Extracting the zero-gravity surface figure of a mirror through multiple clockings in a flightlike hexapod mount.

An elegant and accurate way to determine the zero-gravity surface figure of an optic from ground-based interferometric metrology is to average the figures found in two or more configurations that are rotated with respect to the direction of gravity, so gravity forces in the frame of the optic cancel in the average. In a recent elucidation of this technique, we emphasized that care must be taken to ensure that mount forces at each attachment point similarly cancel, and we presented some specific mounting schemes that gave accurate zero-gravity surface determinations during fabrication and acceptance testing of the Space Interferometry Mission PT-M1 mirror. Here we show that multiconfiguration averaging techniques work well for the most important special case of a mirror in a flightlike hexapod mount clocked into either two or three symmetrically placed positions. We explicitly compute mount forces (axial forces in the six struts of the hexapod) and show that at any attachment point their average over multiple clocked configurations vanishes in the frame of the optic, ensuring the success of zero-gravity surface figure extraction.

Modeling and computer simulation of ultrasonic/sonic driller/corer (USDC).

Simulation and analytical models for the ultrasonic/sonic drill/corer (USDC) are described in this paper. The USDC was developed as a tool for in-situ rock sampling and analysis in support of the NASA planetary exploration program. The USDC uses a novel drive mechanism, which transfers ultrasonic vibrations of a piezoelectric actuator into larger oscillations of a free-flying mass (free-mass). The free-mass impact on the drill bit creates a stress pulse at the drill tip/rock interface causing fracture in the rock. The main parts of the device (transducer, free-mass, bit, and rock) and the interactions between them were analyzed and numerically modeled to explore the drive mechanism. Each of these interactions is normally described by a time-dependent 2- or 3-D model involving slowly converging solutions, which makes the conventional approach unsuitable for USDC optimization studies. A simplified integrated model using tabulated data was developed to simulate the operation of the USDC on desktop PC and successfully predicted the characteristics of the device under a variety of conditions. The simulated results of the model and the experimental data used to verify the model are presented.

Precise determination of the zero-gravity surface figure of a mirror without gravity-sag modeling.

The zero-gravity surface figure of optics used in spaceborne astronomical instruments must be known to high accuracy, but earthbound metrology is typically corrupted by gravity sag. Generally, inference of the zero-gravity surface figure from a measurement made under normal gravity requires finite-element analysis (FEA), and for accurate results the mount forces must be well characterized. We describe how to infer the zero-gravity surface figure very precisely using the alternative classical technique of averaging pairs of measurements made with the direction of gravity reversed. We show that mount forces as well as gravity must be reversed between the two measurements and discuss how the St. Venant principle determines when a reversed mount force may be considered to be applied at the same place in the two orientations. Our approach requires no finite-element modeling and no detailed knowledge of mount forces other than the fact that they reverse and are applied at the same point in each orientation. If mount schemes are suitably chosen, zero-gravity optical surfaces may be inferred much more simply and more accurately than with FEA.