Chapter XXXIX: How to Make an Astronomical Telescope

BY FRANK CHASEMORE.

The investigation of astronomical phenomena can only be made with the aid of a good telescope, the purchase of which is attended with considerable cost. It is my purpose in this chapter to give such directions as will enable any boy with average ingenuity to make for himself, at the cost of a few shillings, an instrument with which he can observe the more interesting of these phenomena.

This telescope will be of the simple non-achromatic class--that is, the colour effect of the unequal refraction of light is not corrected. Object-glasses of the achromatic construction are very expensive. All refracting telescopes were of the simple class up to 1758, when Mr. John Dollond, who had a few years before set up in business in London as optician, discovered a way to correct the colour effect on the image. This was by making the object-glass compound, or of two or more lenses fitted to each other, each being made of a different quality of glass from the other and having a different refracting power, one lens correcting or neutralising the dispersing caused by the other. The lenses for our telescope can be had from Messrs. Dollond and Co., descendants of the above John Dollond, No. 1, Ludgate Hill, who have given me some valuable hints with regard to the construction of this telescope.

A refracting telescope consists of an eye-piece, a tube, and an object-glass; these are mounted on a firm stand. The object-glass at one end of the tube collects the rays of light, reflected from an object, to a point, in the focus of the eyepiece, which magnifies the image that is there formed, enabling the eye, placed at the orifice of the eyepiece, to see an enlarged image of the object.

The stand must be firm, so as not to vibrate when any one passes along the floor of the room, and it must have a vertical and lateral motion connected with it. Fig. 1 shows what our instrument will be like when finished.

The first thing to be made is the tube; this must be thirty-nine inches long and two inches in diameter inside. Get a wooden roller four feet long and two inches wide. A piece of curtain rod will do. Now mix some strong glue. If you have not a glue-pot, mix it in a jar placed in a saucepan of water. Get some sheets of stout brown paper and well damp them. Take a strip of brown paper, that has not been damped, thirty-nine inches long and seven inches wide. Rub the roller all over well with powdered chalk and put this dry paper strip round it to form a case, lapping and gluing the edges together, but being very careful not to let any glue touch the roller. Now take your damped paper and rub it all over on both sides with hot glue, and roll it on the roller; roll it tightly and rub the glue well in, and rub each layer of paper well in to the under one, so that when dry it will form a mass of paper and glue. Put on enough paper to form a casing a quarter of an inch thick. When you have done papering set the whole on one side to get quite dry and hard. While this is drying we can be making the eyepiece and stand. For the eyepiece we shall want a piece of brass tube four inches long and large enough for the larger of the two lenses that form the eyepiece to go inside; that will be a little more than an inch in diameter inside. Get your lenses before getting the tube. This tube can be bought at the ironmonger’s.

Now for the lenses. Go to Messrs. Dollond and ask for a two-inch simple object-glass, forty inches focus. This will be one shilling and sixpence. For the eyepiece ask for two plano-convex lenses--one of one-inch focus, the other of two-inch focus. These will be three shillings and sixpence the pair. The object glass is to be double convex. Now, having got your lenses, we will fix them in the tube.

Cut a piece of cardboard three-quarters of an inch wide, and long enough to go all round inside the tube tightly, and not to lap. Push this in to form a lining at one end, and forming a shelf out of the thickness. This shelf is to be about two inches from the end. Now turn the other end of the tube up and drop the larger of the two lenses--which is called the field lens--on to the shelf with the rounded side downwards. Now push in on the top of it another cardboard lining three-quarters of an inch wide. Push this lining quite down on to the flat side of the lens to keep it firm. On to the shelf formed by this lining place a disc of cardboard the size of the inside of the tube, and with a hole cut in the centre half an inch in diameter. This hole must be cut quite clean. On to this disc push in a cardboard lining one inch wide to keep all firm. Now cut two discs of cardboard, one the exact size of the inside of the tube, with a hole in the centre a trifle smaller in diameter than the small lens, which is called the eye-lens; the other a quarter of an inch smaller, and having a hole in the centre the exact size of the eye-lens. Glue these two discs together (as in Fig. 2), being careful to get them concentric. When this is dry push the eye-lens into the ledge formed, the flat side downwards, and put the cardboard discs on to the lining in the tube, the rounded side of the glass inside the tube. Fasten the disc and lens in place with a narrow strip of cardboard, going all round just inside the tube.

Fig. 3 will show the arrangement of lenses and disc in the tube. The lenses are to be an inch and three-quarters from edge to edge; the disc is to be an inch from the eye-lens. Now get a tinman to make you a cap of thin brass plate (like Fig. 4). This is to fit tightly on the end of the tube over the eye-lens, and is to have a hole in the centre of the top three-eighths of an inch in diameter.

Now the eyepiece is finished, and we will get on with the stand.

