Commissioning the Webb telescope to realize the observatory’s full capability necessitated the development of robust wavefront sensing and control processes. These processes rely on techniques that were adapted or newly innovated for the mission, and further adaptation of these techniques may be expected for future segmented telescopes. Over the course of mission development, these techniques were refined to form a baseline wavefront commissioning plan that assumes several conditions and performance requirements are met. Herein we present efforts carried out to define and develop contingency concepts of operation for Webb telescope commissioning, and the mission-level approach to managing the response to deviations from the baseline plan in the event of significant off-nominal or anomaly scenarios encountered by the wavefront team. An overview of selected contingencies is presented along with more detailed example model cases and instances of interest encountered in flight.
The James Webb Space Telescope (JWST) is a segmented deployable telescope, currently operating at L2. The telescope utilizes 6 degrees of freedom for adjustment of the Secondary Mirror (SM) and 7 degrees of freedom for adjustment of each of its 18 segments in the Primary Mirror (PM). After deployment, the PM segments and the SM arrived in their correct optical positions to within a ~1 mm, with accordingly large wavefront errors. A Wavefront Sensing and Controls (WFSC) process was executed to adjust each of these optical elements in order to correct the deployment errors and produce diffraction-limited images across the entire science field. This paper summarizes the application of the WFSC process.
The James Webb Space Telescope’s (Webb’s) deployable primary and secondary mirrors are actively controlled to achieve and maintain precise optical alignment on-orbit. Each of the 18 primary mirror segment assemblies (PMSAs) and the secondary mirror assembly (SMA) are controlled in six degrees of freedom by using six linear actuators in a hexapod arrangement. In addition, each PMSA contains a seventh actuator that adjusts radius of curvature (RoC). The actuators are of a novel stepper motor-based cryogenic two-stage design that is capable of sub-10 nm motion accuracy over a 20 mm range. The nm-level motion of the 132 actuators were carefully tested and characterized before integration into the mirror assemblies. Using these test results as an initial condition, knowledge of each actuator’s length (and therefore mirror position) has relied on software bookkeeping and configuration control to keep an accurate motor step count from which actuator position can be calculated. These operations have been carefully performed through years of Webb test operations using both ground support actuator control software as well as the flight Mirror Control Software (MCS). While the actuator’s coarse stage length is cross-checked using a linear variable differential transformer (LVDT), no on-board cross-check exists for the nm-level length changes of the actuators’ fine stage. To ensure that the software bookkeeping of motor step count is still accurate after years of testing and to test that the actuator position knowledge was properly handed off from the ground software to the flight MCS, a series of optical tests were devised and performed through the Center of Curvature (CoC) ambient optical test campaigns at the Goddard Space Flight Center (GSFC) and during the thermal-vacuum tests of the entire optical payload that were conducted in Chamber A at Johnson Space Center (JSC). In each test, the actuator Fine Step Count (FSC) value is compared to an external measurement provided by an optical metrology tool with the goal of either confirming the MCS database value, or providing a recommendation for an updated calibration if the measured FSC differs significantly from the MCS-based expectation. During ambient testing of the PMSA hexapods, the nm-level actuator length changes were measured with a custom laser deflectometer by measuring tilts of the PMSA. The PMSA RoC fine stage characterization was performed at JSC using multi-wave interferometric measurements with the CoC Optical Assembly (COCOA). Finally, the SMA hexapod fine stage characterization test was performed at JSC using the NIRCam instrument in the “pass-and-a-half” test configuration using a test source from the Aft-Optics System Source Plate Assembly (ASPA). In this paper, each of these three tests, subsequent data analyses, and uncertainty estimations will be presented. Additionally, a summary of the ensemble state of Webb’s actuator fine stages is provided, along with a comparison to a Wavefront Sensing and Control (WFSC)-based requirement for FSC errors as they relate to the optical alignment convergence of the telescope on-orbit.
