The Dark Energy Spectroscopic Instrument (DESI) is under construction to measure the expansion history of the Universe using the Baryon Acoustic Oscillation technique. The spectra of 35 million galaxies and quasars over 14000 sq. deg will be measured during the life of the experiment. A new prime focus corrector for the KPNO Mayall telescope will deliver light to 5000 fibre optic positioners. The fibres in turn feed ten broad-band spectrographs. We describe the design, production, quality assurance procedures and performance of the DESI slit assemblies.
With the recent development of new ultra fine aluminium alloys and progress in the field of directly machined freeform surfaces, diamond machined freeform gratings could play an important part in future spectrographs or integral field units, particularly at SWIR and LWIR wavelengths where the improved thermal performance of metal optics at cryogenic temperatures is well established. Freeform diamond machined gratings can offer a cost-effective, compact, and flexible alternative to gratings fabricated by other methods such as ion beam etching or complement these technologies. In this paper, both the advantages and limitations of 5 axis diamond machined freeform gratings are presented and potential applications are discussed.
Spectroscopy is a key technique in astronomy and nowadays most major telescopes include at least one spectrograph in their instrument suite. The dispersive element is one of the most important components and it defines the pupil size, spectral resolution and efficiency. Different types of dispersive elements have been developed including prisms, grisms, VPH and echelle gratings. In this paper, we investigate the design and optimization possibilities offered by metallic freeform gratings using diamond machining techniques. The incorporation of power in a diffraction grating enables several functionalities within the same optical component, such as the combination of dispersion, focusing and field reformat. The resulting benefit is a reduction in the number of surfaces and therefore, an improvement in the throughput. Freeform surfaces are also interesting for their enhanced optical performance by allowing extra degree of freedom in the optimization. These degrees of freedom include the shape of the substrate but also additional parameters such as the pitch or the number of blaze angle. Freeform gratings used as single optical component systems also present some limitations such as the trade-off between optical quality versus field of view or the spectral range versus spectral resolution. This paper discusses the possibility offered by the design of freeform gratings for low to medium spectral resolution, in the visible and near-infrared, for potential applications in ultra-compact integral field spectrographs.
A novel concept for the calibration of multi object fiber-fed spectrographs is described for the 4MOST instrument. The 4MOST facility is foreseen to start science operations in 2022 at the ESO VISTA telescope. The calibration system provides intensity, wavelength and resolution calibrations for the 4MOST spectrographs. The heart of the system is a combination of a bright broad band lamp and a Fabry-Perot etalon. The lamp is able to provide sufficient flux to illuminate the VISTA focal plane and the Fabry-Perot etalon provides a regular comb of spectral lines. The Fabry-Perot etalon can be moved in and out of the optical beam to choose between intensity and spectral calibrations. A fiber bundle of 156 fibers is guided to the VISTA spider arms where each fiber is connected to a small integrating sphere. The integrating spheres are attached to the bottom side of the four VISTA telescope spider struts and provide unvignetted illumination of the telescope. The exit port of the integrating spheres is projected on the VISTA focal plane with a small collimator lens. The integrating spheres assure a uniform illumination of the focal plane and are insensitive to FRD effects of the input fibers due to motion and stress during telescope movements. The calibration system illumination only originates from the telescope spiders and therefore the telescope pupil is not fully filled. The calibration system uses the azimuthal scrambling properties of the fibers that connect the telescope focal plane and the spectrometers to completely fill the spectrograph pupil.
The Dark Energy Spectroscopic Instrument (DESI) is under construction to measure the expansion history of the Universe using the Baryon Acoustic Oscillation technique. The spectra of 35 million galaxies and quasars over 14000 sq deg will be measured during the life of the experiment. A new prime focus corrector for the KPNO Mayall telescope will deliver light to 5000 fiber optic positioners. The fibers in turn feed ten broad-band spectrographs. We will describe the extensive preparations of the Mayall telescope and its environs for DESI, and will report on progress-to-date of the installation of DESI itself.
