A high precision, dual drive system has been designed and developed for the Wide Field Upgrade to the Hobby-Eberly
Telescope* at McDonald Observatory in support of the Hobby-Eberly Telescope Dark Energy Experiment. Analysis,
design and controls details will be of interest to designers of large scale, high precision robotic motion devices. The drive
system positions the 19,000 kg star tracker to a precision of less than 5 microns along its 4-meter travel. While
positioning requirements remain essentially equal to the existing HET, tracker mass increases by a factor greater than 5.
The 10.5-meter long tracker is driven at each end by planetary roller screws, each having two distinct drive sources
dictated by the desired operation: one slowly rotates the screw when tracking celestial objects and the second rotates the
nut for rapid displacements. Key results of the roller screw rotordynamics analysis are presented. A description of the
complex bearing arrangement providing required degrees of freedom as well as the impact of a detailed Failure Modes
and Effects Analysis addressing necessary safety systems is also presented. Finite element analysis results demonstrate
how mechanical springs increase the telescope's natural frequency response by 22 percent. The critical analysis and
resulting design is provided.
The Hobby-Eberly Telescope (HET) is an innovative large telescope of 9.2 meter aperture, located in West Texas at the
McDonald Observatory (MDO). The HET operates with a fixed segmented primary and has a tracker which moves the
four-mirror corrector and prime focus instrument package to track the sidereal and non-sidereal motions of objects. A
major upgrade of the HET is in progress that will increase the pupil size to 10 meters and the field of view to 22' by
replacing the corrector, tracker and prime focus instrument package. In addition to supporting the existing suite of
instruments, this wide field upgrade will feed a revolutionary new integral field spectrograph called VIRUS, in support
of the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX). This paper discusses the current status of this
upgrade.
Wind loading can be a detrimental source of vibration and deflection for any large terrestrial optical telescope. The
Hobby-Eberly Telescope* (HET) in the Davis Mountains of West Texas is undergoing a Wide Field Upgrade (WFU) in
support of the Dark Energy Experiment (HETDEX) that will greatly increase the size of the instrumentation subjected
to operating wind speeds of up to 20.1 m/s (45 mph). A non-trivial consideration for this telescope (or others) is to
quantify the wind loads and resulting deflections of telescope structures induced under normal operating conditions so
that appropriate design changes can be made. A quasi-static computational fluid dynamics (CFD) model was generated
using wind speeds collected on-site as inputs to characterize dynamic wind forces on telescope structures under various
conditions. The CFD model was refined until predicted wind speed and direction inside the dome agreed with
experimental data. The dynamic wind forces were then used in static loading analysis to determine maximum
deflections under typical operating conditions. This approach also allows for exploration of operating parameters
without impact to the observation schedule of the telescope. With optimum combinations of parameters (i.e. dome
orientation, tracker position, and louver deployment), deflections due to current wind conditions can be significantly
reduced. Furthermore, the upper limit for operating wind speed could be increased, provided these parameters are
monitored closely. This translates into increased image quality and observing time.
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