The Visible Integral Field Replicable Unit Spectrograph (VIRUS), the instrument for the Hobby Eberly Telescope Dark Energy Experiment (HETDEX), consists of 78 replicable units, each with two integral field spectrographs. The VIRUS design takes advantage of large-scale replication of simple units to significantly reduce engineering and production costs of building a facility instrument of this scale. With VIRUS being 156 realizations of the same spectrograph, this paper uncovers the statistical variations in production of these units. Lab relative throughput measures are compared with independently measured grating and optical element performance allowing for potential diagnosis for the cause of variation due to spectrograph elements. Based on variations in performance of individual optical components, throughput curves are simulated for 156 VIRUS spectrograph channels. Once delivered, each unit is paired with a fiber bundle and throughput measurements are made on sky using twilight flats. We compare throughput variance from on-sky measurements to the simulated throughputs. We find that the variation in throughput matches that predicted by modeling of the individual optics performance. This paper presents the results for the 40 VIRUS units now deployed.
KEYWORDS: Mirrors, Point spread functions, Optical alignment, James Webb Space Telescope, Space telescopes, Telescopes, Cryogenics, Monte Carlo methods
The James Webb Space Telescope is a large, deployable telescope that will operate at cryogenic temperatures at the Earth-Sun Lagrange 2 point. The Webb Optical Telescope Element (OTE) consists of 18 actively controlled Primary Mirror Segment Assemblies (PMSAs), an actively controlled Secondary Mirror Assembly (SMA), and an Aft-Optics Subsystem (AOS) that contains a fixed Tertiary Mirror and a Fine Steering Mirror. The OTE is combined with the Integrated Science Instrument Module (ISIM) to create the full optical train called OTIS (OTE and ISIM).
OTIS has recently undergone cryogenic vacuum testing in Chamber A at Johnson Space Center in Houston, TX. A key outcome of this test was to verify there is adequate range of motion in PMSA and SMA actuators to align them to AOS/ISIM under flight-like conditions. The alignment state of the PMSAs and SMA was measured using photogrammetry and cross-checked optically using a variation of a classical Hartmann test. In the “Pass-and-a-Half” (PAAH) configuration, fiber sources near the Cassegrain focus propagate light through the full optical train and small tilts on the PMSAs create an array of spots on the science instrument detectors, mimicking the effect of a Hartmann mask. Comparison of measured and modeled spot arrays provides the alignment state of the SMA and the global tilt of the primary mirror. This paper will discuss the methodology, testing, and analysis performed to measure the alignment state of OTIS using the Hartmann method and verify the primary and secondary mirrors can be successfully aligned on orbit to meet performance requirements.
The James Webb Space Telescope (JWST) telescope’s Secondary Mirror Assembly (SMA) and eighteen Primary Mirror Segment Assemblies (PMSAs) are each actively controlled in rigid body position via six hexapod actuators. Each of the PMSAs additionally has a radius of curvature actuator. The mirrors are stowed to the mirror support structure to survive the launch environment and then must be deployed 12.5 mm to reach the nominally deployed position before the Wavefront Sensing & Control (WFSC) alignment and phasing process begins. JWST requires testing of the full optical system in a Cryogenic Vacuum (CV) environment before launch. The cryo vacuum test campaign was executed in Chamber A at the Johnson Space Center (JSC) in Houston Texas. The test campaign consisted of an ambient vacuum test, a cooldown test, a cryo stable test at 65 Kelvin, a warmup test, and finally a second ambient vacuum test. Part of that test campaign was the functional and performance testing of the hexapod actuators on the flight mirrors. This paper will describe the testing that was performed on all 132 hexapod and radius of curvature actuators. The test campaign first tests actuators individually then tested how the actuators perform in the hexapod system. Telemetry from flight sensors on the actuators and measurements from external metrology devices such as interferometers, photogrammetry systems and image analysis was used to demonstrate the performance of the JWST actuators. The mirror move commanding process was exercised extensively during the JSC CV test and many examples of accurately commanded moves occurred. The PMSA and SMA actuators performed extremely well during the JSC CV test, and we have demonstrated that the actuators are fully functional both at ambient and cryo temperatures and that the mirrors will go to their commanded positions with the accuracy needed to phase and align the telescope.
