J. R. Stephens
I certainly advise anyone interested in this subject, or in photographic astronomy past and present, to read the memoirs of Dr. Draper (date 1864) and Prof. Ritchey (date 1904). They are bound together in "Smithsonian Contribution to Knowledge, " Vol. 34, and may be ordered though any bookseller. They cost together 75 cents. The address is Smithsonian Institution, Washington, D. C. To throw more light on what has gone before, I will make a few short abstracts from Prof. Ritchey's book. They are as follows:
"No greater mistake could be made than to assume that cheap and poorly annealed discs of glass, or those with large striae or pouring marks, are good enough for mirrors of reflecting telescopes."
"The thinner mirrors suffer much greater temporary change of curvature from the very slight heat generated during the process of polishing, and they are undoubtedly more likely to suffer temporary disturbance of figure from changes of temperature when in use in the telescope."
" All mirrors should be polished, not figured, and silvered on the back, as well as the face, in order that both sides may be similarly affected by temperature changes when in use in the telescope. For the same reason the method of supporting the mirror should be such that the back is as freely exposed to the air as possible.
" Half-size tools, 8-15 of the diameter of the mirror are economical, and are quickly prepared, and a much greater variety of stroke can be used with them, so that, with a well-designed grinding machine, I have found it easier to produce fine-ground surfaces, entirely free from zones, with half size, than with full-size tools. If temperature conditions and uniform rotation of the glass are carefully attended to, the surface of revolution produced by the smaller tools is fully as perfect is that given by the larger ones."
Grinding-tools for concave and convex mirrors are always made in pairs, one concave the other convex. These iron tools, when being cast, are poured face down, so that the face will be clean. They are turned in a lathe, to the proper curvature as shown by templets. The convex tool then has grooves cut across its surface in a planer. The tools are then ground together with the fine grades of carborendum, which is much more effective for this purpose than emery, and water. This enables the optician to secure the exact curvature desired. A very important point is that by grinding with the concave tool on the top, the radii curvature of both tools can be gradually shortened. When the convex too! is used on top, the curvature of both is gradually flattened.
By this means, and the use of very fine grades of carborundum, a most perfect control of the curvature of the tools may be had."
Prof. Ritchey does not use carborundum in grinding the mirror.
"Before beginning the fine-grinding of the face and back, it is well to round the corners at the edge of the glass. This is done by means of a smooth flat strip of sheet brass of the size and shape of a large flat file. This is worked over the corners of the glass by hand while the disc rotates slowly, emery and water being used for cutting. A quarter-round corner is usually made. Finer and finer grades of emery are used for smoothing the quarter-round. This rounding and smoothing are very necessary, as particles of glass from a sharp or rough edge are liable to be drawn in upon the surface during fine-grinding."
The grades of washed flour emery used for grinding are 2, 5, 12, 30, 60, 120 and 240 minutes. " A weight of grinding-tool on the mirror of 1/2|-pound per square inch is not objectionable with emeries down to 5 or 10 min. washed. With 30 min. washed, and all finer grades, scratches are almost certain to occur with this pressure. The pressure on the glass is therefore decreased by counterpoising the tool to 1/2 pound per square inch for 12 to 20 min. emeries; 1/8 pound per square inch for 30 to 60 min. emeries; and about 1-12 pound per square inch for 120 and 240 min. emeries. This obviates to a great extent the danger of scratches in grinding, provided that thorough cleanliness is practised on the preparation and use of fine emeries."
After the rosin squares are stuck to the polisher and warm pressed to shape on the mirror and redressed with a knife, they are ready for coating with wax. "A pound of best beeswax is melted in a large clean cup, and is very carefully strained through several thicknesses of cheesecloth, into a similar clean cup. A brush is made by tying several thicknesses of cheese-cloth around the end of a thin blade of wood 1 1/4 in. wide, the width of the squares. Each rosin square is now coated with a thin layer of wax by a single stroke of the brush. The wax should be very hot - otherwise the layer will be too thick." The polisher is then cold pressed.
"The thin cream of rouge and water is applied to the glass by means of a wide brush, consisting of a thin paddle of wood, with thin cheesecloth wrapped and tied about one end. Brushes of the usual kind should not be used.
By taking these precautions and by the use of the wax surface on the rosin squares, scratches in polishing can be entirely avoided. The wax surface polishes more slowly than a bare rosin one; but it has the very great advantage that its action is more smooth and uniform. The rosin surface often tends to cling to the glass, and this unequally in different parts of the stroke.
When the bare rosin begins to show at the corners or edges of the faces of the squares, which will occur after six or eight hours' use of the tool, a new coat of wax must be applied and the tool again thoroughly cold pressed."
"Weight of polisher for large tools, 1-18 pound for each square inch of area, which is found to work well for all large tools. For tools 18 in. or less in diameter, somewhat greater pressure per square inch may be used."
" My practice has been to fine-grind and polish to a spherical surface free from zones, and then to para-bolize by means of suitable polishing tools."
"On account of the ease of rigorously testing a concave spherical surface, this is the form which should be first attempted by beginners in optical work."
"The tendency of the edge to turn back or down is most pronounced when a long stroke is used to excess, or when the rosin squares are too soft. It is entirely prevented by rounding the extreme outside squares, all around the polisher, in something like a semicircular form, convex to the edge of the polisher."