A major performance upgrade for the Hobby-Eberly Telescope (HET) is in the conceptual design phase. The extensive upgrade will include a wide field optical corrector, a new HET tracker with increased payload capacity, and improved telescope pointing and tracking accuracy. The improvements will support the HET Dark Energy Experiment (HETDEX), which seeks to characterize the evolution of dark energy by mapping the imprint of baryonic oscillations on the large scale structure of the Universe. HETDEX will use the increased field-of-view and payload to feed an array of approximately 145 fiber-fed spectrometers, called VIRUS for "Visible Integral field Replicable Unit Spectrograph". The new corrector will have a science field-of-view diameter of 18 arcminutes, in contrast to the original corrector's 4 arcminute field, a twenty-fold increase in area. A new HET tracker with increased payload capacity will be designed to support the wide field corrector. Improved pointing and tracking will be accomplished using new autocollimation and distance measuring metrology combined with real-time wavefront sensing and correction. The upgrade will maintain operation of the current suite of facility instruments, consisting of low, medium, and high resolution spectrometers.
The HET is a modified Arecibo-style telescope with a segmented spherical primary and a four-mirror spherical
aberration corrector (SAC). Objects are tracked by driving the SAC along the focal sphere of the primary. In the original
design of the telescope the alignment of the SAC was to be maintained passively. In practice, this could not be done to
specifications, leading to degraded imaging quality. We have developed a metrology system to actively control the
alignment of the SAC. An autocollimator maintains the optical axis of the SAC normal to the primary mirror beneath it.
An absolute distance measuring interferometer (DMI) monitors the SAC/primary mirror distance, maintaining focus.
Both systems work at a wavelength of 1.5 microns, well above the operating wavelength of current or planned science
instruments and therefore do not interfere with observations. The performance of the system is measured via Hartmann
testing.
Several upgrades are implemented in the primary mirror control system, including calibration of individual edge
sensors, new control system software, and a new method of setting and controlling the overall radius of curvature of the
primary array. New techniques were developed to efficiently piston the segments onto the proper sphere radius.
The CFHTIR is a large format near IR camera based on the Rockwell HAWAII Array. CFHTIR is designed for both direct imaging at the f/8 Cassegrain focus, as well as spectroscopy on the OSIS multiobject spectrograph. The camera provides 0.21 inch/pixel sampling in both applications with a single set cold transfer optics and pupil mask. The camera includes two eight-position filterwheels driven by cryogenic stepper motors with position control using a novel Hall effect sensor technique. CFHTIR also uses a novel dewar wiring technique employing flexible circuit vacuum feedthrus. CFHTIR is the second large format IR camera based on the Hawaii array constructed at CFHT, the first being the KIR camera for the CFHT Adaptive Optics Bonnette which was commissioned in 1997. This paper describes the system architecture of the CFHTIR highlighting key design concepts and detailing the physical elements.
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