Should the beginner not be willing or able to face the purchase of even a comparatively humble instrument, his case is by no means desperate, for he will find facilities at hand, such as were not thought of a few years ago, for the construction of his own telescope. Two-inch achromatic object-glasses, with suitable lenses for the making up of the requisite eye-pieces, are to be had for a few shillings, together with cardboard tubes of sizes suitable for fitting up the instrument; and such a volume as Fowler's 'Telescopic Astronomy' gives complete directions for the construction of a glass which is capable of a wonderful amount of work in proportion to its cost. The substitution of metal tubes for the cardboard ones is desirable, as metal will be found to be much more satisfactory if the instrument is to be much used. The observer, however, will not long be satisfied with such tools as these, useful though they may be. The natural history of amateur astronomers may be summed up briefly in the words 'they go from strength to strength.' The possessor of a small telescope naturally and inevitably covets a bigger one; and when the bigger one has been secured it represents only a stage in the search for one bigger still, while along with the desire for increased size goes that for increased optical perfection. No properly constituted amateur will be satisfied until he has got the largest and best instrument that he has money to buy, space to house, and time to use.
Let us suppose, then, that the telescope has been acquired, and that it is such an instrument as may very commonly be found in the hands of a beginner—a refractor, say, of 2, 2½, or 3 inches aperture (diameter of object-glass). The question of reflectors will fall to be considered later. Human nature suggests that the first thing to do with it is to unscrew all the screws and take the new acquisition to pieces, so far as possible, in order to examine into its construction. Hence many glasses whose career of usefulness is cut short before it has well begun. 'In most cases,' says Webb, 'a screw-driver is a dangerous tool in inexperienced hands'; and Smyth, in the Prolegomena to his 'Celestial Cycle,' utters words of solemn warning to the 'over-handy gentlemen who, in their feverish anxiety for meddling with and making instruments, are continually tormenting them with screw-drivers, files, and what-not.' Unfortunately, it is not only the screw-driver that is dangerous; the most deadly danger to the most delicate part of the telescope lies in the unarmed but inexperienced hands themselves. You may do more irreparable damage to the object-glass of your telescope in five minutes with your fingers than you are likely to do to the rest of the instrument in a month with a screw-driver. Remember that an object-glass is a work of art, sometimes as costly as, and always much more remarkable than, the finest piece of jewellery. It may be unscrewed, carefully, from the end of its tube and examined. Should the examination lead to the detection of bubbles or even scratches in the glass (quite likely the latter if the instrument be second-hand), these need not unduly vex its owner's soul. They do not necessarily mean bad performance, and the amount of light which they obstruct is very small, unless the case be an extreme one. But on no account should the two lenses of the object-glass itself be separated, for this will only result in making a good objective bad and a bad one worse. The lenses were presumably placed in their proper adjustment to one another by an optician before being sent out; and should their performance be so unsatisfactory as to suggest that this adjustment has been disturbed, it is to an optician that they should be returned for inspection. The glass may, of course, be carefully and gently cleaned, using either soft chamois leather, or preferably an old silk handkerchief, studiously kept from dust; but the cleaning should never amount to more than a gentle sweeping away of any dust which may have gathered on the surface. Rubbing is not to be thought of, and the man whose telescope has been so neglected that its object-glass needs rubbing should turn to some other and less reprehensible form of mischief. For cleaning the small lenses of the eye-pieces, the same silk may be employed; Webb recommends a piece of blotting-paper, rolled to a point and aided by breathing, for the edges which are awkward to get at. Care must, of course, be taken to replace these lenses in their original positions, and the easiest way to ensure this is to take out only one at a time. In replacing them, see that the finger does not touch the surface of the glass, or the cleaning will be all to do over again.
FIG. 8.
a, O.G. in perfect adjustment; b, O.G. defectively centred.