KEYWORDS: Image segmentation, James Webb Space Telescope, Space telescopes, Wavefronts, Telescopes, Wavefront sensors, Control systems, Mirrors, Image processing, Sensors
A subset of the Wavefront Sensing and Controls (WFSC) operations for JWST were demonstrated during its recent cryo-vac testing using the flight telescope and instruments, and a functional simulation of the spacecraft and ground system. The demonstration had three goals: to confirm the operation of the flight data collection scripts, to check the WFSC optical components, and to verify the coordinates and influence functions that will be used for flight WFSC. In this paper, we present the results and lessons learned from this demonstration.
The Visible Integral-field Replicable Unit Spectrograph (VIRUS) consists of 156 identical spectrographs (arrayed as 78 pairs, each with a pair of spectrographs) fed by 35,000 fibers, each 1.5 arcsec diameter, at the focus of the upgraded 10 m Hobby-Eberly Telescope (HET). VIRUS has a fixed bandpass of 350-550 nm and resolving power R~750. The fibers are grouped into 78 integral field units, each with 448 fibers and 20 m average length. VIRUS is the first example of large-scale replication applied to optical astronomy and is capable of surveying large areas of sky, spectrally. The VIRUS concept offers significant savings of engineering effort and cost when compared to traditional instruments. The main motivator for VIRUS is to map the evolution of dark energy for the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), using 0.8M Lyman-alpha emitting galaxies as tracers. The VIRUS array has been undergoing staged deployment starting in late 2015. Currently, more than half of the array has been populated and the HETDEX survey started in 2017 December. It will provide a powerful new facility instrument for the HET, well suited to the survey niche of the telescope, and will open up large spectroscopic surveys of the emission line universe for the first time. We will review the current state of production, lessons learned in sustaining volume production, characterization, deployment, and commissioning of this massive instrument.
KEYWORDS: Adaptive optics, Mirrors, James Webb Space Telescope, Cryogenics, Telescopes, Optical alignment, Space telescopes, Photogrammetry, Monte Carlo methods, Actuators
The James Webb Space Telescope is a large space-based astronomical telescope that will operate at cryogenic temperatures. Because of its size, the telescope must be stowed in an inoperable configuration for launch and remotely reconfigured in space to meet the operational requirements using active Wave Front Sensing and Control (WFSC). Predicting optical performance for the flight system relies on a sequence of incremental tests and analyses that has culminated with the cryogenic vacuum test of the integrated Optical Telescope Element (OTE) and Integrated Science Instrument Module (ISIM) referred to as OTIS. The interplay between the optical budgeting process, test verification results at incrementally increasing levels of integration, use of test validated models, and the WFSC process to produce the final optical performance predictions for final verification by analysis will be presented.
The second generation Low Resolution Spectrograph (LRS2) is a new facility instrument for the Hobby-Eberly Telescope (HET) at McDonald Observatory. Designed as a powerful spectroscopic follow-up platform, LRS2 is based on the design of the HETs new Visible Integral-field Replicable Unit Spectrograph (VIRUS) and provides integral field spectroscopy for two seeing-limited fields of 6”×12” with unity fill factor. The replicable design of VIRUS has been leveraged for LRS2 to gain broad wavelength coverage from 370 nm to 1.0 μm, spread between two fiber-fed dual-channel spectrographs that operate in unison but observe independent fields that are separated by 100”. The blue spectrograph pair, LRS2-B, covers 364≤λ (nm) ≤ 467 and 454 ≤ λ (nm)≤700 at fixed resolving powers of R =λ/δλ≈2500 and 1400, respectively, while the red spectrograph pair, LRS2-R, covers 643≤λ (nm)≤845 and 823≤λ (nm)≤1056 with both of its channels having R≈2500. In this paper, a detailed description of the instrument's design, assembly, and laboratory testing is provided in which the focus is placed on the departures from the basic framework of the design and processes previously established for VIRUS. Both LRS2 spectrograph pairs have been successfully deployed on the HET, and commissioning efforts are ongoing. Using on-sky data, the performance of the spectrograph is compared to models of the instrumental sensitivity. The measured performance of LRS2 indicates that the instrument will provide efficient spectroscopic follow-up observations of individual targets, and will be especially powerful when combined with the extensive survey capabilities of VIRUS for HETDEX.
