M. A. AINSLEY
Before passing on to describe the tube and mounting I employed, I must say something as to the procedure in case of over-correction. If the polisher is made as I suggest, and the pitch is not too soft, overcorrection, i. e., class C3, is not likely to occur until the final figuring is in progress; but when the polisher is graduated, a few minutes polishing makes a great difference to the figure, and it is only too easy to overdo the deepening of the center, and so get the hyperboloid. In such a case it is sufficient to make a new polisher of uniform squares and polish with short strokes. The pitch may also be a little harder with advantage. I found, in working my first 9 in. mirror, that it was quite impossible, with uniform facets, to advance beyond the oblate spheriod, class A, and it was not until I graduated the polisher that I could even get a spherical figure, much less anything like a paraboloid. The mirror, however, was very light, being only 1 in. thick. In the case of the second 9 in. mirror, which is 2 1/2 in. thick, the same thing occurred, the figure showing no signs of altering beyond class A until the polisher was graduated. I think, therefore, that the method I have indicated is fairly safe.
In case of pronounced over-correction, i. e., when the curve is distinctly B, - I think it is best to regrind with, say, one-minute emery, or Dr. Blacklock's method might be adopted. He made a doubly graduated polisher, the central squares being reduced in size as well as those at the margin; while on a zone about half-way between center and margin the facets were kept the same size as before. I have not tried this method, so cannot speak from experience; but I think I should be rather afraid of rings making their appearance on the mirror. In making my second 9 in. mirror I obtained very slight over-correction at the finish; but a new polisher of hard pitch and uniform squares corrected this in ten minutes or so.
One more word about testing. It is very necessary to have behind the mirror, which is transparent, being unsilvered, a uniformly dark surface; otherwise false lights and shadows will show, and confuse the tests. I used black paper; but anything fairly dark will do.
I describe the one made for my second 9 in. mirror, as that used for the first, though fairly satisfactory, proved rather liable to warp and split. My tube is built up of 1/2 in. mahogany screwed to four corner-pieces of teak 1 1/2 in. square, which extends the whole length. This gives a very stiff tube, which, at the same time, is not too heavy to be removed from its stand and carried indoors, or, at least, under shelter. The tube is 10 in. inside measurement, though I think 10 1/2 In. would be better."The length, of course, depends on the focal length of the mirror; in my case it was 73 in. If the tube is made about 3 in. longer than this focal length it will about do.
Drawings of the tube will show, I think, how it was all fitted up. Fig. 1 is partly a section, partly a view, of the left side of the tube. Fig. 2, a view of the front and Fig. 3 of the mirror end.
AA are the corner pieces of 1 1/2 in. teak, to which are screwed BB, the sides of the tube of 1/2 in. mahogany, an opening being left at C to enable the mirror to be covered by a hinged lid.
M is the speculum, supported by two small brass pieces D, on the backing E, which is circular, and made of 1 in. teak, covered with two thicknesses of cloth.
F is the end piece of the tube; also of 1 in. teak to which the long corner pieces A are screwed. The mirror is adjusted by three long brass bolts G, the heads of which are sunk flush with the front of E. These have adjusting nuts outside, while the mirror board E is pressed forward by a central pad of india-rubber, shown to the right of the letter E in the figure. This keeps the screw just taut without bending the board E; the bolts G pass through board and end of tube quite loosely.
is is a hole 2 1/2 in. or so in diameter, the position of which on the side of the tube is decided by the focal length, and by the distance outside the tube at which we desire the image to be formed. If, for example, we want the image 5 in. outside the tube, then the distance from center of tube to image =5÷5 = 10 1/2 and the center of II must be (focal length-10 1/2 in.) from the surface of M. Through the hole can be seen the flat mirror K. This is 1 1/2 in. in minor axis - If would be better - and is supported on a cylinder of teak L, cut off at 45°, to which it is clipped y copper strips. This block L is held at the end of a in. brass rod about 8 in. long, which passes quite lot ely through a short brass tube O, 3 in. by in. the rod N being held at two points by means of six screws, B. This gives almost unlimited power of adjustment in any direction. The tube O is held on a pillar of iron P, 1 in. broad by 1/4 in. thick, which is securely fixed to a plate Q, 6 x 3 x 1/4 in., which is secured to the side of the tube by four small bolts. I am aware that this is not the orthodox way of mounting the flat mirror; but, after trying the ordinary way in which three springs are employed, I have adopted the method shown, as it is far easier to make, to fit, and to adjust, besides being absolutely firm. As regards the definition, or rather the loss of definition, produced by the pillar P, I have not found anything to complain of after careful comparative trials.
