KOSMOS is a low-resolution, long-slit, optical spectrograph that has been upgraded at the University of Washington for its move from Kitt Peak National Observatory’s Mayall 4-m telescope to the Apache Point Observatory’s ARC 3.5-m telescope. One of the additions to KOSMOS is a slitviewer, which requires the fabrication of reflective slits, as KOSMOS previously used matte slits machined via wire electrical discharge machining. We explore an innovative method of slit fabrication using nanofabrication methods and compare the slit edge roughness, width uniformity, and the resulting scattering of the new fabricated slits to the original slits. We find the kerf surface of the chemically etched reflective silicon slits are generally smoother than the machined matte slits, with an upper limit average roughness of 0.42 ± 0.03 μm versus 1.06 ± 0.04 μm, respectively. The etched slits have width standard deviations of 6 ± 3 μm versus 10 ± 6 μm, respectively. The scattering for the chemically etched slits is higher than that of the machined slits, showing that the reflectivity is the major contributor to scattering, not the roughness. This scattering, however, can be effectively reduced to zero with proper background subtraction. As slit widths increase, scattering increases for both types of slits, as expected. Future work will consist of testing and comparing the throughput and spectrophotometric data quality of these nanofabricated slits to the machined slits with on-sky data, in addition to making the etched slits more robust against breakage and finalizing the slit manufacturing process.
We discuss the field retermination of high-fiber count MTP fiber connectors used with the APOGEE spectrograph at Apache Point Observatory (APO) in 2021. We address lessons-learned, wear-analysis of removed MTPs, and throughput of the fiber train with the newly terminated fibers in SDSS-V. For the past decade the spectrograph at APO, as part of multiple incarnations of the Sloan Digital Sky Survey (SDSS), has relied upon rapid changes of ten MTP connectors, each containing 30 terminated fibers, and all contained within a custom gang connector system. These rapid changes enable the iterative plugging of the gang connector into multiple cartridges with different plug plates to observe various survey fields throughout the night. While robotic Focal Plane Systems have been developed for SDSS-V to replace plug plates, which will minimize the fiber connector cycles, we nonetheless reterminated the most heavily used MTP connectors. The connector cycles had far exceeded manufacturer lifetimes and the overall system throughput was degrading.
MegaMapper is a 6.5m Magellan-like telescope fitted with a wide-field-corrector (WFC) and atmospheric-dispersion-corrector (ADC) that delivers a 3° diameter corrected field-of-view. The telescope’s focal surface is populated by ∼25,000 robotic fiber-positioners feeding a cluster of 36 DESI-like medium resolution spectrographs. We present the facility concept for MegaMapper including: conceptual optical and opto-mechanical designs for the telescope and WFC/ADC that deliver ≲ 0.4” image quality over the full FOV for zenith distances ≤ 50°; the development of a new and modular robotic fiber-positioner focal plane design that can populate the focal surface at high densities (6.2 mm pitch or ∼1 per arcmin2); and concepts for hosting the MegaMapper spectrograph cluster under environmentally controlled conditions inside the telescope enclosure. Building on existing and proven designs and technologies, MegaMapper aims to minimize the project’s technical risk and cost while delivering a competitive next-generation massively multiplexed spectroscopic facility. MegaMapper will lead the study of inflation, dark energy, dark matter, and time-domain astronomy over the next decades by carrying out wide-field cosmological galaxy-redshift surveys, massive spectroscopic surveys of stars in the Milky Way halo and satellites, and by providing a spectroscopic follow-up counterpart to wide field imaging facilities like the Vera C. Rubin Observatory and the Nancy Grace Roman space telescope.
We present measurements of the adapted SDSS-V BOSS fiber slit, as well as measurements of the APOGEE fiber tributaries in order to better our understanding of throughput loss at multi-fiber termination junctions. The BOSS fiber slit was adapted from the original slit used on sky, and multi fiber termination connectors were added in order to extend the fiber length for the new SDSS-V FPS configuration. Testing on multi fiber termination junctions was conducted with APOGEE tributaries to characterize the expected throughput loss due to multi fiber terminations for the APOGEE spectrograph. Multi fiber connectors were tested with and without index matching gel to evaluate whether throughput loss could be mitigated
The Sloan Digital Sky Survey V (SDSS-V) is an all-sky spectroscopic survey of <6 million objects, designed to decode the history of the Milky Way, reveal the inner working of the stars, investigate the origin of solar systems, and track the growth of supermassive black holes across the Universe. Here we describe the mathematical frame work behind the SDSS-V robotic Focal Plane System (FPS), specifically the coordinate systems and transforms used to propagate locations of targets from astronomical catalogs to coordinates on a robotically positioned fiber or guide camera. This framework provides the backbone for the design and operations of SDSS-V’s MOS surveys. All coordinate systems and transformations between them are provided in SDSS-V’s coordio software package.