The Dark Energy Spectroscopic Instrument (DESI) is under construction to measure the expansion history of the Universe using the Baryon Acoustic Oscillation technique. The spectra of 35 million galaxies and quasars over 14000 sq deg will be measured during the life of the experiment. A new prime focus corrector for the KPNO Mayall telescope will deliver light to 5000 fiber optic positioners. The fibers in turn feed ten broad-band spectrographs. We will describe the design and performance of the DESI fiber system. This includes 5000 custom positioner fiber assemblies, spliced to 10 fiber cables terminated in a slit array.
The Dark Energy Spectroscopic Instrument (DESI) is under construction to measure the expansion history of the Universe using the Baryonic Acoustic Oscillation technique. The spectra of 35 million galaxies and quasars over 14000 sq deg will be measured during the life of the experiment. A new prime focus corrector for the KPNO Mayall telescope will deliver light to 5000 fibre optic positioners. The fibres in turn feed 10 broad-band spectrographs. We will describe the design and production progress on the fibre cables, strain relief system and preparation of the slit end. In contrast to former projects, the larger scale of production required for DESI requires teaming up with industry to find a solution to reduce the time scale of production as well as to minimise the stress on the optical fibres.
The Dark Energy Spectroscopic Instrument (DESI) is under construction to measure the expansion history of the Universe using the Baryon Acoustic Oscillation technique. The spectra of 35 million galaxies and quasars over 14000 square degrees will be measured during the life of the experiment. A new prime focus corrector for the KPNO Mayall telescope will deliver light to 5000 fiber optic positioners. The fibers in turn feed ten broad-band spectrographs. We present an overview of the instrumentation, the main technical requirements and challenges, and the current status of the project.
The Dark Energy Spectroscopic Instrument (DESI) is under construction to measure the expansion history of the Universe using the Baryon Acoustic Oscillation technique. The spectra of 40 million galaxies over 14000 sq deg will be measured during the life of the experiment. A new prime focus corrector for the KPNO Mayall telescope will deliver light to 5000 fiber optic positioners. The fibers in turn feed ten broad-band spectrographs. We will describe the Fiber Systems design with specific emphasis on novel approaches and essential elements that lead to exceptional performance.
The Dark Energy Survey Instrument (DESI) is a 5000-fibre optical multi object spectrograph for the 4m Mayall telecope at the Kitt Peak National Observatory. Ten identical three channel spectrographs will be equipped with 500-element fibre slits. Here we focus on the architecture of the science slits and the interchangeable auxiliary slits required for calibration.
Euclid is a medium class European Space Agency mission scheduled for launch in 2020. The goal of the survey is to examine the nature of Dark Matter and Dark Energy in the Universe. One of the cosmological probes used to analyze Euclid’s data, the weak lensing technique, measures the distortions of galaxy shapes and this requires very accurate knowledge of the system point spread function (PSF). Therefore, to ensure that the galaxy shape is not affected, the detector chain of the telescope’s VISible Instrument (VIS) needs to meet specific performance performance requirements. Each of the 12 VIS readout chains consisting of 3 CCDs, readout electronics (ROE) and a power supply unit (RPSU) will undergo a rigorous on-ground testing to ensure that these requirements are met. This paper reports on the current status of the warm and cold testing of the VIS Engineering Model readout chain. Additionally, an early insight to the commissioning of the Flight Model calibration facility and program is provided.
The Southern African Large Telescope (SALT) High Resolution Spectrograph (HRS) is a fibre-fed R4 échelle
spectrograph employing a white pupil design with red and blue channels for wavelength coverage from 370–890nm.
The instrument has four modes, each with object and sky fibres: Low (R~15000), Medium (R~40000) and High
Resolution (R~65000), as well as a High Stability mode for enhanced radial velocity precision at R~65000. The High
Stability mode contains a fibre double-scrambler and offers optional simultaneous Th-Ar arc injection, or the inclusion
of an iodine cell in the beam. The LR mode has unsliced 500μm fibres and makes provision for nod-and-shuffle for
improved background subtraction. The MR mode also uses 500μm fibres, while the HR and HS fibres are 350μm. The
latter three modes employ modified Bowen-Walraven image-slicers to subdivide each fibre into three slices. All but the
High Stability bench is sealed within a vacuum tank, which itself is enclosed in an interlocking Styrostone enclosure, to
insulate the spectrograph against temperature and atmospheric pressure variations. The Fibre Instrument Feed (FIF)
couples the four pairs of fibres to the telescope focal plane and allows the selection of the appropriate fibre pair for a
given mode, and adjustment of the fibre separation to optimally position the sky fibre. The HRS employs a
photomultiplier tube for an exposure meter and has a dedicated auto-guider attached to the FIF. We report here on the
commissioning results and overall instrument performance since achieving first light on 28 September 2013.
