Large-amplitude anomalous events have been observed in CCDs due to radioactive emission from high-index optical glasses, some producing charge-trapping like artefacts. We have identified the source of these events from one glass (Ohara S-YGH51) as α-particles from trace amounts of nuclides in the actinium decay series, parent 235U. We present measurements of the anomalous event rate for samples of 15 separate optical glasses with nd ≥ 1.6. There is a variation in anomalous count rates of orders of magnitude range over these materials. Care should be taken in the selection of optical glasses to be located in close proximity to detectors.
OCTOCAM is the new large Gemini instrument in building. It is an imaging spectrograph with 8 cameras covering the range 370 nm to 2350 nm at a typical resolution of 3000-4000. It will have 2 IFUs, one for normal operation over all wavelengths, the other for AO in the NIR only and with a smaller field but a higher spectral resolution. Currently, no IFU exists that covers the entire range of VIS and NIR in a single observation. Such an IFU would have a number of applications: It can be used for resolved studies of HII regions over a broad wavelength range and emission line galaxies over a broad redshift range using the same set of emission lines. Another application is to observe transients with only arcseconds localization very early without waiting for a sub-arcsecond position, hence allowing to obtain very valuable early data. For bright transients such as SNe and GRBs we can study the immediate environment in detail, and even use the actual transient as AO tip-tilt star to study the environment at high spectral and very high angular resolutions. The IFUs will be Advanced Image Slicers, a proven concept now in use in many instruments around the world including Gemini NIFS, VLT MUSE and KMOS, and JWST NIRSpec. The normal operation slicer will have a field of 9.7" x 6.8" with 17 slices 0.4" wide giving 0.18" x 0.4" spaxels. The slices are smaller than the standard slit size of 0.54" (3 pixels) so will deliver higher spectral resolution. This IFU will deliver much higher performances than the GMOS IFU and NIFS with a larger field of view and spectral range but also considerably fewer pixels per arsec2 then reducing the readout noise. With its wavelength range starting at 370 nm, diamond machining cannot be used. A glass slicer system will have to be used as in MUSE. The wavelength range will however be much larger covering the whole VIS and NIR range. Modern reflection coatings as UV enhanced silver can be used but a trade-off may be better by starting at a longer wavelength to get higher transmission. Special consideration is necessary for the fore-optics which cannot be diamond machined and for the overall design due to the limited space envelope. The AO slicer will have a field of 2.5" x 3.6" with 31 slices 0.08" wide imaged on 2 pixels in the spectral direction to get proper sampling. The fore-optics will magnify the beam in both directions but with different magnifications to get spaxels of 0.08" x 0.08". The smaller slice image width will give a spectral resolution of about 5000 including aberrations, about the same than NIFS but covering all 4 NIR bands at once. This slicer uses a slit 60% longer than OCTOCAM is designed for. It is possible because the magnification reduces the beam size so the aberrations and vignetting.
GHOST is a high resolution spectrograph system currently being built for the Gemini South Observatory in Chile. In the Cassegrain unit, the observational targets are acquired on integral field units and guided during science exposures, feeding the fiber cable to the temperature-stabilized echelle spectrograph. The Cassegrain unit is mounted on the Gemini telescope, and consists of a main structural plate, the two object positioners and ballast frame. The image from each of the two science beams passes through a field lens and a mini-atmospheric dispersion corrector and is then captured by the integral field unit. The positioner moves each corrector-integral field unit assembly across the focal surface of the telescope. The main structural plate provides the interface for the positioner and ballast frame to the telescope structure. In this paper we describe the final design and assembly of the GHOST Cassegrain unit and report on the outcome of on-sky testing at the telescope in Chile.
In this paper we present the Australian Astronomical Observatory’s concept design for Sphinx - a fiber positioner with 4,332 “spines” on a 7.77mm pitch for CFHT’s Mauna Kea Spectroscopic Explorer (MSE) Telescope. Based on the Echidna technology used with FMOS (on Subaru) and 4MOST (on VISTA), the next evolution of the tilting spine design delivers improved performance and superior allocation efficiency. Several prototypes have been constructed that demonstrate the suitability of the new design for MSE. Results of prototype testing are presented, along with an analysis of the impact of tilting spines on the overall survey efficiency. The Sphinx fiber positioner utilizes a novel metrology system for spine position feedback. The metrology design and the careful considerations required to achieve reliable, high accuracy measurements of all fibers in a realistic telescope environment are also presented.
The Gemini High-Resolution Optical SpecTrograph (GHOST) is the newest instrument chosen for the Gemini South telescope. It is being developed by a collaboration between the Australian Astronomical Observatory (AAO), the NRC - Herzberg in Canada and the Australian National University (ANU). Using recent technological advances and several novel concepts it will deliver spectroscopy with R>50,000 for up to 2 objects simultaneously or R>75,000 for a single object. GHOST uses a fiber-image-slicer to allow use of a much smaller spectrograph than that nominally required by the resolution-slit–width product. With its fiber feed, we expect GHOST to have a sensitivity in the wavelength range between 363-950 nm that equals or exceeds that of similar directly-fed instruments on world-class facilities. GHOST has entered the build phase. We report the status of the instrument and describe the technical advances and the novel aspects, such as the lenslet-based slit reformatting. Finally, we describe the unique scientific role this instrument will have in an international context, from exoplanets through stellar elemental abundances to the distant Universe. Keywords: Gemini, spectrograph, spectroscopy, ́echelle, high resolution, radial velocity, fiber image slicer, integral field unit.
