Design and development of a long-travel positioning actuator and tandem constant force actuator safety system for the Hobby Eberly Telescope wide-field upgrade

Abstract

The Wide Field Upgrade presents a five-fold increase in mass for the Hobby-Eberly Telescope's * tracker system. The design of the Hobby-Eberly Telescope places the Prime Focus Instrument Package (PFIP) at a thirty-five degree angle from horizontal. The PFIP and its associated hardware have historically been positioned along this uphill axis (referred to as the telescope's Y-axis) by a single screw-type actuator. Several factors, including increased payload mass and design for minimal light obscuration, have led to the design of a new and novel configuration for the Y-axis screw-drive as part of the tracker system upgrade. Typical screw-drive designs in this load and travel class (approximately 50 kilonewtons traveling a distance of 4 meters) utilize a stationary screw with the payload translating with the moving nut component. The new configuration employs a stationary nut and translating roller screw affixed to the moving payload, resulting in a unique drive system design. Additionally, a second cable-actuated servo drive (adapted from a system currently in use on the Southern African Large Telescope) will operate in tandem with the screw-drive in order to significantly improve telescope safety through the presence of redundant load-bearing systems. Details of the mechanical design, analysis, and topology of each servo drive system are presented in this paper, along with discussion of the issues such a configuration presents in the areas of controls, operational and failure modes, and positioning accuracy. Findings and results from investigations of alternative telescope safety systems, including deformable crash barriers, are also included.

Design of the fiber optic support system and fiber bundle accelerated life test for VIRUS

“…The operational limits for HETDEX occur at tracker X/Y positions of +/-1870.08 mm from the center of the primary mirror 12,13 . At this position the focal surface and collector mirror are tilted at +/-9 degrees from the tracker plane.…”

HETDEX tracker control system design and implementation

“…The main design goal of the CFD is to carry a set amount of the total carriage weight, at any position and at any rate of speed without causing any degradation of the desired carriage motion. The CFD functionality is implemented on the HFC 11 .…”

Design and analysis of the tracker bridge for the Hobby-Eberly Telescope wide-field upgrade

“…A second pair of linear bearings mounts on top of the two main bridge beams allowing the payload to translate in the Y-axis via a third roller screw 12 . The Z-axis is normal to the upper hex and main bridge beams, which is 35 degrees from the gravity vector.…”

“…• The four anti-skew blocks 11 (165 kg each), four Y-axis hard stops 11 (50 kg each), two FMS carriages (305 kg each), Y-drive slew housing 12 (178 kg) and CFD (584 kg) are all modeled as simple beams in their respective locations. The density and CG locations of each component are manipulated within the beam's properties to match the actual hardware.…”

“…Due to the mass and geometry changes, the entire tracker, drive systems 11,12 , and control architecture 13 are being replaced, including the bridge which serves as the backbone of the tracker. Experience with the HET tracker created new requirements for the WFU system which influences the bridge design, such as required frequency response, obscuration limits, and hexapod location.…”