KMOS is a multi-object near-infrared integral field spectrograph built by a consortium of UK and German institutes for
the ESO Paranal Observatory. We report on the on-sky performance verification of KMOS measured during three
commissioning runs on the ESO VLT in 2012/13 and some of the early science results.
A study for a spectrograph delivering at least 10000 slits for galaxies and 20000 for stars over a 2.5 deg2 field have been
completed as an answer to the call for proposal for future VISTA MOS instrumentation. In a single night, 65000 galaxy
redshifts can be measured to z~0.7 and beyond for measuring the Baryon Acoustic Oscillation (BAO) scale and many
other science goals. The design features ten cloned spectrographs which give a smaller total weight and length than a
unique spectrograph to make it placable in the space envelope of the Cassegrain focus. The clones use a transparent
design including a grism in which all optics are about the size or smaller than the clone rectangular subfield so that they can be tightly packed with little gaps between subfields. Only low cost glasses are used; the variations in chromatic aberrations between bands are compensated by changing a box containing the grism and two adjacent lenses. Two bands cover the 550nm to 900nm wavelength range at resolution of 1100 for blue end and 3000 for red end while another cover the Calcium triplet at 5000. An optional box does imaging but we studied different innovative methods for acquisition without imaging. A new 2.3° corrector was designed that places the pupil before and relatively near the focal plane which permits to give more space at the back of the spectrographs by placing them in a hedgehog configuration. An offaxis field lens in each spectrograph permits to control the pupil position.
KMOS is a multi-object near-infrared integral field spectrograph being built by a consortium of UK and German
institutes. We report on the final integration and test phases of KMOS, and its performance verification, prior to
commissioning on the ESO VLT later this year.
SALT HRS is a fibre-fed, high dispersion échelle spectrograph currently being constructed for the Southern African
Large Telescope (SALT). In this paper we highlight the performance of key optical components, describe the integration
tasks that have taken place and present some first light results from the laboratory. The instrument construction is well
advanced and we report on the attainment of the required mechanical and thermal stability and provide a measurement of
the input optics performance (including the fibre feed). The initial optical alignment of both the fibre input optics,
including image slicers, and the spectrograph optics has taken place and is described.
The ORIGIN concept is a space mission with a gamma ray, an X-ray and an optical telescope to observe the gamma ray
bursts at large Z to determine the composition and density of the intergalactic matter in the line of sight. It was an answer
to the ESA M3 call for proposal. The optical telescope is a 0.7-m F/1 with a very small instrument box containing 3
instruments: a slitless spectrograph with a resolution of 20, a multi-imager giving images of a field in 4 bands
simultaneously, and a cross-dispersed Échelle spectrograph giving a resolution of 1000. The wavelength range is 0.5 μm
to 1.7 μm. All instruments fit together in a box of 80 mm x 80 mm x 200 mm. The low resolution spectrograph uses a
very compact design including a special triplet. It contains only spherical surfaces except for one tilted cylindrical
surface to disperse the light. To reduce the need for a high precision pointing, an Advanced Image Slicer was added in
front of the high resolution spectrograph. This spectrograph uses a simple design with only one mirror for the collimator
and another for the camera. The Imager contains dichroics to separate the bandwidths and glass thicknesses to
compensate the differences in path length. All 3 instruments use the same 2k x 2k detector simultaneously so that
telescope pointing and tip-tilt control of a fold mirror permit to place the gamma ray burst on the desired instrument
without any other mechanism.