The High Efficiency and Resolution Multi Element Spectrograph, HERMES is a facility-class optical spectrograph for the AAT. It is designed primarily for Galactic Archeology, the first major attempt to create a detailed understanding of galaxy formation and evolution by studying the history of our own galaxy, the Milky Way. The goal of the Galactic Archeology with Hermes (GALAH) survey is to reconstruct the mass assembly history of the Milky Way, through a detailed spatially tagged abundance study of one million stars. The spectrograph is based at the Anglo Australian Telescope (AAT) and is fed by the existing 2dF robotic fiber positioning system. The spectrograph uses VPH-gratings to achieve a spectral resolving power of 28,000 in standard mode and also provides a high-resolution mode ranging between 40,000 to 50,000 using a slit mask. The GALAH survey requires a SNR greater than 100 for a star brightness of V=14. The total spectral coverage of the four channels is about 100nm between 370 and 1000nm for up to 392 simultaneous targets within the 2- degree field of view. Hermes was commissioned in late 2013, with the GALAH Pilot starting in parallel with the commissioning. The GALAH survey started in early 2014 is currently about 33% complete. We present a description of the motivating science; an overview the instrument; and a status report on GALAH Survey.
ULTIMATE is an instrument concept under development at the AAO, for the Subaru Telescope, which will have the unique combination of ground layer adaptive optics feeding multiple deployable integral field units. This will allow ULTIMATE to probe unexplored parameter space, enabling science cases such as the evolution of galaxies at z ~ 0:5 to 1.5, and the dark matter content of the inner part of our Galaxy. ULTIMATE will use Starbugs to position between 7 and 13 IFUs over a 14 × 8 arcmin field-of-view, pro- vided by a new wide-field corrector. All Starbugs can be positioned simultaneously, to an accuracy of better than 5 milli-arcsec within the typical slew-time of the telescope, allowing for very efficient re-configuration between observations. The IFUs will feed either the near-infrared nuMOIRCS or the visible/ near-infrared PFS spectrographs, or both. Future possible upgrades include the possibility of purpose built spectrographs and incorporating OH suppression using fibre Bragg gratings. We describe the science case and resulting design requirements, the baseline instrument concept, and the expected performance of the instrument.
Gemini High-Resolution Optical SpecTrograph (GHOST) is a fiber-fed spectrograph being developed for the Gemini telescope. GHOST is a white pupil échelle spectrograph with high efficiency and a broad continuous wavelength coverage (363-1000nm) with R>50,000 in two-object mode and >75,000 in single-object mode. The design incorporates a novel zero-Petzval sum white pupil relay to eliminate grating aberrations at the cross-dispersers. Cameras are based on non-achromatic designs with tilted detectors to eliminate the need for exotic glasses. This paper outlines the optical design of the bench-mounted spectrograph and the predicted spectrograph resolution and efficiency for the spectrograph.
The High Efficiency and Resolution Multi Element Spectrograph, HERMES, is a facility-class optical spectrograph for the Anglo-Australian Telescope (AAT). It is designed primarily for Galactic Archaeology, the first major attempt to create a detailed understanding of galaxy formation and evolution by studying the history of our own galaxy, the Milky Way. The goal of the GALAH survey is to reconstruct the mass assembly history of the Milky Way through a detailed chemical abundance study of one million stars. The spectrograph is based at the AAT and is fed by the existing 2dF robotic fiber positioning system. The spectrograph uses volume phase holographic gratings to achieve a spectral resolving power of 28,000 in standard mode and also provides a high-resolution mode ranging between 40,000 and 50,000 using a slit mask. The GALAH survey requires an SNR greater than 100 for a star brightness of V=14 in an exposure time of one hour. The total spectral coverage of the four channels is about 100 nm between 370 and 1000 nm for up to 392 simultaneous targets within the 2-degree field of view. HERMES has been commissioned over three runs, during bright time in October, November, and December 2013, in parallel with the beginning of the GALAH pilot survey, which started in November 2013. We present the first-light results from the commissioning run and the beginning of the GALAH survey, including performance results such as throughput and resolution, as well as instrument reliability.
We present advances in the patented Echidna 'tilting spine' fiber positioner technology that has been in operation since 2007 on the SUBARU telescope in the FMOS system. The new Echidna technology is proposed to be implemented on two large fiber surveys: the Dark Energy Spectroscopic Instrument (DESI) (5000 fibers) as well the Australian ESO Positioner (AESOP) for 4MOST, a spectroscopic survey instrument for the VISTA telescope (~2500 fibers). The new 'superspine' actuators are stiffer, longer and more accurate than their predecessors. They have been prototyped at AAO, demonstrating reconfiguration times of ~15s for errors of <5 microns RMS. Laboratory testing of the prortotype shows accurate operation at temperatures of -10 to +30C, with an average heat output of 200 microwatts per actuator during reconfiguration. Throughput comparisons to other positioner types are presented, and we find that losses due to tilt will in general be outweighed by increased allocation yield and reduced fiber stress FRD. The losses from spine tilt are compensated by the gain in allocation yield coming from the greater patrol area, and quantified elsewhere in these proceedings. For typical tilts, f-ratios and collimator overspeeds, Echidna offers a clear efficiency gain versus current r-that or theta-phi positioners.
4MOST is a wide-field, high-multiplex spectroscopic survey facility under development for the VISTA telescope of the European Southern Observatory (ESO). Its main science drivers are in the fields of galactic archeology, high-energy physics, galaxy evolution and cosmology. 4MOST will in particular provide the spectroscopic complements to the large
area surveys coming from space missions like Gaia, eROSITA, Euclid, and PLATO and from ground-based facilities like VISTA, VST, DES, LSST and SKA. The 4MOST baseline concept features a 2.5 degree diameter field-of-view with ~2400 fibres in the focal surface that are configured by a fibre positioner based on the tilting spine principle. The fibres feed two types of spectrographs; ~1600 fibres go to two spectrographs with resolution R<5000 (λ~390-930 nm) and
~800 fibres to a spectrograph with R>18,000 (λ~392-437 nm and 515-572 nm and 605-675 nm). Both types of spectrographs are fixed-configuration, three-channel spectrographs. 4MOST will have an unique operations concept in which 5 year public surveys from both the consortium and the ESO community will be combined and observed in parallel during each exposure, resulting in more than 25 million spectra of targets spread over a large fraction of the
southern sky. The 4MOST Facility Simulator (4FS) was developed to demonstrate the feasibility of this observing
concept. 4MOST has been accepted for implementation by ESO with operations expected to start by the end of 2020.