Several external hardware upgrades have been developed for the APOGEE Spectrographs as part of the Sloan Digital Sky Survey-V (SDSS-V) to improve their radial velocity (RV) precision from a floor of 100-200 m/sec to approx. 30 m/sec. The upgrades include: (1) Back Pressure Regulator (BPR) systems to stabilize the internal instrument LN2 tank boil-off pressure, lessening induced movement of the APOGEE optical bench; (2) Fabry-Perot Interferometer (FPI) calibration sources to improve wavelength calibration; and (3), the use of octagonal core fiber segments in the fiber train to improve radial scrambling. We discuss the fabrication, commissioning, and early performance of these upgrades.
We describe the on-sky performance of the robotic Focal Plane System (FPS) units that replace the fiber plug-plate systems at the Sloan and duPont telescopes for the SDSS-V survey. The first FPS was arrived at Apache Point in December 2021, and the second unit will be delivered to Las Campanas in spring 2022. Each FPS carries 500 zonal fiber positioners carrying three fibers: two science fibers for the BOSS and APOGEE spectrographs and a back-illuminated metrology fiber. The FPS enables the SDSS-V Milky Way and Black Hole Mapper surveys that will begin survey operations in 2022.
We present measurements of 40 fiber tapers created as a possible approach to feed the BOSS spectrographs for SDSS-V in the Southern hemisphere at Las Campanas observatory. The fibers are designed with 180µm core at the input (telescope feed) end, and tapering to a 120µm core at the output (spectrograph) end. The taper is located in close proximity to the output end and is protected by metal strain relief held in place by the ferrule. We find the fiber tapers to provide adequate throughput but to underperform related to standard (untapered) fibers when it comes to FRD. We present here measurements from three different test bench setups, located at the University of Washington, Yale University, and SSL at the University of California, Berkeley. We find that although the overall throughput is high, the FRD is poor in the tapers. The current fiber taper performance is not adequate for the SDSS-V fiber requirements.
The Sloan Digital Sky Survey V (SDSS-V) is an all-sky spectroscopic survey of > 6 million objects, designed to decode the history of the Milky Way, reveal the inner workings of stars, investigate the origin of solar systems, and track the growth of supermassive black holes across the Universe.1 This paper describes the design of the theta/phi fiber positioner robots that are being produced to be integrated in the Focal Plane System (FPS) of the SDSS-V telescopes. For each installation, 500 robots are being manufactured, more than 800 units have already been received from the manufacturer and validated. Mechanical, electronic and firmware designs are presented in the current paper in detail. We will expose the different iterations of the prototypes that were developed, built and tested and ultimately allowed to achieve the end version that meets the requested science requirements. The fiber positioner robot is carrying 3 optical fibers integrated into a single snowflake ferrule. Two of the fibers are science fibers connected to two different spectrographs, and the third fiber is used for metrology. The robot is capable of positioning the fibers with a planar accuracy better than 50 microns with a first blind move within its workspace of a diameter of 44.8mm. With a complementary fiber viewing camera (FVC) and the backlighted metrology fiber to perform a few small corrections moves, the positioner can reach a sub 5-micron precision on the fiber position.
The Sloan Digital Sky Survey V (SDSS-V) is an all-sky spectroscopic survey of < 6 million objects, designed to decode the history of the Milky Way, reveal the inner workings of stars, investigate the origin of solar systems, and track the growth of supermassive black holes across the Universe. Collaboratively, organizations across both academia and industry have partnered to overcome technical challenges and execute operational directives associated with commissioning the various mechanical, electrical, and software subsystems of SDSS-V. While this type of collaboration is not unique, the scale and complexity of next generation astronomical instruments is an emerging challenge that requires industrial systems and process engineering practices at a quasi-industrial scale. Driven by the success of multiplexed spectroscopic surveys, instrumentation is evolving to include systems with hundreds to thousands of components and sub-assemblies procured or produced from various sources. This trend requires the adoption of new and existing processes and best practices in the design, integration, and test of next generation astronomical instruments. The following discussion outlines those industrial systems and process engineering processes, methods, and practices, currently in the operational phase, for the design, integration, and test of the SDSS-V Focal Plane System (FPS). An emphasis is placed on processes, methods, and practices related to coordination of multiple contract manufacturing vendors and operational execution of small batch manufacturing.