Next comes the question of testing the quality of the objective. (The stand is meanwhile assumed, but will be spoken of later.) Point the telescope to a star of about the third magnitude, and employ the eye-piece of highest power, if more than one goes with the instrument—this will be the shortest eye-piece of the set. If the glass be of high quality, the image of the star will be a neat round disc of small size, surrounded by one or two thin bright rings (Fig. 8, a). Should the image be elliptical and the rings be thrown to the one side (Fig. 8, b), the glass may still be quite a good one, but is out of square, and should be readjusted by an optician. Should the image be irregular and the rings broken, the glass is of inferior quality, though it may still be serviceable enough for many purposes. Next throw the image of the star out of focus by racking the eye-piece in towards the objective, and then repeat the process by racking it again out of focus away from the objective. The image will, in either case, expand into a number of rings of light, and these rings should be truly circular, and should present precisely the same appearance at equal distances within and without the focus. A further conception of the objective's quality may be gained by observing whether the image of a star or the detail of the moon or of the planets comes sharply to a focus when the milled head for focussing is turned. Should it be possible to rack the eye-tube in or out for any distance without disturbing the distinctness of the picture to any extent, then the glass is defective. A good objective will admit of no such range, but will come sharply up to focus, and as sharply away from it, with any motion of the focussing screw. A good glass will also show the details of a planet like Saturn, such as are within its reach, that is, with clearness of definition, while an inferior one will soften all the outlines, and impart a general haziness to them. The observer may now proceed to test the colour correction of his objective. No achromatic, its name notwithstanding, ever gives an absolutely colourless image; all that can be expected is that the colour aberration should have been so far eliminated as not to be unpleasant. In a good instrument a fringe of violet or blue will be seen around any bright object, such as Venus, on a dark sky; a poor glass will show red or yellow. It is well to make sure, however, should bad colour be seen, that the eye-piece is not causing it; and, therefore, more than one eye-piece should be tried before an opinion is formed. Probably more colour will be seen at first than was expected, more particularly with an object so brilliant as Venus. But the observer need not worry overmuch about this. He will find that the eye gets so accustomed to it as almost to forget that it is there, so that something of a shock may be experienced when a casual star-gazing friend, on looking at some bright object, remarks, as friends always do, 'What beautiful colours!' Denning records a somewhat extreme case in which a friend, who had been accustomed to observe with a refractor, absolutely resented the absence of the familiar colour fringe in the picture given by a reflector, which is the true achromatic in nature, though not in name. The beginner is recommended to read the article 'The Adjustment of a Small Equatorial,' by Mr. E. W. Maunder, in the Journal of the British Astronomical Association, vol. ii., p. 219, where he will find the process of testing described at length and with great clearness.
In making these tests, allowance has, of course, to be made for the state of the atmosphere. A good telescope can only do its best on a good night, and it is not fair to any instrument to condemn it until it has been tested under favourable conditions. The ideal test would be to have its performance tried along with that of another instrument of known good quality and of as nearly the same size as possible. If this cannot be arranged for, the tests must be made on a succession of nights, and good performance on one of these is sufficient to vindicate the reputation of the glass, and to show that any deficiency on other occasions was due to the state of the air, and not to the instrument. Should his telescope pass the above tests satisfactorily, the observer ought to count himself a happy man, and will until he begins to hanker after a bigger instrument.
The mention of the pointing of the telescope to a star brings up the question of how this is to be done. It seems a simple thing; as a matter of fact, with anything like a high magnifying power it is next to impossible; and there are few things more exasperating than to see a star or a planet shining brightly before your eyes, and yet to find yourself quite unable to get it into the field of view. The simple remedy is the addition of a finder to the telescope. This is a small telescope of low magnifying power which is fastened to the larger instrument by means of collars bearing adjusting screws, which enable it to be laid accurately parallel with the large tube (Fig. 10). Its eye-piece is furnished with cross-threads, and a star brought to the intersection of these threads will be in the field of the large telescope. In place of the two threads crossing at right angles there may be substituted three threads interlacing to form a little triangle in the centre of the finder's field. By this device the star can always be seen when the glass is being pointed instead of being hidden, as in the other case, behind the intersection of the two threads. A fine needle-point fixed in the eye-piece will also be found an efficient substitute for the cross-threads. In the absence of a finder the telescope may be pointed by using the lowest power eye-piece and substituting a higher one when the object is in the field; but beyond question the finder is well worth the small addition which it makes to the cost of an instrument. A little care in adjusting the finder now and again will often save trouble and annoyance on a working evening.
The question of a stand on which to mount the telescope now falls to be considered, and is one of great importance, though apt to be rather neglected at first. It will soon be found that little satisfaction or comfort can be had in observing unless the stand adopted is steady. A shaky mounting will spoil the performance of the best telescope that ever was made, and will only tantalize the observer with occasional glimpses of what might be seen under better conditions. Better have a little less aperture to the object-glass, and a good steady mounting, than an extra inch of objective and a mounting which robs you of all comfort in the using of your telescope. Beginners are indeed rather apt to be misled into the idea that the only matters of importance are the objective and its tube, and that money spent on the stand is money wasted. Hence many fearful and wonderful contrivances for doing badly what a little saved in the size of the telescope and expended on the stand would have enabled them to do well. It is very interesting, no doubt, to get a view of Jupiter or Saturn for one field's-breadth, and then to find, on attempting to readjust the instrument for another look, that the mounting has obligingly taken your star-gazing into its own hands, and is now directing your telescope to a different object altogether; but repetition of this form of amusement is apt to pall. A radically weak stand can never be made into a good one; the best plan is to get a properly proportioned mounting at once, and be done with it.