Let two circular pieces of beech five inches in diameter and one and a half inches thick, each to have a hole bored in the centre an inch in diameter, and going right through the wood. Take one of these pieces, and on the under side fasten, by three strong brass hinges that work stiffly, three legs, made of inch and a half square pine (Fig. 5). These legs are to be four feet six inches long.

Take the other piece of beech and fasten a rod or roller of wood, two feet and a half long and one inch diameter, into the hole, so that one end is flush with the top side of the wood (Fig. 6). Now fasten two uprights to the top, letting them into the wood (Fig. 6). These are to be three-quarters of an inch thick, and four and a half inches high, and are to be three and a half inches apart.

Bore a hole in each upright about half an inch from the top and about one quarter of an inch in diameter. Now get nine screw stair-eyes at the ironmonger’s, and about seven feet of brass wire one eighth of an inch thick. Get this wire straightened. Into the inside of each leg, and one foot from the top, screw one of the brass stair-screws. Now get a piece of wood, circular, three inches diameter and one and a half inches thick (a ribbon roller will do very well), and bore a hole right through the centre, one and a quarter inches in diameter. Round this piece of wood at equal distances screw three more of the screw-eyes. Now cut from your brass wire three lengths of fourteen inches, and turn a ring at each end of each piece, and hook one end of each piece into a screw-eye in the circular block, and the other end into the eye in each leg, closing up the rings so that they will not come unhooked. This arrangement will keep the top from tilting. Now the stand is finished, and we will take the tube in hand if it is quite dry and hard. Before drawing out the roller, cut the ends off quite square with a sharp knife, leaving the tube thirty-seven and a half inches long. Now draw out the roller without breaking the tube. We must next fix the object-glass. Cut a strip of cardboard half an inch wide, and long enough to go all round inside the tube without lapping, and to fit tightly. Push this inside, so that it will form a shelf half an inch inside the end of the tube. Glue this in its place. Upon this place the object-glass, and fix it there by gluing a strip of cardboard all round inside the tube on the top of the lens. To fix the eyepiece, cut from the roller used to make the tube on, a piece one inch long, and bore a hole right through the centre of it the exact size of the eyepiece tube. Glue this block in the other end of tube. Push the brass tube in this hole with the field-lens inwards. The telescope can be focussed by pushing in or drawing out the brass tube. Get a piece of deal eight inches long and three and a half inches wide and two inches deep. Cut a groove along the top as Fig. 7, one inch deep and a little more than two and a quarter inches wide, to fit the outside of the tube. Glue this block on the tube, so that one end is thirteen inches from the front end of the tube (the eyepiece end). Put the block in its place between the uprights on the stand, and fix it there by two screws passing through the holes in the uprights and screwing into the block. Pass the rod attached to the uprights through the hole in the top of stand and through the hole in the block underneath.

Now we have only to make the arrangement for elevating the telescope. For this you will want the rest of the brass wire and the remaining three stair-eyes as well as two pieces of thin brass plate, four and a half inches long, half an inch wide, and one-sixteenth of an inch thick. Bend these pieces of plate as in Fig. 8, making the bent parts one inch long, and get the ends cut as in the figure, and have holes drilled in the bent parts a little larger than the brass wire. Cut your wire into two lengths of eighteen and a half inches, and take them to the tinman and get him to cut a screw-thread nearly the whole length of each, leaving about two inches to each. At this end of each get him to turn a ring, and get him to close these two rings into one of the screw-eyes. Get him to make a screw-nut for each wire about the size of a farthing, but about twice the thickness. Screw the eye carrying these wires through the tube into the eyepiece block, screw the remaining two screw-eyes into two of the legs of the stand, on the outside of each leg and about one foot from the top of each. Bend the cut part of the brass plates into rings and close them in these screw-eyes. Now put the screw-nuts in their places in the brass plates, and put the screw wires through the holes in the top, and turn the nuts to the left, which will draw down the wires and with them the eyepiece of the telescope. To turn the telescope to the left turn the right-hand nut to the right, and the other to the left, and to turn it to the right reverse the action of the nuts. In making this telescope you must be very careful in fixing the lenses. They are to be placed so that the centres are to be in one straight line, which line is to be at right angles to the lenses. You can cover your tube with coloured paper to give it a finish. It will be advantageous, in using the instrument, by keeping out all light not wanted, to make a cardboard tube about six inches long and large enough to slide easily on the end over the object-glass and to project about five inches. The telescope is now finished, and will with ordinary care last for years.

Appearance of the Moon as seen through one of these telescopes on June 9th]

It may be well to add that astronomical telescopes show the image inverted; this is done to save the use of erecting glasses which absorb light, and consequently make the image faint. If you wish to make yours useful for terrestrial purposes, you must insert in front of the field lens, and about four inches from it, a lens of the same focal length; this will turn the image right way up. The blurring of the image is sometimes caused by the two lenses composing the eye-piece _not_ being at the distance apart proper for their focal lengths.