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The Large Binocular Telescope (LBT) consists of two 8.4-meter primary mirrors on a common mount. When the telescope is complete, to complement the two primaries there will be two 0.9-meter adaptive secondaries and two tertiary mirror flats that all work to support a variety of Gregorian focal stations, as well as prime focus. A fundamental goal of the telescope is to perform interferometric observations, and therefore, there is a critical need for the ability to co-point the individual telescopes to high precision. Further, a unique aspect of the LBT is the comparatively large range over which the optics can be adjusted which provides flexibility for the acquisition of targets. In the most general case, an observer could be performing an observation using different targets, within constraints, with different instruments on each of the two telescope sides, with different observing duty cycles. As a consequence of the binocular nature of the telescope and the number of possible observing combinations, there are unique requirements imposed on the Telescope Control System (TCS), and in particular, on the Pointing Control Subsystem (PCS). It is the responsibility of the PCS to arbitrate the pointing requests made on the two sides of the telescope by the observers, incorporate guide updates, and generate tracking trajectories for the mount and the rotators, in conjunction with providing tip/tilt demands on the subsystem controlling the optical elements, and ensure each target remains on the specified location (i.e., pointing origin) in the focal plane during an active observation. This paper describes the current design and implementation of the LBT PCS.

We have studied two mark geometries for possible use in a projection e-beam lithography system using SCALPEL (scattering with angular limitation in projection electron lithography). These are V-grooves and vertically etched geometries, pedestals or trenches. We report results of measurements of backscattered electron (BSE) contrast form topographic marks of varying size and as a function of energy up to 100 kV. The marks were fabricated on silicon wafers. The measurements were taken both in a scanning electron microscope and in an experimental SCALPEL machine operating in focused probe mode. The V-grooves ranged from 1.0 to 30 micrometers wide. The vertical etched features ranged from 2 to 30 micrometers wide and 0.6 and 50 micrometers depth. The results depended not only on the feature width and depth, but also on whether the features were isolated or in line and space patterns. Using a BSE ratio of 1.05 as a criterion for acceptable contrast from an alignment mark, V-grooves and vertical etched features had acceptable contrast with exception of the smallest and shallowest features for both geometries.