so severe a test. The belts, crape ring, Cassini’s division, ought to be revealed in any telescope of moderate aperture. If, with regard to any of these objects, the details apparently run into each other and there is a “fuzzy” or woolly aspect about them which cannot be eliminated by careful focusing, then either the atmosphere or the telescope is in fault. If the former, another opportunity must be awaited. An observer of experience will see at a glance whether the cause lies in the air or the instrument. The images will be agitated by obnoxious currents, if the defects are due to the atmosphere, but if the glass itself is in error, then the objects will be comparatively tranquil but merged in hazy outlines, and a general lack of distinctness will be apparent. Perhaps the best test of all as to the efficiency of a telescope is that of a moderately bright star, say of the 2nd or 3rd magnitude. With a high power the image should be very small, circular, and surrounded by two or three rings of light lying perfectly concentric with each other. No rays, wings, or extraneous appearance other than the diffraction rings should appear.
This, however, specially applies to refractors, for in reflectors the arms of the flat occasion rays from any bright star; I have also seen them from Mars, but of course this does not indicate an imperfect mirror. If there is any distortion on one side of the image, then the lenses are inaccurately centred though the instrument may be otherwise good, and a little attention may soon set matters right. When testing a glass the observer should choose objects at fairly high altitudes, and not condemn a telescope from a single night’s work unless the evidence is of unusually convincing character. If false colour is seen in a silver-on-glass reflector it is originated by the eyepiece, though not necessarily so in a refractor. The object-glass of the latter will be sure to show some uncorrected colour fringing a bright object. A good lens, when exactly focused, exhibits a claret tint, but within the focus purple is seen and beyond the focus green comes out. In certain cases the secondary spectrum of an object-glass is so inadequately corrected that the vivid colouring of the images is sometimes attributed by inexperienced observers to a real effect. A friend who used a 3-inch refractor once called on me to have a glimpse of Jupiter through my 10-inch With-reflector. On looking at the planet he at once exclaimed “But where are the beautiful colours, Mr. Denning?” I replied to his question by asking another, viz., “What colours?” he answered, “Why, the bright colours I see round Jupiter in my refractor?” I said, “Oh, they exist in your telescope only!” He looked incredulous, and when he left me that night did not seem altogether pleased with the appearance of Jupiter shorn of his false hues!
Mounting.—Too much care cannot be given to the mounting of telescopes, for the most perfectly figured glass will be rendered useless by an inefficient stand; a faulty lens, if thoroughly well mounted, will do more than a really good one on a shaky or unmanageable mounting. Whatever form is adopted, the arrangement should ensure the utmost steadiness, combined with every facility for readily following objects. A man who has every now and then to undergo a great physical exertion in bodily shifting the instrument is rendered unfit for delicate work. The telescope should be provided with every requisite for carrying on prolonged work with slight exertion on the part of the observer. Unless the stand is firm there will be persistent vibrations, especially if the instrument is erected in the open, for there are very few nights in the year when the air is quite calm. These contingencies should be provided against with scrupulous attention if the observer would render his telescope most effective for the display of its powers, and avoid the constant annoyance that must otherwise follow.
Fig. 14.
Huygens’s negative eyepiece.
Fig. 15.
Ramsden’s positive eyepiece.
Eyepieces.—Good eyepieces are absolutely essential. Many object-glasses and specula have been deprecated for errors really originated by the eyepiece. Again, telescopes have not unfrequently been blamed for failures through want of discrimination in applying suitable powers. A consistent adaptation of powers according to the aperture of the telescope, the character of the object, the nature of the observation, and the atmospheric conditions prevailing at the time, is necessary to ensure the best results. If it is required to exhibit a general view of Jupiter and his satellites to a friend, we must utilize a low power with a large field; if, on the other hand, we desire to show the red spot and its configuration in detail, we must apply the highest power that is satisfactorily available. The negative or Huygenian eyepiece is the one commonly used, and it forms good colourless images, though the field is rather small. The positive or Ramsden eyepiece gives a flatter and larger field, but it is not often achromatic. A Kellner eyepiece, the feature of which is a very large field, is often serviceable in observations of nebulæ, clusters, and comets. Telescopes are sometimes stated to bear 100 to the inch on planets, but this is far beyond their capacities even in the very best condition of air. Amateurs soon find from experience that it is best to employ those powers which afford the clearest and most comprehensive views of the particular objects under scrutiny. Of course when abnormally high powers are mentioned in connection with an observation, they have an impressive sound, but this is all, for they are practically useless for ordinary work. I find that 40, or at the utmost 50 to the inch, is ample, and generally beyond the capacities of my 10-inch reflector. A Barlow lens used in front of the eyepiece raises the power about one third, and thus a whole set of eyepieces may be increased by its insertion. It is said to improve the definition, while the loss of light is very trifling. I formerly used a Barlow lens in all planetary observations, but finally dispensed with it, as I concluded the improved distinctness did not compensate for the fainter image. A great advantage, both in light and definition, results from the employment of a single lens as eyepiece. True, the field is very limited, and, owing to the spherical aberration, the object so greatly distorted near the edges that it must be kept near the centre, but, on the whole, the superiority is most evident. By many careful trials I find it possible to glimpse far more detail in planetary markings than with the ordinary eyepiece. Dawes, and other able observers, also found a great advantage in the single lens, and Sir W. Herschel, more than a century ago, expressed himself thus:—“I have tried both the double and single lens eye-glass of equal powers, and always found that the single eye-glass had much the superiority in light and distinctness.”
Requisite Powers.—For general purposes I believe three eyepieces are all that is absolutely requisite, viz., a low power with large field for sweeping up nebulæ and comets; a moderate power for viewing the Moon and planets; and a high power for double stars and the more delicate forms on the planets. For a 3-inch refractor, eyepieces of about 15, 75, and 150 would be best, and for a 10-inch reflector 40, 150, and 300. For very difficult double stars a still higher power will be occasionally useful, say 250 for the refractor, and 500 for the reflector. The definition usually suffers so much under high powers, and the tremors of the atmosphere are brought out so conspicuously, that the greater expansion of the image of a planet does not necessarily enable it to present more observable detail. The features appear diluted and merged in hazy outlines, and there is a lack of the bright, sharply determinate forms so steadily recognized under lower magnifiers. In special cases great power may become essential, and, under certain favourable circumstances, will prove really serviceable, but, in a general way, it is admitted that the lowest power which shows an object well is always the best. I have occasionally obtained very fair views of Saturn with a power of 865, but find that I can perceive more of the detail with 252. Some daylight observations of Venus were also effected under very high power, and, though the definition remained tolerably good, I found as the result of careful comparison that less power answered more satisfactorily. But it would be absurd to lay down inviolable rules in such cases. Special instruments, objects, and circumstances require special powers, and