The Centre for Advanced Instrumentation (CfAI) of Durham University (UK) has recently successfully completed the
development of 24 Integral Field Units (IFUs) for the K-band Multi-Object Spectrometer (KMOS). KMOS is a second
generation instrument for ESO’s Very Large Telescope (VLT) which is due for delivery during the summer of 2012. The
KMOS IFU is based on the Advanced Image Slicer Concept developed by the CfAI and previously successfully
implemented on the Gemini Near-InfraRed Spectrograph and JWST NIRSpec. Each IFU contains 14 channels which
have to be accurately aligned. In addition, all 24 IFUs have to be co-aligned requiring the accurate alignment of an
unprecedented grand total of 1152 optical surfaces. In this paper we describe how this has been achieved through the use
of complex monolithic multi-faceted metal mirror arrays, which were fabricated in-house by means of freeform diamond
machining. We will summarise the results from the metrology performed on each of the optical components and describe
how these were integrated and aligned into the system. We will also summarise the results from the system level
acceptance tests, which demonstrate the excellent performance of the IFUs. Each of the 24 IFUs is essentially diffraction
limited across the entire field (Strehl ratios ~ 0.8) with throughput predictions (based on measurements of the surface
roughness) rising from 86% at a wavelength of 1 micron to 93% at 2.5 micron. We believe that this level of performance
has not previously been achieved in any image slicing IFU and showcases the potential of the current state-of-the-art
We developed the technology of microslice integral field units some years ago as the next step in SAURON type
microlens IFU design with typically 5 times more spatial elements (spaxels) for the same spectrograph and spectral
length aiming at 1,000,000 spaxels IFUs. A full instrument for laboratory demonstration composed of the fore-optics, the
IFU, the spectrograph and the detector has now been built and tested. It has about 10,000 spatial elements and spectra
150 pixel long. Our IFU has 5 cylindrical microlens arrays along the optical axis as opposed to one hexagonal array in
the previous design. Instead of imaging pupils on the spectrograph input focal plane, our IFU images short slitlets 17
pixel long that keep the spatial information along the spatial direction then giving 17 spaxels per slitlet instead of one in
pupil imaging. This removes most of the lost space between spectra leaving place for more and keeps the spatial
information over the element size while pupil images lose it. The fore-optics re-images the field on the input of the IFU.
They are made of cylindrical optics to get the desired different magnifications in both directions. All the optics and
detector fit in a cylinder 35 mm in diameter and 280 mm long. With a different set of fore-optics on a 4-m telescope, a
field of 43" x 6.7" with spatial elements of 0.14" x 0.22" could be observed so 12 of these mini-spectrographs would
cover a field surface area of about 1 arcmin2 and 120,000 spaxels.
EUCLID, the ESA Dark Energy Mission, contains a NIR and a visible imagers (NIP & VIS), and an NIR spectrograph
(NIS). Different designs of the NIS have been studied especially a slitless design, a Digital Micromirror Device (DMD)
design using grisms and another using prisms, and more recently a combination of the NIP and NIS into one instrument.
We present the design of the prism DMD NIS. This design has the advantage over the slitless design of having a DMD
mask which reduces the background by a factor of more than 100 and all the advantages over the grism DMD NIS that a
prism gives over a grism as a higher and more uniform transmission, the absence of parasite orders, and a choice of the
slope of the spectral resolution with wavelength. The field per spectrograph was made sufficiently large to reduce the
number of spectrographs to two. The design was made so that the mapping of the sky of the NIS is easily compatible
with the mapping strategy of the NIP and VIS. Two designs were made. In one, the field is larger but the surface shapes
of the optics are complex which makes manufacturing more challenging. In the other, the design was made to be fully
compatible with the manufacturing criteria of SESO after extensive discussions to carefully understand the
manufacturing limitations especially the formula for highly aspheric surface shapes as biconics. This was done by
directly integrating the criteria into the optimization process of ZEMAX. A calibration system that uses the DMD with
the micromirrors in their OFF positions was also developed.
