Abstract

It is possible to change the figure of reflecting surfaces quantitatively by controlled deposition of metals, evaporated in vacuum. The theory for calculating screens and the experimental technique are described. A spherical 12-inch mirror has been repeatedly parabolized, obtaining a surface perfect within 0.05 wave-length. A 558inch spherical mirror has been parabolized with the optical axis outside of its periphery. A defective parabolic mirror was corrected for turned up and down edges. A convex spherical secondary mirror was hyperbolized. The limitations and possible applications of the method are discussed.

© 1936 Optical Society of America

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References

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  1. Part of the results contained in this paper were reported at the Meeting of the Astronomical Society of the Pacific on June 26, 1935.
  2. Fellow of the John Simon Guggenheim Memorial Foundation.
  3. The Evaporation Process and its Application to Aluminizing of Large Telescope Mirrors by John Strong, A.J.

Other (3)

Part of the results contained in this paper were reported at the Meeting of the Astronomical Society of the Pacific on June 26, 1935.

Fellow of the John Simon Guggenheim Memorial Foundation.

The Evaporation Process and its Application to Aluminizing of Large Telescope Mirrors by John Strong, A.J.

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Figures (12)

F. 1
F. 1

Circle and parabolas important in figuring.

F. 2(a)
F. 2(a)

Central zone screen with 90° maximum aperture; (b) the same with 360° maximum aperture; (c) intermediary zone screen, 2 percent maximum aperture; (d) screen used for correcting the 7-inch parabolic mirror; (e) hyperbolizing baffle employed with the convex Cassegrain mirror; (f) correcting screen for the off-axis mirror.

F. 3
F. 3

Experimental arrangement.

F. 4(a)
F. 4(a)

Spherical test mirror at the center of curvature. Notice the zones at 0.37 and 0.79 from the center. Their elevation is less than 1/20 of a wave-length. Contrast film and contrast paper was used to make them appear conspicuous. (b) The same mirror taken at the parabolic focus. The 0.37 zone is visible, the other one has disappeared. In this and in following focograms the direction of movement of the knife-edge is indicated by an arrow.

F. 5
F. 5

12-inch Spherial mirror parabolized on half its Surface. (a) Focograph taken at the mean center of curvature of the parabolic half P. (b) At the center of curvature of the spherical part S. Notice the 0.37 and 0.79 zones conspicuous in Fig. 4a.

F. 6
F. 6

The partially parabolized mirror seen from the parabolic focus after the 4th layer of aluminum was deposited. The edge of the parabolic half P appears turned up, due to “nontheoretical” distribution of sources.

F. 7
F. 7

12-inch mirror parabolized with the intermediary zone screen. (a) Amount deposited just right. The 0.37 and 0.79 zones of the spherical mirror appear after the parabolizing (compare with Fig. 4a), showing that the treatment has introduced no new zones of any importance. Taken at the parabolic focus with a flat. (b) Same overcorrected: too much aluminum deposited. (c) Same undercorrected.

F. 9
F. 9

Spherical mirror parabolized off-axis. (a) Focograph taken at the parabolic focus, 9 mm away from its edge, with the help of a flat. (b) Microphotographs of the image of a pinhole formed by the mirror in the off-axis set-up before (two pictures at different focal distance) and after parabolizing (the image is enlarged and blurred by vibrations of the building).

F. 11
F. 11

Defective 7-inch parabolic mirror. (a) After correcting with the screen Fig. 2d. The scratches visible are due to the testing flat and not to the 7-inch disk.

Equations (7)

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X C = Y 2 / 2 R + Y 4 / 8 R 3 .
X P 1 = Y 2 / 2 R + τ ,
X P 2 = Y 2 / 2 R + Y 0 2 Y 2 / 8 R 3 .
Δ X 1 = τ Y 2 / 8 R 3 ,
Δ X 2 = Y 2 ( Y 0 2 Y 2 ) / 8 R 3 .
Δ X max = Y 0 4 / 32 R 3 = D / 4096 f 3
Δ X = e 2 Y 2 ( Y 0 2 Y 2 ) / 8 R 3 ,