The Visible Integral-field Replicable Unit Spectrograph (VIRUS) consists of 156 identical spectrographs (arrayed as 78 pairs) fed by 35,000 fibers, each 1.5 arcsec diameter, at the focus of the upgraded 10 m Hobby-Eberly Telescope (HET). VIRUS has a fixed bandpass of 350-550 nm and resolving power R~700. VIRUS is the first example of industrial-scale replication applied to optical astronomy and is capable of surveying large areas of sky, spectrally. The VIRUS concept offers significant savings of engineering effort, cost, and schedule when compared to traditional instruments. The main motivator for VIRUS is to map the evolution of dark energy for the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX‡), using 0.8M Lyman-alpha emitting galaxies as tracers. The VIRUS array is undergoing staged deployment during 2016 and 2017. It will provide a powerful new facility instrument for the HET, well suited to the survey niche of the telescope, and will open up large spectroscopic surveys of the emission line universe for the first time. We will review the production, lessons learned in reaching volume production, characterization, and first deployment of this massive instrument.
The Visible Integral Field Replicable Unit Spectrograph (VIRUS), the instrument for the Hobby Eberly Telescope Dark Energy Experiment (HETDEX), consists of 78 replicable units, each with two integral field spectrographs. Each spectrograph has its own 2kx2k CCD detector with 15 micron pixels. Following alignment, the final stage prior to deployment of each unit is characterization of the 156 spectrograph channels and their CCDs. We describe the laboratory calibration system and scripting that automates this process. Both fiber and continuous (non-spatially modulated) input slits are utilized. Photon transfer curves are made to measure the gain and read noise of each CCD. Pixel flats are also made to correct for pixel-to-pixel QE variations. Relative throughput measurements of each unit are made using the same lab fiber bundle for consistency, and fiber profiles are characterized for later use by the CURE data reduction package. Replicable unit instruments provide a cost effective solution for scaling up instruments for large and extremely large class telescopes. Because VIRUS is the first massively replicated instrument, we have the opportunity to examine the end result of variations in the manufacturing processes that go into production. This paper presents the characterization setup for VIRUS units and compares the performance and variability of processed units with specifications for HETDEX.
The Low Resolution Spectrograph 2 (LRS2) was recently deployed on the Hobby-Eberly Telescope (HET). LRS2 consists of four spectrographic channels, each covering adjacent wavelength bands from 360-1050nm which are fed with a fiber optic integral field unit (IFU). The integral field unit developed for this instrument represents a transformative approach to expanding the wavelength coverage of integral field spectrographs. The unique input feed of the IFU serves two functions; combining the wavelength coverage of two spectrographic channels to one spatial field on sky, and expanding the field of view of each individual fiber to eliminate the interstitial space between fibers on sky. The spectral multiplexing is achieved with dichroic beam splitter and collimator, while the focal reduction is achieved with a pair of micro lens arrays. The optical components required micron scale alignment precision in a compact mechanical package to allow integration on the telescope focal surface. Here we report on the design, assembly, and performance of the IFU.