This paper provides a top-level overview of the 4MOST facility, while other papers in these proceedings provide more
detailed descriptions of the instrument concept, the instrument requirements development, the systems engineering implementation, the instrument model, the fibre positioner concepts, the fibre feed, and the spectrographs.
The 4MOST instrument is a concept for a wide-field, fibre-fed high multiplex spectroscopic instrument facility on the
ESO VISTA telescope designed to perform a massive (initially >25x106 spectra in 5 years) combined all-sky public
survey. The main science drivers are: Gaia follow up of chemo-dynamical structure of the Milky Way, stellar radial
velocities, parameters and abundances, chemical tagging; eROSITA follow up of cosmology with x-ray clusters of
galaxies, X-ray AGN/galaxy evolution to z~5, Galactic X-ray sources and resolving the Galactic edge;
Euclid/LSST/SKA and other survey follow up of Dark Energy, Galaxy evolution and transients. The surveys will be
undertaken simultaneously requiring: highly advanced targeting and scheduling software, also comprehensive data
reduction and analysis tools to produce high-level data products. The instrument will allow simultaneous observations of
~1600 targets at R~5,000 from 390-900nm and ~800 targets at R<18,000 in three channels between ~395-675nm
(channel bandwidth: 45nm blue, 57nm green and 69nm red) over a hexagonal field of view of ~ 4.1 degrees. The initial
5-year 4MOST survey is currently expect to start in 2020. We provide and overview of the 4MOST systems: optomechanical,
control, data management and operations concepts; and initial performance estimates.
The new HERMES spectrograph represents the first foray by AAO into the use of commercial off-the-shelf industrial field bus technology for instrument control, and we regard the final system, with its relatively simple wiring requirements, as a great success. However, both software and hardware teams had to work together to solve a number of problems integrating the chosen CANopen/CAN bus system into our normal observing systems. A Linux system running in an industrial PC chassis ran the HERMES control software, using a PCI CAN bus interface connected to a number of distributed CANopen/CAN bus I/O devices and servo amplifiers. In the main, the servo amplifiers performed impressively, although some experimentation with homing algorithms was required, and we hit a significant hurdle when we discovered that we needed to disable some of the encoders used during observations; we learned a lot about how servo amplifiers respond when their encoders are turned off, and about how encoders react to losing power. The software was based around a commercial CANopen library from Copley Controls. Early worries about how this heavily multithreaded library would work with our standard data acquisition system led to the development of a very low-level CANopen software simulator to verify the design. This also enabled the software group to develop and test almost all the control software well in advance of the construction of the hardware. In the end, the instrument went from initial installation at the telescope to successful commissioning remarkably smoothly.
The High Efficiency and Resolution Multi Element Spectrograph, HERMES, was an approximately $12 million dollar project to provide a new facility class instrument for the Anglo-Australian Telescope (AAT). It was commissioned in Q4 2013. This paper examines how software challenges presented by HERMES were handled, including: minimizing cost through reusing the existing AAT 2dF/AAOmega facility software as far as possible; using instrument and data simulators to ensure new software was almost ready before any hardware had been seen; extensive upgrading of our fiber data reduction software; dealing with the tighter calibration and alignment tolerances of a high-resolution spectrograph.
The Robert Stobie Spectrograph Near Infrared Instrument (RSS-NIR), a prime focus facility instrument for the 11-meter
Southern African Large Telescope (SALT), is well into its laboratory integration and testing phase. RSS-NIR will
initially provide imaging and single or multi-object medium resolution spectroscopy in an 8 arcmin field of view at
wavelengths of 0.9 - 1.7 μm. Future modes, including tunable Fabry-Perot spectral imaging and polarimetry, have been
designed in and can be easily added later. RSS-NIR will mate to the existing visible wavelength RSS-VIS via a dichroic
beamsplitter, allowing simultaneous operation of the two instruments in all modes. Multi-object spectroscopy covering a
wavelength range of 0.32 - 1.7 μm on 10-meter class telescopes is a rare capability and once all the existing VIS modes
are incorporated into the NIR, the combined RSS will provide observational modes that are completely unique.
The VIS and NIR instruments share a common telescope focal plane, and slit mask for spectroscopic modes, and
collimator optics that operate at ambient observatory temperature. Beyond the dichroic beamsplitter, RSS-NIR is
enclosed in a pre-dewar box operating at -40 °C, and within that is a cryogenic dewar operating at 120 K housing the
detector and final camera optics and filters. This semi-warm configuration with compartments at multiple operating
temperatures poses a number of design and implementation challenges. In this paper we present overviews of the RSSNIR
instrument design and solutions to design challenges, measured performance of optical components, detector
system optimization results, and an update on the overall project status.
The High Efficiency and Resolution Multi Element Spectrograph, HERMES is an facility-class optical spectrograph for
the AAT. It is designed primarily for Galactic Archeology , the first major attempt to create a detailed
understanding of galaxy formation and evolution by studying the history of our own galaxy, the Milky Way. The goal of
the GALAH survey is to reconstruct the mass assembly history of the of the Milky Way, through a detailed spatially
tagged abundance study of one million stars. The spectrograph is based at the Anglo Australian Telescope (AAT) and is
fed by the existing 2dF robotic fiber positioning system. The spectrograph uses VPH-gratings to achieve a spectral
resolving power of 28,000 in standard mode and also provides a high-resolution mode ranging between 40,000 to 50,000
using a slit mask. The GALAH survey requires a SNR greater than 100 for a star brightness of V=14. The total spectral
coverage of the four channels is about 100nm between 370 and 1000nm for up to 392 simultaneous targets within the 2
degree field of view. Hermes has been commissioned over 3 runs, during bright time in October, November and
December 2013, in parallel with the beginning of the GALAH Pilot survey starting in November 2013. In this paper we
present the first-light results from the commissioning run and the beginning of the GALAH Survey, including
performance results such as throughput and resolution, as well as instrument reliability. We compare the abundance
calculations from the pilot survey to those in the literature.
