Paraboloid Figured by Ion Bombardment L H. Narodny and M. Tarasevich

Kollsman Instrument Corporation, 575 Underhill Boulevard, Syosset, New York 11791. Received 19 June 1967.

The work of Hines1 outlined the use of argon ions in eroding glass surfaces. Since early 1965, we have conducted theoretical and experimental investigations on the controlled erosion of various dielectric, crystalline, and metallic substrates with argon ions using low accelerating potentials up to 50 kV. Low voltage erosion by heavy ions using currents of the order of 0.1 mA has. yielded highly reproducible results on a variety of ma­terials. The work of Meinel2 was performed at relatively high energies of the order of 1 MeV. We are reporting here on an ƒ/6, 10-cm diam Pyrex paraboloid figured by argon ions.

Figure 1 shows the interference pattern of the completed

Fig. 1. Interference pattern of paraboloid at its center of curvature.

2010 APPLIED OPTICS / Vol. 6, No. 11 / November 1967

paraboloid at its center of curvature. The interferogram was obtained by the Burch method of scatter plate interferometry.3

Erosion was performed on a long radius (120-cm) spheroid blank, using mechanical scanning techniques. The depth of material to be eroded at any radial zone was calculated from the best fit be­tween the spheroid and a reference paraboloid tangent to the spheroid at the central axis and intersecting it at the edge of the aperture. This case represents the least amount of material to be removed and therefore affords the greatest economy of time in the use of the ion beam accelerator facility. The ion beam dwell time as a function of mirror radius was programmed to yield the calculated depth of erosion. Up to four or five wave­lengths of material thickness can be precisely removed in this way.

The mirror blank was exposed to a focused argon ion beam approximately 3 mm in diameter which traced out a spiral path on the substrate surface. The exposure time was 11 h at 100 μA beam current. The exposure time could have been reduced to approximately 1.5 h by increasing the beam current although no attempt was made to do so in this project. A small central zone approximately 1 cm in diameter was left uneroded to serve as a reference area for metrological purposes.

A computer program was devised to obtain a contour plot of figure deviations at selected X-Y coordinates on the mirror using data obtained from the scatter plate interferograms of the figured surface. The results of the computations showed that an rms value of 0.046 wavelengths (at 5500 A) figure deviation from a best fit paraboloid has been obtained.

We plan to use the data outputs of similar computer programs in the synthesis of a completely automatic ion beam figuring system.

We wish to acknowledge the skillful work of Lewis Golden in the formulation of the mathematical techniques used and in the evaluation of figure and wavefront deviations from the inter-ferometric data.

References 1. R. L. Hines, J. Appl. Phys. 28, 587 (1957). 2. A. B. Meinel, S. Bashkin, and D. A. Loomis, Appl. Opt. 4,

1674 (1965). 3. J. M. Burch, Nature 171, 889 (1953).