VIRUS is a massively replicated spectrograph built for HETDEX, the Hobby Eberly Telescope Dark Energy Experiment. It consists of 156 channels within 78 units fed by 34944 fibers over the 22 arcminute field of the upgraded HET. VIRUS covers a relatively narrow bandpass (350-550nm) at low resolution (R ~ 700) to target the emission of Lyman-alpha emitters (LAEs) for HETDEX. VIRUS is a first demonstration of industrial style assembly line replication in optical astronomy. Installation and testing of VIRUS units began in November of 2015. This winter we celebrated the first on sky instrument activity of the upgraded HET, using a VIRUS unit and LRS2-R (the upgraded facility Low Resolution Spectrograph for the HET). Here we describe progress in VIRUS installation and commissioning through June 2016. We include early sky data obtained to characterize spectrograph performance and on sky performance of the newly upgraded HET. As part of the instrumentation for first science light at the HET, the IFU fed spectrographs were used to test a full range of telescope system functionality including the field calibration unit (FCU).We also use placement of strategic IFUs to map the new HET field to the fiber placement, and demonstrate actuation of the dithering mechanism key to HETDEX observations.
The Visible Integral-field Replicable Unit Spectrograph (VIRUS) is a baseline array of 150 copies of a simple, fiber-fed integral field spectrograph that will be deployed on the Hobby-Eberly Telescope (HET). VIRUS is the first optical astronomical instrument to be replicated on an industrial scale, and represents a relatively inexpensive solution for carrying out large-area spectroscopic surveys, such as the HET Dark Energy Experiment (HETDEX). Each spectrograph contains a volume phase holographic (VPH) grating with a 138 mm diameter clear aperture as its dispersing element. The instrument utilizes the grating in first-order for 350 < λ (nm) < 550. Including witness samples, a suite of 170 VPH gratings has been mass produced for VIRUS. Here, we present the design of the VIRUS VPH gratings and a discussion of their mass production. We additionally present the design and functionality of a custom apparatus that has been used to rapidly test the first-order diffraction efficiency of the gratings for various discrete wavelengths within the VIRUS spectral range. This device has been used to perform both in-situ tests to monitor the effects of adjustments to the production prescription as well as to carry out the final acceptance tests of the gratings' diffraction efficiency. Finally, we present the as-built performance results
for the entire suite of VPH gratings.
VIRUS is the massively replicated fiber-fed spectrograph being built for the Hobby-Eberly Telescope to support
HETDEX (the Hobby-Eberly Telescope Dark Energy Experiment). The instrument consists of 156 identical
channels, fed by 34,944 fibers contained in 78 integral field units, deployed in the 22 arcminute field of the
upgraded HET. VIRUS covers 350-550nm at R ≈ 700 and is built to target Lyman α emitters at 1.9 < z < 3.5 to
measure the evolution of dark energy. Here we present the assembly line construction of the VIRUS spectrographs,
including their alignment and plans for characterization. We briefly discuss plans for installation on the telescope.
The spectrographs are being installed on the HET in several stages, and the instrument is due for completion
by the end of 2014.
The Visible Integral-field Replicable Unit Spectrograph (VIRUS) consists of a baseline build of 150 identical
spectrographs (arrayed as 75 unit pairs) fed by 33,600 fibers, each 1.5 arcsec diameter, at the focus of the upgraded 10
m Hobby-Eberly Telescope (HET). VIRUS has a fixed bandpass of 350-550 nm and resolving power R~700. VIRUS is
the first example of industrial-scale replication applied to optical astronomy and is capable of surveying large areas of
sky, spectrally. The VIRUS concept offers significant savings of engineering effort, cost, and schedule when compared
to traditional instruments.
The main motivator for VIRUS is to map the evolution of dark energy for the Hobby-Eberly Telescope Dark Energy
Experiment (HETDEX), using 0.8M Lyman-α emitting galaxies as tracers. The full VIRUS array is due to be deployed starting at the end of 2014 and will provide a powerful new facility instrument for the HET, well suited to the
survey niche of the telescope, and will open up large area surveys of the emission line universe for the first time.
VIRUS is in full production, and we are about half way through. We review the production design, lessons learned in
reaching volume production, and preparation for deployment of this massive instrument. We also discuss the application
of the replicated spectrograph concept to next generation instrumentation on ELTs.