At the sides of the tube are pointed pieces TT, 9x1 x 1/4 in., which take its weight when on the stand. SS are 1/2 in. holes drilled in the side of the tube to take the fittings for carrying the finder. These are shown in Fig. 2.
I have made no provision for covering either of the mirrors, as I found that if care is taken to prevent violent changes of temperature the silver film is little affected by time. So long as the whole tube is kept clear of dust and protected from direct impact of rain, the mirrors do not dew over, and I have found the films on them last well over a year without renewal. The silvering process I have adopted is so quick and simple that resilvering is virtually no trouble. A cover for the end of the tube, which is stowed in a vertical position in a porch or under an open shed, is all I have been using.
The fittings to carry the eyepiece I have not drawn. Almost anything will do in the way of rackwork, such as the front of an old magic lantern, or the mount of an old portrait or other photographic lens. A visit to an optician will most likely furnish what is needful for a few shillings. Even rackwork is not absolutely essential, and I have found that a simple sliding tube about 5 or 6 in. long (Fig 2, W) with a screw thread to take the standard eyepiece screw, and sliding in another tube about 2 in. long, with a flange by which it can be fixed over the hole II (which need not be of any particular size) is practically quite sufficient. If the tube is made to slide easily, and kept lubricated with vaseline and protected from blows, which would dent it, focussing is just as easy as rackwork after a little practice. This tube is of course also used in the testing apparatus before mentioned. It will also be convenient to have a disc of brass with a hole about 1-16 in. in diameter in the center, to screw into the end of the tube in place of an eyepiece for the purpose of adjustment, as will be described later.
For merely testing the mirror on a star, a stout hoard, not less than 1 1/2 in. thick, to which the flat mounting, etc., can be attached, would be sufficient, but as I said before, I recommend that tube and mounting be made as soon as the focal length is accurately known - certainly as soon as the polish is coming up. In this way we may take advantage of any fine weather that may occur for testing on a star and after all, the image of a star is the " final court of appeal " as regards the perfection, or otherwise, of the mirror.
For purposes of testing, the mounting may be very simple. I have found that an ordinary kitchen chair balanced on its hind legs and carrying the tube across the back and seat, makes an efficient temporary mounting if steadied in some way by cords, struts, etc., or the telescope may have its mirror end on the ground and the front end on a trestle; in fact, a number of ways in which the telescope can be kept pointing to the Pole Star will occur to the reader, and the Pole Star is the best for the purpose on account of its brightness and its very slow motion in the field of view.
The finder is carried on two iron rings with stems passing through the holes S and secured by nuts inside. Each ring, as shown in Fig. 2 at V, has three screws, similar, but larger than those used for securing the flat, and with this arrangement adjustment is easy and quick. A finder with cross wires complete, may be picked up second-hand, or an ordinary cheap telescope, such as is sold for $1.25 or so, can be utilized. If the second or third lenses, counting from the object-glass, be removed, the telescope becomes an inverting one of lower power, and if the small tube containing the two lenses nearest to the eye be drawn out, a diaphragm will be found be placed across this, and secured, as nearly as possible at right angles and crossing in the center, by means of seccotine, an efficient finder will be provided - not that a properly constructed telescope with the necessary adjustments is not better. In any case, the provision of an efficient finder will be a great help in the testing.
In my next I will describe the method of adjusting mirrors and finder, and the mounting I have employed.