The Gemini High-Resolution Optical SpecTrograph (GHOST) is the newest instrument being developed for the Gemini telescopes, in a collaboration between the Australian Astronomical Observatory (AAO), the NRC - Herzberg in Canada and the Australian National University (ANU). We describe the process of design optimisation that utilizes the unique strengths of the new partner, NRC - Herzberg, the design and need for the slit viewing camera system, and we describe a simplification for the lenslet-based slit reformatting. Finally, we out- line the updated project plan, and describe the unique scientific role this instrument will have in an international context, from exoplanets through to the distant Universe.
The High Efficiency and Resolution Multi Element Spectrograph, HERMES is an optical spectrograph designed
primarily for the GALAH, Galactic Archeology Survey, the first major attempt to create a detailed understanding of
galaxy formation and evolution by studying the history of our own galaxy, the Milky Way1. The goal of the GALAH
survey is to reconstruct the mass assembly history of the of the Milky way, through a detailed spatially tagged
abundance study of one million stars in the Milky Way. The spectrograph will be based at the Anglo Australian
Telescope (AAT) and be fed with the existing 2dF robotic fibre positioning system. The spectrograph uses VPH-gratings
to achieve a spectral resolving power of 28,000 in standard mode and also provides a high resolution mode ranging
between 40,000 to 50,000 using a slit mask. The GALAH survey requires a SNR greater than 100 aiming for a star
brightness of V=14. The total spectral coverage of the four channels is about 100nm between 370 and 1000nm for up to
392 simultaneous targets within the 2 degree field of view.
Current efforts are focused on manufacturing and integration. The delivery date of spectrograph at the telescope is
scheduled for 2013. A performance prediction is presented and a complete overview of the status of the HERMES
spectrograph is given. This paper details the following specific topics:
The approach to AIT, the manufacturing and integration of the large mechanical frame, the opto-mechanical slit
assembly, collimator optics and cameras, VPH gratings, cryostats, fibre cable assembly, instrument control hardware and
software, data reduction.
We report on the status of the detector system for the Robert Stobie Spectrograph Near Infrared Arm (RSS-NIR) for the
Southern African Large Telescope (SALT). The detector is a HAWAII-2RG array with a 1.7 μm cutoff wavelength.
The controller incorporates a Teledyne cryogenic SIDECAR ASIC board inside the dewar and an FPGA interface card,
developed by the Inter-University Centre for Astronomy and Astrophysics (IUCAA), outside the dewar. Data
acquisition software written by IUCAA runs under a Linux operating system and communicates to the detector system
through USB to fiber optic converters for electrical isolation on the telescope. System characterization is performed at
the University of Wisconsin RSS-NIR Lab in a liquid nitrogen cooled test dewar. The test dewar contains a thermal
control system that emulates operation of the cryocooler used in the instrument dewar and maintains a stable detector
operating temperature of 120 K. Light is provided to the detector with near infrared LEDs mounted inside the dewar.
We present preliminary data on system noise and plans for further characterization tests.
We describe the design, construction, and expected performance of two new fiber integral field units (IFUs) -
HexPak and GradPak - for the WIYN 3.5m Telescope Nasmyth focus and Bench Spectrograph. These are the
first IFUs to provide formatted fiber integral field spectroscopy with simultaneous sampling of varying angular
scales. HexPak and GradPak are in a single cable with a dual-head design, permitting easy switching between
the two different IFU heads on the telescope without changing the spectrograph feed: the two heads feed a
variable-width double-slit. Each IFU head is comprised of a fixed arrangement of fibers with a range of fiber
diameters. The layout and diameters of the fibers within each array are scientifically-driven for observations
of galaxies: HexPak is designed to observe face-on spiral or spheroidal galaxies while GradPak is optimized
for edge-on studies of galaxy disks. HexPak is a hexagonal array of 2.9 arcsec fibers subtending a 40.9 arcsec
diameter, with a high-resolution circular core of 0.94 arcsec fibers subtending 6 arcsec diameter. GradPak is a
39 by 55 arcsec rectangular array with rows of fibers of increasing diameter from angular scales of 1.9 arcsec to
5.6 arcsec across the array. The variable pitch of these IFU heads allows for adequate sampling of light profile
gradients while maintaining the photon limit at different scales.
The Gemini High-Resolution Optical SpecTrograph (GHOST) will fill an important gap in the current suite of Gemini
instruments. We will describe the Australian Astronomical Observatory (AAO)-led concept for GHOST, which consists
of a multi-object, compact, high-efficiency, fixed-format, fiber-fed design. The spectrograph itself is a four-arm variant
of the asymmetric white-pupil echelle Kiwispec spectrograph, Kiwisped, produced by Industrial Research Ltd. This
spectrograph has an R4 grating and a 100mm pupil, and separate cross-disperser and camera optics for each of the four
arms, carefully optimized for their respective wavelength ranges. We feed this spectrograph with a miniature lensletbased
IFU that sub-samples the seeing disk of a single object into 7 hexagonal sub-images, reformatting this into a slit
with a second set of double microlenses at the spectrograph entrance with relatively little loss due to focal-ratio
degradation. This reformatting enables high spectral resolution from a compact design that fits well within the relatively
tight GHOST budget. We will describe our baseline 2-object R~50,000 design with full wavelength coverage from the
ultraviolet to the silicon cutoff, as well as the high-resolution single-object R~75,000 mode.