Jennifer Marshall, Darren DePoy, Travis Prochaska, Richard Allen, Patrick Williams, Jean-Philippe Rheault, Ting Li, Daniel Nagasawa, Christopher Akers, David Baker, Emily Boster, Caitlin Campbell, Erika Cook, Alison Elder, Alex Gary, Joseph Glover, Michael James, Emily Martin, Will Meador, Nicholas Mondrik, Marisela Rodriguez-Patino, Steven Villanueva, Gary Hill, Sarah Tuttle, Brian Vattiat, Hanshin Lee, Taylor Chonis, Gavin Dalton, Mike Tacon
The Visual Integral-Field Replicable Unit Spectrograph (VIRUS) instrument is a baseline array 150 identical fiber fed
optical spectrographs designed to support observations for the Hobby-Eberly Telescope Dark Energy Experiment
(HETDEX). The collimator subassemblies of the instrument have been assembled in a production line and are now
complete. Here we review the design choices and assembly practices used to produce a suite of identical low-cost
spectrographs in a timely fashion using primarily unskilled labor.
The second generation Low Resolution Spectrograph (LRS2) is a new facility instrument for the Hobby-Eberly Telescope (HET). Based on the design of the Visible Integral-field Replicable Unit Spectrograph (VIRUS), which is the new flagship instrument for carrying out the HET Dark Energy Experiment (HETDEX), LRS2 provides integral field spectroscopy for a seeing-limited field of 12" x 6". For LRS2, the replicable design of VIRUS has been leveraged to gain broad wavelength coverage from 370 nm to 1.0 μm, spread between two fiber-fed dual- channel spectrographs, each of which can operate as an independent instrument. The blue spectrograph, LRS2-B, covers 370 λ (nm) ≤ 470 and 460 ≤ λ (nm) ≤ 700 at fixed resolving powers of R = λ/δλ ≈ 1900 and 1100, respectively, while the red spectrograph, LRS2-R, covers 650 ≤ λ (nm) ≤ 842 and 818 ≤ λ (nm) ≤ 1050 with both of its channels having R ≈ 1800. In this paper, we present a detailed description of the instrument’s design in which we focus on the departures from the basic VIRUS framework. The primary modifications include the fore-optics that are used to feed the fiber integral field units at unity fill-factor, the cameras’ correcting optics and detectors, and the volume phase holographic grisms. We also present a model of the instrument’s sensitivity and a description of specific science cases that have driven the design of LRS2, including systematically studying the spatially resolved properties of extended Lyα blobs at 2 < z < 3. LRS2 will provide a powerful spectroscopic follow-up platform for large surveys such as HETDEX.
The upcoming Wide-Field Upgrade (WFU) has ushered in a new era of instrumentation for the Hobby-Eberly Telescope (HET). Here, we present the design, construction progress, and lab tests completed to date of the blue-optimized second generation Low Resolution Spectrograph (LRS2-B). LRS2-B is a dual-channel, fiber fed instrument that is based on the design of the Visible Integral Field Replicable Unit Spectrograph (VIRUS), which is the new flagship instrument for carrying out the HET Dark Energy eXperiment (HETDEX). LRS2-B utilizes a microlens-coupled integral field unit (IFU) that covers a 7”x12” area on the sky having unity fill-factor with ~300 spatial elements that subsample the median HET image quality. The fiber feed assembly includes an optimized dichroic beam splitter that allows LRS2-B to simultaneously observe 370 <λ(nm) < 470 and 460 < λ(nm) < 700 at fixed resolving powers of R ≈ λ/Δλ ≈ 1900 and 1200, respectively. We discuss the departures from the nominal VIRUS design, which includes the IFU, fiber feed, camera correcting optics, and volume phase holographic grisms. Additionally, the motivation for the selection of the wavelength coverage and spectral resolution of the two channels is briefly discussed. One such motivation is the follow-up study of spectrally and (or) spatially resolved Lyα emission from z ≈ 2.5 star-forming galaxies in the HETDEX survey. LRS2-B is planned to be a commissioning instrument for the HET WFU and should be on-sky during quarter 4 of 2013. Finally, we mention the current state of LRS2-R, the red optimized sister instrument of LRS2-B.