The Euclid Near-Infrared Spectrometer (E-NIS) Instrument was conceived as the spectroscopic probe on-board the ESA
Dark Energy Mission Euclid. Together with the Euclid Imaging Channel (EIC) in its Visible (VIS) and Near Infrared
(NIP) declinations, NIS formed part of the Euclid Mission Concept derived in assessment phase and submitted to the
Cosmic Vision Down-selection process from which emerged selected and with extremely high ranking. The Definition
phase, started a few months ago, is currently examining a substantial re-arrangement of the payload configuration due to
technical and programmatic aspects. This paper presents the general lines of the assessment phase payload concept on
which the positive down-selection judgments have been based.
Astrophotonics offers a solution to some of the problems of building instruments for the next generation of telescopes
through the use of photonic devices to miniaturise and simplify instruments. It has already proved its worth in
interferometry over the last decade and is now being applied to nightsky background suppression. Astrophotonics offers
a radically different approach to highly-multiplexed spectroscopy to the benefit of galaxy surveys such as are required to
determine the evolution of the cosmic equation of state. The Astrophotonica Europa partnership funded by the EU via
OPTICON is undertaking a wide-ranging survey of the technological opportunities and their applicability to high-priority
astrophysical goals of the next generation of observatories. Here we summarise some of the conclusions.
XMS is a multi-channel wide-field spectrograph designed for the prime focus of the 3.5m Calar-Alto telescope. The
instrument is composed by four quadrants, each of which contains a spectrograph channel. An innovative mechanical
design -at concept/preliminary stage- has been implemented to: 1) Minimize the separation between the channels to
achieve maximal filling factor; 2) Cope with the very constraining space and mass overall requirements; 3) Achieve very
tight alignment tolerances; 4) Provide lens self-centering under large temperature excursions; 5) Provide masks including
4000 slits (edges thinner than 100μ). An overview of this very challenging mechanical design is here presented.
Wide-field multi-object spectroscopy is a high priority for European astronomy over the next decade. Most 8-10m
telescopes have a small field of view, making 4-m class telescopes a particularly attractive option for wide-field
instruments. We present a science case and design drivers for a wide-field multi-object spectrograph (MOS) with
integral field units for the 4.2-m William Herschel Telescope (WHT) on La Palma. The instrument intends to take
advantage of a future prime-focus corrector and atmospheric-dispersion corrector (Agocs et al, this conf.) that will
deliver a field of view 2 deg in diameter, with good throughput from 370 to 1,000 nm. The science programs cluster into
three groups needing three different resolving powers R: (1) high-precision radial-velocities for Gaia-related Milky Way
dynamics, cosmological redshift surveys, and galaxy evolution studies (R = 5,000), (2) galaxy disk velocity dispersions
(R = 10,000) and (3) high-precision stellar element abundances for Milky Way archaeology (R = 20,000). The multiplex
requirements of the different science cases range from a few hundred to a few thousand, and a range of fibre-positioner
technologies are considered. Several options for the spectrograph are discussed, building in part on published design
studies for E-ELT spectrographs. Indeed, a WHT MOS will not only efficiently deliver data for exploitation of
important imaging surveys planned for the coming decade, but will also serve as a test-bed to optimize the design of
MOS instruments for the future E-ELT.
The high-resolution échelle spectrograph, SALT HRS, is at an advanced stage of construction and will shortly become
available to the user community of the Southern African Large Telescope (SALT). This paper presents a commentary on
the construction progress to date and gives the instrument's final specification with refined estimates for its performance
based on the initial testing of the optics and the science-grade detectors. It also contributes a discussion of how the fibre
input optics have been tailored to specific scientific aspirations to give four distinct operational modes. Finally, the use of
the instrument is discussed in the context of the most common science cases.
KMOS is a near-infrared multi-object integral-field spectrometer which is one of a suite of second-generation
instruments under construction for the VLT. The instrument is being built by a consortium of UK and German
institutes working in partnership with ESO and is now in the manufacture, integration and test phase. In this paper
we present an overview of recent progress with the design and build of KMOS and present the first results from the
subsystem test and integration.
