How Flat a Diagonal
How Flat a Diagonal? We have taken great pains to produce
a curve of remarkable precision on our mirror. We should be
equally concerned that nothing will interfere with the formation of
a perfect image. In order to keep within the Rayleigh limit, each
area of the diagonal that may be engaged in the formation of a
point image may not depart from a true plane by more than 1/10
of a wave length of light. Such an area, elliptical in outline, is
illustrated in Fig. 46.
At a distance of 6½" inside the focus of a 6-inch f/8 mirror, the minor and major axes of this ellipse are 0.8" and 1.12" , respectively; therefore a diagonal of double these dimensions would have to be flat to within 1/5 of a wave, measured along
its major axis. The order of surface precision will, of course, depend on the dimensions of the ellipse shown in Fig. 46, which may vary according to the focal ratio of the mirror and the location of the secondary mirror in the cone. The surfaces of a prism must be flat to a similar degree.
Through examination of Fig. 50, it can be seen that if the diagonal is one wave length convex, the axial ray, aside from any other disturbances to which it may be subject, will be advanced from its proper position by 1.4 wave lengths. Hence, there is need for paying particular attention to the figure of the diagonal.
Untested plate glass should never be used. Among several pieces of plate glass, each about 2" square, seldom will one be found that is flat to better than 0.00004". In general, the surfaces of such pieces also have different degrees of curvature on different diameters; in other words, they are astigmatic.
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