The Visible Integral-field Replicable Unit Spectrograph (VIRUS) consists of a baseline build of 150 identical
spectrographs (arrayed as 75 units, each with a pair of spectrographs) fed by 33,600 fibers, each 1.5 arcsec diameter,
deployed over the 22 arcminute field of the upgraded 10 m Hobby-Eberly Telescope (HET). The goal is to deploy 82
units. VIRUS has a fixed bandpass of 350-550 nm and resolving power R~700. VIRUS is the first example of
industrial-scale replication applied to optical astronomy and is capable of spectral surveys of large areas of sky. This
approach, in which a relatively simple, inexpensive, unit spectrograph is copied in large numbers, offers significant
savings of engineering effort, cost, and schedule when compared to traditional instruments.
The main motivator for VIRUS is to map the evolution of dark energy for the Hobby-Eberly Telescope Dark Energy
Experiment (HETDEX) using 0.8M Lyman-α emitting galaxies as tracers. The full VIRUS array is due to be deployed
by early 2014 and will provide a powerful new facility instrument for the HET, well suited to the survey niche of the
telescope. VIRUS and HET will open up wide-field surveys of the emission-line universe for the first time. We present
the production design and current status of VIRUS.
The Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) uses a novel technique of replicated spectrographs (VIRUS) to measure dark energy at intermediate redshifts (2 < z < 4). VIRUS contains over 30,000 fibers and over 160 independent and identical channels. Here we report on the construction and characterization of the initial batch of VIRUS spectrograph cameras. Assembly of the first batch of 16 is in progress. A brief overview of the assembly is presented, and where available performance is compared to specification.
The Visible Integral field Replicable Unit Spectrograph (VIRUS) is an array of at least 150 copies of a simple, fiber-fed integral field spectrograph that will be deployed on the Hobby-Eberly Telescope (HET) to carry out the HET Dark Energy Experiment (HETDEX). Each spectrograph contains a volume phase holographic grating as its dispersing element that is used in first order for 350 < λ(nm) < 550. We discuss the test methods used to evaluate the performance of the prototype gratings, which have aided in modifying the fabrication prescription for achieving the specified batch diffraction efficiency required for HETDEX. In particular, we discuss tests in which we measure the diffraction efficiency at the nominal grating angle of incidence in VIRUS for all orders accessible to our test bench that are allowed by the grating equation. For select gratings, these tests have allowed us to account for < 90% of the incident light for wavelengths within the spectral coverage of VIRUS. The remaining light that is unaccounted for is likely being diffracted into reflective orders or being absorbed or scattered within the grating layer (for bluer wavelengths especially, the latter term may dominate the others). Finally, we discuss an apparatus that will be used to quickly verify the first order diffraction efficiency specification for the batch of at least 150 VIRUS production gratings.
In the era of the Hobby-Eberly Telescope (HET) Wide-Field Upgrade (WFU), the current Low-Resolution Spectrograph
(LRS) will be replaced with a more capable red-optimized fiber instrument, called LRS2. This new spectrograph will be
based on the Visible Integral-field Replicable Unit Spectrograph (VIRUS) that was designed to be easily adapted to a
wide range of spectral resolutions, and wavelength ranges. The current snapshot of LRS2, fed by a 7x12 sq. arcsec fiber
integral-field unit (IFU), covers 350-1100 nm, simultaneously at a fixed resolving power R~1800, with the wavelength
range split into two pairs of spectrographs, one for the blue to red wavelength range (350-630 nm) and the other for the
red and far-red range (630-1100 nm). These units are designated LRS2-B and LRS2-R, respectively. Only minimal
modification from the base VIRUS design in gratings (for both pairs) and in the detector (for the red pair only) is
required. In addition to this flexibility, the generic nature and massively replicable characteristic of the instrument can
allow us to adapt the instrument to a wide range of not only telescope diameters (1 m ~ 40 m), but also observing modes
(single to multiple objects). We discuss the current snapshot of the LRS2 design.