The Sloan Digital Sky Survey V (SDSS-V) is an all-sky spectroscopic survey of <6 million objects, designed to decode the history of the Milky Way, reveal the inner workings of stars, investigate the origin of solar systems, and track the growth of supermassive black holes across the Universe. This paper describes the design and construction of two robotic Focal Plane System (FPS) units that will replace the traditional SDSS fiber plug-plate systems at the Sloan and du Pont telescopes for SDSS-V. Each FPS deploys 500 zonal fiber positioners that allow us to reconfigure the fibers onto a new target field within 2-3 minutes of acquisition. Each positioner carries three fibers: two science fibers that feed the BOSS and APOGEE spectrographs and a third back-illuminated metrology fiber is used in conjunction with a telescopemounted Fiber Viewing Camera (FVC) to measure the absolute positions of the fiber heads. The 300 APOGEE fibers are distributed among the 500 positioners to maximize common field coverage. A set of fiber-illuminated fiducials distributed in and around the positioner array establish a fixed reference frame for the FVC system. Finally, six CCD cameras mounted around the periphery of the focal plane provide acquisition, guiding, and focus monitoring functions. The FPS is a key enabling technology of the SDSS-V Milky Way and Black Hole Mapper surveys.
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.
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 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.
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.
Teague introduced a phase retrieval method that uses the image shape moments. More recently, an independent
study arrived at a similar technique, which was then applied to in-situ full-field image-quality evaluation of
spectroscopic systems. This moment-based wavefront sensing (MWFS) method relies on the geometric relation
between the image shape moments and the geometric wavefront modal coefficients. The MWFS method allows
a non-iterative determination of the modal coefficients from focus-modulated images at arbitrary spatial resolutions.
The determination of image moments is a direct extension of routine centroid and image size calculation,
making its implementation easy. Previous studies showed that the MWFS works well in capturing large low-order
modes, and is quite suitable for in-situ alignment diagnostics. At the Astronomical Instrumentation conference
in 2012, we presented initial results of the application of the moment-based wavefront sensing to a fiber-fed astronomical
spectrograph, called VIRUS (a set of replicated 150 identical integral-field unit spectrographs contained
in 75 unit pairs). This initial result shows that the MWFS can provide accurate full-field image-quality assessment
for efficiently aligning these 150 spectrographs. Since then, we have assembled more than 24 unit pairs
using this technique. In this paper, we detail the technical update/progress made so far for the moment-based
wavefront sensing method and the statistical estimates of the before/after alignment aberrations, image-quality,
and various efficiency indicators of the unit spectrograph alignment process.
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.
The Hobby-Eberly Telescope (HET) is an innovative large telescope located in West Texas at the McDonald Observatory. The HET operates with a fixed segmented primary and has a tracker, which moves the four-mirror optical 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 substantially increase the pupil size to 10 meters (from 9.2 m) and the field of view to 22 arcminutes (from 4 arcminutes) by replacing the corrector, tracker, and prime focus instrument package. In addition to supporting existing instruments, and a new low resolution spectrograph, 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§). The upgrade is being installed and this paper discusses the current status.
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.
VIRUS is the visible, integral-field replicable unit spectrograph for the Hobby-Eberly-Telescope (HET) consisting of 75
integral-field-units that feed 150 spectrographs. The full VIRUS instrument features over 33,000 fibres, each projecting
to 1.5 arcseconds diameter on sky, deployed at the prime focus of the upgraded 10m HET. The assembly and acceptance
testing for all IFUs includes microscopic surface quality inspections, astrometry of fibre positions, relative throughput
measurements, focal-ratio-degradation evaluation, and system acceptance using a VIRUS reference spectrograph to
verify the image quality, spectral transmission, stability, or to detect any stray light issues.
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.
Jennifer Marshall, Travis Prochaska, Richard Allen, Darren DePoy, Gary Hill, Sarah Tuttle, Ting Li, Brian Vattiat, Taylor Chonis, Emily Boster, David Baker, Marisela Rodriguez-Patino, Patrick Williams, Jean-Philippe Rheault, Daniel Nagasawa, Christopher Akers, Caitlin Campbell, Erika Cook, Alison Elder, Alex Gary, Joseph Glover, Michael James, Emily Martin, Will Meador, Nicholas Mondrik, Steven Villanueva, Hanshin Lee, 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 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.
We describe the mechanical assembly and optical alignment processes used to construct the Visual Integral-Field
Replicable Unit Spectrograph (VIRUS) instrument. VIRUS is a set of 150+ optical spectrographs designed to support
observations for the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX). To meet the instrument's
manufacturing constraints, a production line will be set up to build subassemblies in parallel. To aid in the instrument's
assembly and alignment, specialized fixtures and adjustment apparatuses have been developed. We describe the design
and operations of the various optics alignment apparatuses, as well as the mirrors' alignment and bonding fixtures.