Two feasibility studies for spectrographs that can deliver at least 4000 MOS slits over a 1° field at the prime focuses of
the Anglo-Australian and Calar Alto Observatories have been completed. We describe the design and science case of the
Calar Alto eXtreme Multiplex Spectrograph (XMS) for which an extended study, half way between feasibility study and
phase-A, was made. The optical design is quite similar than in the AAO study for the Next Generation 1 degree Field
(NG1dF) but the mechanical design of XMS is quite different and much more developed. In a single night, 25000 galaxy
redshifts can be measured to z~0.7 and beyond for measuring the Baryon Acoustic Oscillation (BAO) scale and many
other science goals. This may provide a low-cost alternative to WFMOS for example and other large fibre spectrographs.
The design features four cloned spectrographs which gives a smaller total weight and length than a unique spectrograph
to makes it placable at prime focus. The clones use a transparent design including a grism in which all optics are about
the size or smaller than the clone rectangular subfield so that they can be tightly packed with little gaps between
subfields. Only low cost glasses are used; the variations in chromatic aberrations between bands are compensated by
changing a box containing the grism and two adjacent lenses. Three bands cover the 420nm to 920nm wavelength range
at 10A resolution while another cover the Calcium triplet at 3A. An optional box does imaging. We however also studied
different innovative methods for acquisition without imaging. A special mask changing mechanism was also designed to
compensate for the lack of space around the focal plane. Conceptual designs for larger projects (AAT 2º field, CFHT,
VISTA) have also been done.
The benefits Astronomy could gain by performing multi-slit spectroscopy in a space mission is renown. Digital
Micromirror Devices (DMD), developed for consumer applications, represent a potentially powerful solution. They are
currently studied in the context of the EUCLID project. EUCLID is a mission dedicated to the study of Dark Energy
developed under the ESA Cosmic Vision programme. EUCLID is designed with 3 instruments on-board: a Visual
Imager, an Infrared Imager and an Infrared Multi-Object Spectrograph (ENIS). ENIS is focused on the study of Baryonic
Acoustic Oscillations as the main probe, based on low-resolution spectroscopic observations of a very large number of
high-z galaxies, covering a large fraction of the whole sky. To cope with these challenging requirements, a highmultiplexing
spectrograph, coupled with a relatively small telescope (1.2m diameter) has been designed. Although the
current baseline is to perform slit-less spectroscopy, an important option to increase multiplexing rates is to use DMDs as
electronic reconfigurable slit masks. A Texas Instrument 2048x1080 Cinema DMD has been selected, and space
validation studies started, as a joint ESA-ENIS Consortium effort. Around DMD, a number of suited optical systems has
been developed to project sky sources onto the DMD surface and then, to disperse light onto IR arrays. A detailed study
started, both at system and subsystem level, to validate the initial proposal. Here, main results are shown, making clear
that the use of DMD devices has great potential in Astronomical Instrumentation.
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.
SALT HRS is a fiber-fed cross-dispersed echelle spectrograph designed for high resolution and high efficiency seeing-limited
spectroscopy on the Southern African Large Telescope. The spectrograph, which has a dual channel white
pupil design, uses a single R4 echelle grating, a dichroic beam-splitter, and volume phase holographic gratings
as cross-dispersers. The echelle grating has 41.6 grooves/mm and is illuminated with a 200mm diameter beam.
This allows R = 16,000 with a 2.2" fiber and complete wavelength coverage from 370 nm to 890 nm. Resolving
powers of R ≈ 37,000 and 67,000 are obtained using image slicers. The dichroic beam-splitter is used to split the
wavelength coverage between two fully dioptric cameras. The white pupil transfer optics are used to demagnify
the pupil to 111mm which ensures that the camera dimensions are kept reasonable whilst also allowing the
efficient use of VPH gratings. The spectrograph optics are enclosed inside a vacuum tank to ensure immunity to
atmospheric pressure and temperature changes. The entire spectrograph is mechanically and thermally insulated.
Construction of SALT HRS began at Durham University's Centre for Advanced Instrumentation in August 2007
and is expected to be complete in 2009. The spectrograph optical design is largely based on work completed at
the University of Canterbury's Department of Physics and Astronomy.