Hyperspectral imaging has important benefits in remote sensing and target discrimination applications. This paper
describes a class of snapshot-mode hyperspectral imaging systems which utilize a unique optical processor that provides
video-rate hyperspectral datacubes. This system consists of numerous parallel optical paths which collect the full threedimensional
(two spatial, one spectral) hyperspectral datacube with each video frame and are ideal for recording data
from transient events, or on unstable platforms.
We will present the results of laboratory and field-tests for several of these imagers operating at visible, near-infrared,
MWIR and LWIR wavelengths. Measurement results for nitrate detection and identification as well as additional
chemical identification and analysis will be presented.
The Robert Stobie Spectrograph Near Infrared (RSS/NIR) upgrade for the Southern African Large Telescope (SALT)
extends the capabilities of the visible arm of RSS into the NIR. The RSS/NIR instrument is at the prime focus of SALT.
It is a versatile spectrograph with broadband imaging, spectropolarimetric, and Fabry-Perot imaging capabilities. The
multiple modes and prime focus location introduce interesting engineering considerations. The spectrograph has an
ambient temperature collimator, cooled (-40ºC) dispersers and camera and a cryogenic detector. Many of the
mechanisms are required to operate within the cooled and cryogenic environments. The RSS/ NIR upgrade includes the
following mechanisms; an active flexure compensating fold mirror, a filter exchange mechanism, a Volume Phase
Holographic VPH grating exchange and rotation mechanism, an etalon inserter, a beam splitter inserter, an articulating
camera, internal camera focus and a cutoff filter exchange wheel. This paper gives an overview of the mechanical design
and focuses on some of the unique testing and prototyping tasks.
The Robert Stobie Spectrograph near infrared arm will provide high throughput, low to medium resolution long slit and
multi-object spectroscopy with broadband, spectropolarimetric, and Fabry-Perot imaging modes over a 8' diameter field
of view. The wavelength range of the instrument is 0.9-1.7 microns, and can be operated simultaneously with the visible
arm to extend the short wavelength limit to 0.32 microns. Once fielded, RSS-NIR will be the only facility instrument on
an 8-10 meter class telescope with multi-object spectroscopy capability covering this spectral range simultaneously.
RSS-NIR is scheduled to be commissioned on the 11-meter Southern African Large Telescope in late 2012. This is an
upgrade to the existing visible instrument, with which it shares the slit plane and an ambient temperature collimator.
Beyond the collimator, the NIR arm is cooled to -40 °C, with a cryogenic dewar containing the detector, long
wavelength blocking filters, and final camera optics. This semi-warm configuration has required extensive upfront
analysis of the instrumental thermal background levels, which have been incorporated into the instrument performance
simulator. We present the performance predictions for spectroscopic modes of RSS-NIR and preliminary performance
estimates and NIR issues still being addressed in the design for Fabry-Perot and polarimetric modes.
We report on the detector testing status for the Robert Stobie Spectrograph's near-infrared arm. The instrument utilizes a
Teledyne HAWAII-2RG HgCdTe detector array with a 1.7 μm cutoff wavelength. We have selected an operating
temperature of 120 K. The characterization effort will take place in our detector-testing laboratory at the University of
Wisconsin-Madison. The laboratory is equipped with a test dewar, vacuum system, temperature controller,
monochromator, and warm detector test enclosure. We will measure detector performance characteristics such as readout
noise, gain, dark current, linearity, quantum efficiency, and persistence, and develop calibration strategies. Persistence
could have a substantial impact on the spectrograph's science data, and therefore, the development of mitigation
techniques for this effect will be emphasized.
The Robert Stobie Spectrograph Near Infrared Arm (RSS-NIR) is a new instrument on the 11-meter Southern African
Large Telescope (SALT), scheduled to begin commissioning in 2012. This versatile instrument will add capabilities that
are unique to large telescopes. The main instrument modes include NIR imaging, medium resolution long slit
spectroscopy over an 8 arcminute field of view (FOV), multi-object spectroscopy with custom slit masks over an 8x8
arcminute FOV, Fabry-Perot narrowband imaging over an 8 arcminute diameter FOV, and polarimetry and
spectropolarimetry over a 4x8 arcminute FOV. Limiting magnitude predictions are 21.1 and 20.1 for J and H band for
S/N = 10 per spectral resolution element in 1 hour for 1 arcsec2at an R=7000. All instrument modes can be operated
simultaneously with the RSS visible arm, providing spectral coverage from 0.32-1.7 microns. We list the science drivers
and describe the way in which they have guided the design for this instrument. We also present a more detailed
description of some several planned science programs that will take advantage of the unique capabilities of RSS-VISNIR
and the queue-scheduled SALT telescope. Lastly we give a brief description of predicted instrumental
performance, along with a comparison to several other NIR instruments at other observatories.
Hyperspectral imaging has important benefits in remote sensing and material identification.
This paper describes a class of hyperspectral imaging systems which utilize a novel optical
processor that provides video-rate hyperspectral datacubes. These systems have no moving
parts and do not operate by scanning in either the spatial or spectral dimension. They are
capable of recording a full three-dimensional (two spatial, one spectral) hyperspectral datacube
with each video frame, ideal for recording data on transient events, or from unstabilized
platforms. We will present the results of laboratory and field-tests for several of these imagers
operating in the visible, near-infrared, mid-wavelength infrared (MWIR) and long-wavelength
infrared (LWIR) regions.
