We present the mechanical and opto-mechanical design for the Prototype Visible Integral-Field Unit Spectrograph
(VIRUS-P). The VIRUS-P instrument is the single unit prototype for the planned VIRUS instrument which consists of
192 spectrographs for the Hobby Eberly Telescope Dark Energy Experiment (HETDEX). The VIRUS Prototype is a test
bed for the design and will be used for a survey on the McDonald 2.7m Harlan J. Smith Telescope. The mechanical
design is driven by the need for high stability. The structure of the instrument is aluminum but the internal optical
elements of the collimator and the camera are held in alignment with respect to each other using Invar metering rods.
The spectrograph is fiber fed with 246 fibers in a hexagonal packing pattern at the telescope focal plane Integral Field
Unit (IFU) and arranged in a slit at the input to the spectrograph. The reverse-Schmidt collimator articulates, and the
Volume Phase Holographic (VPH) grating rotates independently relative to the fixed Schmidt camera to allow for
versatile grating configurations during the prototype testing. Since the VIRUS spectrograph units will be mounted in a
gravity neutral configuration on the HET, the prototype instrument is mounted on a gimbal at the folded cassegrain port
of the 2.7m Smith Telescope to negate gravity vector changes.
We describe the design, construction, and performance of VIRUS-P (Visible Integral-field Replicable Unit
Spectrograph - Prototype), the prototype for 150+ identical fiber-fed integral field spectrographs for the Hobby-Eberly
Telescope Dark Energy Experiment (HETDEX). VIRUS-P was commissioned in 2007, is in regular service on the
McDonald Observatory 2.7 m Smith telescope, and offers the largest field of any integral field spectrograph. The 246-fiber IFU uses a densepak-type fiber bundle with a 1/3 fill factor. It is fed at f/3.65 through a telecentric, two-group
dioptric focal reducer. The spectrograph's double-Schmidt optical design uses a volume phase holographic grating at
the pupil between the articulating f/3.32 folded collimator and the f/1.33 cryogenic prime focus camera. High on-sky
throughput is achieved with this catadioptric system by the use of high reflectivity dielectric coatings, which set the
340-670 nm bandwidth. VIRUS-P is gimbal-mounted on the telescope to allow short fibers for high UV throughput,
while maintaining high mechanical stability. The instrument software and the 18 square arcmin field, fixed-offset guider
provide rapid acquisition, guiding, and precision dithering to fill in the IFU field. Custom software yields Poisson noise
limited, sky subtracted spectra. The design characteristics are described that achieved uniformly high image quality with
low scattered light and fiber-to-fiber cross talk. System throughput exceeds requirements and peaks at 40%. The
observing procedures are described, and example observations are given.
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 Hobby-Eberly Telescope (HET) is a fixed-elevation, 9.2-m telescope with a spherical primary mirror and a tracker at prime focus to follow astronomical objects. The telescope was constructed for $13.9M over the period 1994-1997. A series of extensive engineering upgrades and corrective actions have been completed recently, resulting in significantly improved delivered image quality and increased operational efficiency. The telescope's Spherical Aberration Corrector (SAC) optics were recoated with a highly reflective and durable broadband coating at Lawrence Livermore National Laboratory. The software mount model that maintains optical alignment of the SAC with the 11-m primary mirror array was recalibrated and improved. The acquisition and guiding optics for both the High Resolution Spectrograph (HRS) and the Low Resolution Spectrograph (LRS) were reworked and improved, allowing for better focus and SAC alignment monitoring and control. Recoating of the primary mirror segment array was begun. Telescope images of 0.82 arcseconds have been recorded for sustained periods in preliminary testing following the engineering upgrade, an improvement of 50% over previous best performance. Additional engineering upgrades are scheduled to consolidate these performance gains and to continue improving delivered image quality, throughput, and telescope operational efficiency. The HET is now capable of the science performance for which it was designed.
The Hobby-Eberly Telescope (HET) imposes unique constraints on the design of a spectral calibration system. Its 9.2 m aperture and queue scheduled operation make traditional dome screens impractical. Furthermore, the changing pupil of the HET's tilted Aricebo design is far more drastic than the simple rotation of traditional alt-azimuth telescopes. Given these constraints we elected to build an internal spectral calibration system (SCS) common to all instruments.
The SCS can feed all HET instruments from a uniformly illuminated Lambertian screen located within the spherical abberation corrector (SAC) at the telescope's second pupil. A moving baffle installed at the third pupil will reproduce, during calibration, the actual HET pupil seen in a science exposure. We eliminated all heat sources at the SAC by locating the lamps in the basement below the telescope and coupling source to screen through 12 600 μm diameter 35 m long fibers.
KEYWORDS: Mirrors, Image segmentation, Mars, Telescopes, Simulation of CCA and DLA aggregates, Beam splitters, Wavefront sensors, Collimators, Sensors, Image quality
The Mirror Alignment Recovery System (MARS) is a Shack-Hartmann based sensor at the center of curvature (CoC) of the Hobby-Eberly Telescope (HET) spherical primary mirror used to align the 91 mirror segments. The instrument resides in a CoC tower next to the HET dome, a location which provides a challenging set of problems including wind shake and seeing from two different domes. The system utilizes an internal light source to illuminate the HET and a reference mirror to provide focused spot locations from a spherical surface. A custom lenslet array is sized to the HET pupil image, matching a single hexagonal lenslet to each mirror segment. Centroids of the HET mirror segment spots are compared to the reference spot locations to measure tip/tilt misalignments of each segment. A MARS proof-of-concept (POC) instrument, tested on the telescope in 2001, utilized a commercial wavefront sensor from Adaptive Optics Associates. The final system uses the same concept, but is customized for optimal performance on the HET.
MARS replaces previous burst-antiburst alignment techniques and provides a more intuitive method of aligning the primary mirror for telescope operators. The POC instrument has improved median HET stack sizes by 0.3" EE50, measured at the CoC tower. The current alignment accuracy is 0.14" rms (0.28" rms on the sky), resolution is 0.014", measurement precision is 0.027" rms, and segment capture range is ± 5". With continuing improvements in HET dome ventilation and the addition of software customized for removal of tower motion during measurement, the alignment accuracy is expected to reach approximately 0.04" rms in the final MARS, to be installed in late 2002.
The Hobby-Eberly Telescope (HET) is a revolutionary large telescope of 9.2 meter aperture, located in West Texas at McDonald Observatory. The Low Resolution Spectrograph [LRS, an international collaboration between the University of Texas at Austin (UT), the Instituto de Astronomia de la Universidad Nacional Autonoma de Mexico (IAUNAM), Stanford University, Ludwig-Maximillians-Universitat, Munich (USM), and Georg- August-Universitat, Gottingen (USG)] is a high throughput, imaging grism spectrograph which rides on the HET tracker at prime focus. The remote location and tight space and weight constraints make the LRS a challenging instrument, built on a limited budget. The mechanical design and fabrication were done in Germany, and the camera and CCD system in Texas. The LRS is a grism spectrograph with three modes of operation: imaging, longslit, and multi-object. Here we present a detailed description of the mechanical design of the LRS. Fabrication, assembly and testing of the LRS will be completed by mid 1998. First light for the LRS on the HET is expected in the summer of 1998.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.