We report on extensive testing carried out on the optical fibers for the VIRUS instrument. The primary result of
this work explores how 10+ years of simulated wear on a VIRUS fiber bundle affects both transmission and focal
ratio degradation (FRD) of the optical fibers. During the accelerated lifetime tests we continuously monitored
the fibers for signs of FRD. We find that transient FRD events were common during the portions of the tests
when motion was at telescope slew rates, but dropped to negligible levels during rates of motion typical for
science observation. Tests of fiber transmission and FRD conducted both before and after the lifetime tests
reveal that while transmission values do not change over the 10+ years of simulated wear, a clear increase in
FRD is seen in all 18 fibers tested. This increase in FRD is likely due to microfractures that develop over time
from repeated flexure of the fiber bundle, and stands in contrast to the transient FRD events that stem from
localized stress and subsequent modal diffusion of light within the fibers. There was no measurable wavelength
dependence on the increase in FRD over 350 nm to 600 nm. We also report on bend radius tests conducted
on individual fibers and find the 266 μm VIRUS fibers to be immune to bending-induced FRD at bend radii
of R 10 cm. Below this bend radius FRD increases slightly with decreasing radius. Lastly, we give details
of a degradation seen in the fiber bundle currently deployed on the Mitchell Spectrograph (formally VIRUS-P)
at McDonald Observatory. The degradation is shown to be caused by a localized shear in a select number of
optical fibers that leads to an explosive form of FRD. In a few fibers, the overall transmission loss through the
instrument can exceed 80%. These results are important for the VIRUS instrument, and for both current and
proposed instruments that make use of optical fibers, particularly when the fibers are in continual motion during
an observation, or experience repeated mechanical stress during their deployment.≥
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 image moment-based wavefront sensing (IWFS) utilizes moments of focus-modulated focal plane images to
determine modal wavefront aberrations. This permits fast, easy, and accurate measurement of wavefront error (WFE) on
any available finite-sized isolated targets across the entire focal plane (FP) of an imaging system, thereby allowing not
only in-situ full-field image quality assessment, but also deterministic fine alignment correction of the imaging system.
We present an experimental demonstration where fine alignment correction of a fast camera system in a fiber-fed
astronomical spectrograph, called VIRUS, is accomplished by using IWFS.
The Faint Intergalactic Redshifted Emission Balloon (FIREBALL) had its first scientific flight in June 2009.
The instrument combines microchannel plate detector technology with fiber-fed integral field spectroscopy on an
unstable stratospheric balloon gondola platform. This unique combination poses a series of calibration and data
reduction challenges that must be addressed and resolved to allow for accurate data analysis. We discuss our
approach and some of the methods we are employing to accomplish this task.
The Faint Intergalactic Redshifted Emission Balloon (FIREBALL) had its first scientific flight in June 2009.
The instrument is a 1 meter class balloon-borne telescope equipped with a vacuum-ultraviolet integral field
spectrograph intended to detect emission from the inter-galactic medium at redshifts 0.3 < z < 1.0. The
scientific goals and the challenging environment place strict constraints on the pointing and tracking systems of
the gondola. In this manuscript we briefly review our pointing requirements, discuss the methods and solutions
used to meet those requirements, and present the aspect reconstruction results from the first successful scientific
flight.
FIREBall (the Faint Intergalactic Redshifted Emission Balloon) is a balloon-borne 1m telescope coupled to an
ultraviolet fiber-fed spectrograph. FIREBall is designed to study the faint and diffuse emission of the warm hot
intergalactic medium, until now detected primarily in absorption. FIREBall is a pathfinding mission to test new
technology and make new constraints on the temperature and density of this gas. FIREBall has flown twice,
the most recent flight (June 2009) a fully functioning science flight. Here we describe the spectrograph design,
current setup, and calibration measurements from the campaign.
FIREBALL (the Faint Intergalactic Redshifted Emission Balloon) is a balloon-borne 1m telescope coupled to an
ultraviolet fiber-fed spectrograph. FIREBALL is designed to study the faint and diffuse emission of the intergalactic
medium, until now detected primarily in absorption. FIREBALL is a path finding mission to test new technology
and make new constraints on the temperature and density of this gas. We report on the first successful science flight
of FIREBALL, in June 2009, which proved every aspect of the complex instrument performance, and provided the
strongest measurements and constraints on IGM emission available from any instrument.
FIREBall (Faint Intergalactic Redshifted Emission Balloon) had a successful first engineering flight in July of 2007 from
Palestine, Texas. Here we detail the design and construction of the spectrograph. FIREBall consists of a 1m telescope
coupled to a fiber-fed ultraviolet spectrograph flown on a short duration balloon. The spectrograph is designed to map
hydrogen and metal line emission from the intergalactic medium at several redshifts below z=1, exploiting a small
window in atmospheric oxygen absorption at balloon altitudes. The instrument is a wide-field IFU fed by almost 400
fibers. The Offner mount spectrograph is designed to be sensitive in the 195-215nm window accessible at our altitudes
of 35-40km. We are able to observe Lyα, as well as OVI and CIV doublets, from 0.3 < z < 0.9. Observations of UV
bright B stars and background measurements allow characterization of throughput for the entire system and will inform
future flights.
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