The Southern African Large Telescope is nearing the end of its commissioning phase and scientific performance
verification programmes began in 2006 with two of its First Generation UV-visible instruments, the imaging camera,
SALTICAM, and the multi-mode Robert Stobie Spectrograph (RSS). Both instruments are seeing limited and designed to
operate in the UV-visible region (320 - 900 nm). This paper reviews the innovative aspects of the designs of these
instruments and discusses the commissioning experience to date, illustrated by some initial scientific commissioning
results. These include long-slit and multi-object spectroscopy, spectropolarimetry, Fabry-Perot imaging spectroscopy and
high-speed photometry. Early spectroscopic commissioning results uncovered a serious underperformance in the
throughput of RSS, particularly at wavelengths < 400nm. We discuss the lengthy diagnosis and eventual removal of this
problem, which was traced to a material incompatibility issue involving index-matching optical coupling fluid. Finally,
we briefly discuss the present status of the third and final First Generation instrument, a vacuum enclosed fibre-fed high
resolution, dual beam, white pupil echelle spectrograph, SALT HRS, currently under construction.
In traditional seeing-limited observations the spectrograph aperture scales with telescope aperture, driving sizes
and costs to enormous proportions. We propose a new solution to the seeing-limited spectrograph problem. A
massively fiber-sliced congfiguration feeds a set of small diffraction-limited spectrographs. We present a prototype,
tunable, J-band, diffraction grating, designed specifically for Astronomical applications: The grating sits at the
heart of a spectrograph, no bigger than a few inches on a side. Throughput requirements dictate using tens-of-thousands
of spectrographs on a single 10 to 30 meter telescope. A full system would cost significantly less than
typical instruments on 10m or 30m telescopes.
We present a cost-effective solution for adaptive optics (AO) correction on the Southern African Large Telescope
(SALT), where each primary mirror segment has compensation for tip-tilt atmospheric errors and a slower, active
optic loop for sensing piston and correcting for focus drift. By treating the telescope as 91 independent tip-tilt
corrected units, we compute the encircled energy gains for different seeing conditions at the SALT. Finally,
the optical design for a simple AO demonstrator camera is presented, using seven tip-tilt correctors to directly
measure and compare closed loop and open loop performances, which will help lead a full SALT AO system
The University of Wisconsin Astronomy Department and the Space Astronomy Lab at UW are designing
an SHS spectrometer for the WIYN 3.5-meter telescope on Kitt Peak and the SALT 10-meter telescope in
South Africa. The new device will be mated to the Sparsepak, (Bershady et al, 2004, 2005) and/or the
Hydra fiber array at WIYN, and fed by either the prime focus image at SALT or the High Resolution
Spectrograph fiber-feed at SALT. The spectrograph will produce spectra at a reciprocal dispersion, R =
25,000 in 20 orders, each order covering an average wavelength band 250 km/s wide, for a total
wavelength range of 5000 km/s. Spectra from approximately 82 fibers will be resolved. Once the system is
proven at WIYN, and because the aperture size for this spectrometer does not scale with telescope size, we
will be able to test this same prototype at the SALT 10-meter telescope. This will be the first application of
this technique to large aperture astronomical observations.
The near infrared (NIR) upgrade to the Robert Stobie Spectrograph (RSS) on the Southern African Large Telescope
(SALT), RSS/NIR, extends the spectral coverage of all modes of the optical spectrograph. The RSS/NIR is a low to
medium resolution spectrograph with broadband, spectropolarimetric, and Fabry-Perot imaging capabilities. The optical
and NIR arms can be used simultaneously to extend spectral coverage from 3200 Å to approximately 1.6 μm. Both arms
utilize high efficiency volume phase holographic gratings via articulating gratings and cameras. The NIR camera
incorporates a HAWAII-2RG detector with an Epps optical design consisting of 6 spherical elements and providing subpixel
rms image sizes of 7.5 ± 1.0 μm over all wavelengths and field angles. The NIR spectrograph is semi-warm,
sharing a common slit plane and partial collimator with the optical arm. A pre-dewar, cooled to below ambient
temperature, houses the final NIR collimator optic, the grating/Fabry-Perot etalon, the polarizing beam splitter, and the
first three camera optics. The last three camera elements, blocking filters, and detector are housed in a cryogenically
cooled dewar. The semi-warm design concept has long been proposed as an economical way to extend optical
instruments into the NIR, however, success has been very limited. A major portion of our design effort entails a detailed
thermal analysis using non-sequential ray tracing to interactively guide the mechanical design and determine a truly
realizable long wavelength cutoff over which astronomical observations will be sky-limited. In this paper we describe
our thermal analysis, design concepts for the staged cooling scheme, and results to be incorporated into the overall
mechanical design and baffling.
We report on the design, development and commissioning of an Integral Field Unit (IFU) that has been built for the Echellette Spectrograph and Imager (ESI) at the W.M. Keck Observatory. This image slicer-based IFU, which was commissioned in the spring of 2004 covers a contiguous field of 5.65 x 4.0 arcseconds in 5 slices that are 1.13 arcseconds wide. The IFU passes a spectral range of 0.39-1.1 um with a throughput of between 45% and 60% depending on wavelength and field position. The IFU head resides in an ESI slit mask holder, so that ESI may be converted to the IFU mode remotely by selecting the appropriate slit mask position. This IFU is the first of a family of designs for the spectrograph, providing a range of field-coverages and dispersions.
We describe a new concept for a MEMS-based active spatial filter for astronomical spectroscopy. The goal of this device is to allow the use of a diffraction-limited spectrometer on a seeing limited observation at improved throughput over a comparable seeing-limited spectrometer, thus reducing the size and cost of the spectrometer by a factor proportional to r0/D (For the case of a 10 meter telescope this size reduction will be approximately a factor of 25 to 50). We use a fiber-based integral field unit (IFU) that incorporates an active MEMS mirror array to feed an astronomical spectrograph. A fast camera is used in parallel to sense speckle images at a spatial resolution of λ/D and at a temporal frequency greater than that of atmospheric fluctuations. The MEMS mirror-array is used as an active shutter to feed speckle images above a preset intensity threshold to the spectrometer, thereby increasing the signal-to-noise ratio (SNR) of the spectrogram. Preliminary calculations suggests an SNR improvement of a factor of about 1.4. Computer simulations have shown an SNR improvement of 1.1, but have not yet fully explored the parameter space.