The upcoming Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) has provided motivation for upgrading
the Hobby-Eberly Telescope (HET) at the McDonald Observatory. This upgrade includes an increase in
the field-of-view to accommodate the new and revolutionary Visible Integral-field Replicable Unit Spectrograph
(VIRUS). VIRUS is the instrument designed to conduct the HETDEX survey and consists of 150 individual
integral-field spectrographs fed by 33,600 total optical fibers covering the 22 arc-minute field-of-view of the
upgraded HET. The spectrographs are mounted in four enclosures, each 6.0×3.0×1.4 meters in size. Each
spectrograph contains a CCD detector that must be cryogenically cooled, presenting an interesting cryogenic
and vacuum challenge within the distribution system. In this paper, we review the proposed vacuum jacketed,
thermal siphon, liquid nitrogen distribution system used to cool the array of detectors and discuss recent developments.
We focus on the design, prototyping, and testing of a novel "make-break" thermal connector, built
from a modified cryogenic bayonet, that is used to quickly detach a single spectrograph pair from the system.
The Infrared Imaging System (IRIS) is a 0.8m telescope and a 1024×1024 pixels camera (IRISCAM) with a HAWAII-1
detector array. IRIS is located at the Cerro Armazones Observatory in Chile that is operated by the Ruhr University
Bochum jointly with the Universidad Católica del Norte in Antofagasta. It will be used primarily to survey star-forming
regions for variability. Our goal is to discover young stellar objects undergoing accretion instabilities or rotational
modulation of star spots, eclipsing binaries, and variable reflection nebulae. The telescope and the infrared camera are
completed and first light was achieved in May of 2010. IRIS is currently being tested and characterized, before the longterm
monitoring project will commence.
The Visible Integral-field Replicable Unit Spectrograph (VIRUS) consists of a baseline build of 150 identical
spectrographs (arrayed as 75 units, each with a pair of spectrographs) fed by 33,600 fibers, each 1.5 arcsec diameter,
deployed over the 22 arcminute field of the upgraded 10 m Hobby-Eberly Telescope (HET). The goal is to deploy 96
units. VIRUS has a fixed bandpass of 350-550 nm and resolving power R~700. VIRUS is the first example of
industrial-scale replication applied to optical astronomy and is capable of spectral surveys of large areas of sky. The
method of industrial replication, in which a relatively simple, inexpensive, unit spectrograph is copied in large numbers,
offers significant savings of engineering effort, cost, and schedule when compared to traditional instruments.
The main motivator for VIRUS is to map the evolution of dark energy for the Hobby-Eberly Telescope Dark Energy
Experiment (HETDEX+) using 0.8M Lyman-α emitting galaxies as tracers. The full VIRUS array is due to be deployed
in late 2011 and will provide a powerful new facility instrument for the HET, well suited to the survey niche of the
telescope. VIRUS and HET will open up wide field surveys of the emission-line universe for the first time. We present
the design, cost, and current status of VIRUS as it enters production, and review performance results from the VIRUS
prototype. We also present lessons learned from our experience designing for volume production and look forward to
the application of the VIRUS concept on future extremely large telescopes (ELTs).
The Infrared Imaging System (IRIS) is a 0.8 m telescope equipped with a 1024×1024 pixels near-infrared camera using
a HAWAII-1 detector array. IRIS will be located at the Cerro Armazones Observatory in Chile that is operated by the
Ruhr University Bochum jointly with the Universidad Catolica del Norte in Antofagasta. The system is specifically
designed to survey star forming regions and to search for deeply embedded variable young stars.
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