The near infrared (NIR) upgrade to the Robert Stobie Spectrograph (RSS) on the Southern African Large Telescope (SALT), RSS/NIR, extends the spectral coverage of all modes of the visible arm. The RSS/NIR is a low to medium resolution spectrograph with broadband imaging, spectropolarimetric, and Fabry-Perot imaging capabilities. The visible and NIR arms can be used simultaneously to extend spectral coverage from approximately 3200 Å to 1.6 μm. Both arms utilize high efficiency volume phase holographic gratings via articulating gratings and cameras. The NIR camera is designed around a 2048x2048 HAWAII-2RG detector housed in a cryogenic dewar. The Epps optical design of the camera consists of 6 spherical elements, providing sub-pixel rms image sizes of 7.5 ± 1.0 μm over all wavelengths and field angles. The exact long wavelength cutoff is yet to be determined in a detailed thermal analysis and will depend on the semi-warm instrument cooling scheme. Initial estimates place instrument limiting magnitudes at J = 23.4 and H(1.4-1.6 μm) = 21.6 for S/N = 3 in a 1 hour exposure well below the sky noise.
The objective of this project is to seamlessly integrate multiple spectral-band focal plane detector arrays into a single multi-band imaging sensor. The resulting product can be applied to a telescope or a microscope, as simply as changing a lens on a camera. The video stream output provides a set of co-registered digital images from a multiple of spectral bands spanning the Visible, NIR, MWIR and LWIR radiation regions (.4um to 14um). These images have a format that is suitable both for direct observation by a human operator and as a data feed for Automated Target Recognition (ATR) algorithms.
"Extreme" adaptive optics systems are optimized for ultra-high-contrast applications, such as ground-based extrasolar planet detection. The Extreme Adaptive Optics Testbed at UC Santa Cruz is being used to investigate and develop technologies for high-contrast imaging, especially wavefront control. A simple optical design allows us to minimize wavefront error and maximize the experimentally achievable contrast before progressing to a more complex set-up. A phase shifting diffraction interferometer is used to measure wavefront errors with sub-nm precision and accuracy. We have demonstrated RMS wavefront errors of <1.3 nm and a contrast of >10-7 over a substantial region using a shaped pupil. Current work includes the installation and characterization of a 1024-actuator Micro-Electro-Mechanical-Systems (MEMS) deformable mirror, manufactured by Boston Micro-Machines, which will be used for wavefront control. In our initial experiments we can flatten the deformable mirror to 1.8-nm RMS wavefront error within a control radius of 5-13 cycles per aperture. Ultimately this testbed will be used to test all aspects of the system architecture for an extrasolar planet-finding AO system.
High dynamic range coronagraphy targeted at discovering planets around nearby stars is often associated with monolithic, unobstructed aperture space telescopes. With the advent of extreme adaptive optics (ExAO) systems with thousands of sensing and correcting channels, the benefits of placing a near-infrared coronagraph on a large segmented mirror telescope become scientifically interesting. This is because increased aperture size produces a tremendous gain in achievable contrast at the same angular distance from a point source at Strehl ratios in excess of 90\% (and at lower Strehl ratios on future giant telescopes such as the Thirty Meter Telescope). We outline some of the design issues facing such a coronagraph, and model a band-limited coronagraph on an aperture with a Keck-like pupil. We examine the purely diffractive challenges facing the eXtreme AO Planetary Imager (XAOPI) given the Keck pupil geometry, notably its inter-segment gap spacing of 6~mm.
Classical Lyot coronagraphs, with hard-edged occulting stops, are not efficient enough at suppressing diffracted light, given XAOPI's scientific goal of imaging a young Jupiter at a separation as close as 0.15 arcseconds (4λD at H on Keck) from its parent star. With a 4000 channel ExAO system using an anti-aliased spatially-filtered wavefront sensor planned for XAOPI, we wish to keep diffracted light due to coronagraphic design at least as low as the noise floor set by AO system limitations. We study the band-limited Lyot coronagraph (BLC) as a baseline design instead of the classical design because of its efficient light suppression, as well as its analytical simplicity. We also develop ways of investigating tolerancing coronagraphic mask fabrication by utilizing the BLC design's mathematical tractability.
As adaptive optics (AO) matures, it becomes possible to envision AO systems oriented towards specific important scientific goals rather than general-purpose systems. One such goal for the next decade is the direct imaging detection of extrasolar planets. An "extreme" adaptive optics (ExAO) system optimized for extrasolar planet detection will have very high actuator counts and rapid update rates - designed for observations of bright stars - and will require exquisite internal calibration at the nanometer level. In addition to extrasolar planet detection, such a system will be capable of characterizing dust disks around young or mature stars, outflows from evolved stars, and high Strehl ratio imaging even at visible wavelengths. The NSF Center for Adaptive Optics has carried out a detailed conceptual design study for such an instrument, dubbed the eXtreme Adaptive Optics Planet Imager or XAOPI. XAOPI is a 4096-actuator AO system, notionally for the Keck telescope, capable of achieving contrast ratios >107 at angular separations of 0.2-1". ExAO system performance analysis is quite different than conventional AO systems - the spatial and temporal frequency content of wavefront error sources is as critical as their magnitude. We present here an overview of the XAOPI project, and an error budget highlighting the key areas determining achievable contrast. The most challenging requirement is for residual static errors to be less than 2 nm over the controlled range of spatial frequencies. If this can be achieved, direct imaging of extrasolar planets will be feasible within this decade.
Ground based adaptive optics is a potentially powerful technique for direct imaging detection of extrasolar planets. Turbulence in the Earth's atmosphere imposes some fundamental limits, but the large size of ground-based telescopes compared to spacecraft can work to mitigate this. We are carrying out a design study for a dedicated ultra-high-contrast system, the eXtreme Adaptive Optics Planet Imager (XAOPI), which could be deployed on an 8-10m telescope in 2007. With a 4096-actuator MEMS deformable mirror it should achieve Strehl >0.9 in the near-IR. Using an innovative spatially filtered wavefront sensor, the system will be optimized to control scattered light over a large radius and suppress artifacts caused by static errors. We predict that it will achieve contrast levels of 107-108 at angular separations of 0.2-0.8" around a large sample of stars (R<7-10), sufficient to detect Jupiter-like planets through their near-IR emission over a wide range of ages and masses. We are constructing a high-contrast AO testbed to verify key concepts of our system, and present preliminary results here, showing an RMS wavefront error of <1.3 nm with a flat mirror.
We present the design and status report on the development of an Integral Field Unit (IFU) for the Echellette spectrograph and imager (ESI), a recently developed R=13000, Cassegrain spectrograph at Keck II. We have designed a family of IFU’s for the spectrograph, providing a range of field-coverages and dispersions. The optical designs are based on the Advanced Image Slicer concept of Content. We describe the completely monolithic, passive, and modular implementation of this design as an IFU head. Each IFU head resides in an ESI slit mask holder, so that it is completely selectable/deselectable as an observing mode during a nights observing run.
The Echelle Spectrograph and Imager (ESI) is a multipurpose instrument which has been delivered by the Instrument Development Laboratory of Lick Observatory for use at the Cassegrain focus of the Keck II telescope. ESI saw first light on August 29, 1999. The optical performance of the instrument has been measured using artificial calibration sources and starlight. Measurements of the average image FWHM in echelle mode are 22 microns, 16 to 18 microns in broad band imaging mode, and comparable in the low- dispersion prismatic mode. Images on the sky, under best seeing conditions show FWHM sizes of 34 microns. Maximum efficiencies are measured to be 30 percent for echelle and anticipated to be greater than 38 percent for low dispersion prismatic mode including atmospheric, telescope and detector losses. In this paper we describe the instrument and its specifications. We discuss the testing that led to the above conclusions.
All Cassegrain spectrographs suffer from gravitationally- induced flexure to some degree. While such flexure can be minimized via careful attention to mechanical design and fabrication, further performance improvements can be achieved if the spectrograph has been designed to minimize hysteresis and has active compensation for any residual flexure. The Echellette Spectrograph and Imager (ESI), built at UCO/Lick Observatory for use at Cassegrain focus on Keck II, compensates for such residual flexure via its collimator mirror. The collimator is driven by three actuators that provide control of collimator focus, tip, and tilt. The ESI control software utilizes a mathematical model of gravitationally-induced flexure to periodically compute and apply flexure corrections by commanding the corresponding tip and tilt motions to the collimator. In addition, the ESI control software provides an optional, manual, closed-loop method for adjusting the collimator position to compensate for any non-repeatable errors. Such errors may result from mechanical hysteresis or from thermally-induced structural deformations of the instrument and are thus not accounted for by the gravitational flexure model. This method relies on measuring the centroid position of fiducial spots within each echellete image. The collimator is adjusted so that the positions of these spots match those in a reference image. These spots are produced by a small round hole in the slit mask located near one end of the slit. We discuss the design and calibration of this flexure compensation system and report on its performance ont he telescope.
The Echellete Spectrograph and Imager (ESI), currently being completed for use at the cassegrain focus of the Keck II telescope, employs two moderate size translating fold mirrors. These mirrors are used to shift between the three instrument modes; medium resolution echellete mode; low resolution prismatic mode; and imaging mode. In order to maintain the optical stability and calibration of these three modes the mirrors must be removed and repeatably located to within 1.3 arcsecs of tip and tilt. In addition, the mirrors must maintain a fixed orientation relative to the telescope axis under a variety of gravity and thermal loads. In this paper we describe a novel concept for moving and locating these mirrors. Analytical analysis of the mounts is presented. Optical and mechanical testing is described.
The Echellette Spectrograph and Imager (ESI), currently being delivered for use at the Cassegrain focus of the Keck II telescope employs an all-spherical, 308 mm focal length f/1.07 Epps camera. The camera consists of 10 lens elements in 5 groups: an oil-coupled doublet; a singlet, an oil- coupled triplet; a grease-coupled triplet; and a field flattener, which also serves as the vacuum-dewar window. A sensitivity analysis suggested that mechanical manufacturing tolerances of order +/- 25 microns were appropriate. In this paper we discuss the sensitivity analysis, the assembly and the testing of this camera.
The Echellette Spectrograph and Imager (ESI) is being built at UCO/Lick Observatory for the Cassegrain focus of the Keck II telescope. The collimator mirror is optimally constrained by a space-frame structure. It will be actively moved to provide the focus and flexure (tip and tilt) control for the instrument. Careful attention to space-frame geometry has simplified the mechanical design. Analytical and Finite Element Analysis (FEA) are presented to demonstrate how a simple but very stiff structure is used to provide support, flexure control, and focus.
The Echellette Spectrograph and Imager (ESI), currently being developed for use at the Cassegrain focus of the Keck II 10-m telescope, employs two large (25 kg) prisms for cross dispersion. In order to maintain optical stability in the spectroscopic modes, these prisms must maintain a fixed angle relative to the nominal spectrograph optical axis under a variety of flexural and thermal loads. In this paper, we describe a novel concept for mounting large prisms that has been developed to address this issue. Analytical and finite element analyses (FEA) of the mounts are presented. Optical and mechanical tests are also described.
SC906: Introduction to Visible and NIR Spectrograph Design and Development for Astronomy
This course provides attendees with an introduction to aerial spectrograph design and development for astronomy. The course concentrates on system configurations and performance optimization and analysis. Specific concepts to be addressed include: image quality, throughput, flexure, performance modeling and system testing.