A Deformable Mirror Controller (DMC) has been devised to overcome the open-loop nature of Multi Object Adaptive
Optics (MOAO), in particular for AO systems with update rates of 1 ms or less. The system is based on a figure sensor,
which uses a monochromatic illumination source and a Shack-Hartmann (SH) wavefront sensor (WFS) to obtain a fine
sampling of DM's 3D surface. The sensor's beam is optically separated from the science path in order to not interfere
with science observations. The DMC incorporates a real-time controller in charge of driving the DM. This controller
runs in a dedicated Field-Programmable-Gate-Array (FPGA) based processor to keep up with stringent speed
requirements. The DMC is being tested in the laboratory and is part of CANARY, an MOAO on-sky demonstrator to be
installed at the William Hershel Telescope.
The SPACE and DUNE proposals for the ESA Cosmic Vision 2015-2025 have been pre-selected for a Dark Energy
Mission. An assessment study was performed in the past few months resulting in a merged mission called EUCLID. The
study led to a possible concept for the mission and the payload, paving the way for the industrial studies. SPACE has
now become the EUCLID spectrograph channel (EUCLID-spectro). We will discuss its science and give a description of
the different studied optical designs. EUCLID-spectro aims to produce the largest three-dimensional map of the Universe
by taking near-IR spectra at R=400 and 0.9μm<λ<1.7μm for ~200 million galaxies at z<2 and H<22 over 20,000 deg2. It
will measure the expansion history of the Universe and the growth rate of structure using Baryonic Acoustic Oscillations,
redshift-space distortions and clusters of galaxies. It will distinguish true dark energy from a modification of Einstein's
gravity. The original design had 4 channels each re-imaging with mirrors a sub-field from the Casgrain focus onto a
Digital Micromirror Device (DMD). A prism spectrograph followed each array. This design was modified to adapt
EUCLID-spectro to a DUNE-type telescope, to reduce the number of optics and spectrographs, and add an imaging
capability. We studied grism spectrographs, especially for a slitless backup solution that have less optics but a smaller
field; we also studied compact prism and lens spectrographs, telescope corrector combined with micromirror arrays at
the Casgrain focus then eliminating the re-imaging, and TIR prisms over the arrays to help with packaging.
The alignment of the JWST NIRSpec spectrograph will use a customised set of optical light sources, imagers and wavefront sensors, which form part of the Optical Ground Support Equipment (OGSE). This has been developed by the Mullard Space Science Laboratory (MSSL) and the Centre for Advanced Instrumentation (CfAI) to be used at the Astrium GmbH, Ottobrunn (Germany) during NIRSpec integration. This paper describes the five precision illumination sources which form a key part of NIRSpec OGSE, and the optomechanical design of the three Shack-Hartmann wavefront sensors used.
We present a novel technique for the design of DM controllers in high spatial resolution adaptive optics systems, operating in open-loop. It consists of a Shack-Hartmann (SH) figure sensor and multiple overlapping MIMO controllers based on the H∞ synthesis method. The controller synthesis can be carried out periodically using a linearized representation of a continuously adjusted model that accounts for varying physical or ambient conditions and incorporates the spatial geometry of the SH. The figure sensor uses a bright reference source and a fast CMOS detector
to sample the DM surface sequentially with an optical arrangement that does not interfere with the main corrected beam.
Taking full advantage of such robust techniques, the controller can successfully handle the dynamics and non-linearity of
the DM, allowing one to decouple, from the main AO control loop standpoint, the turbulence estimation errors from
those originating in the DM servo-loop. It can also implement noise and vibration rejection without compromising the
loop stability, pushing the control bandwidth to the physical limits imposed by hardware and software components. By
splitting the control function into several overlapping controllers, implementation complexity is reduced and continuous
updating of the controller can be easily achieved. Simulations show its ability to successfully control the DM shape, in
spite of partial and non-simultaneous sampling of the SH figure sensor due to detector speed limitations.
Durham University's Centre for Advanced Instrumentation (CfAI) are currently prototyping key components for the KMOS and JWST NIRSpec Integral Field Units (IFUs). These next-generation IFUs will make extensive use of complex monolithic multi-faceted metal mirror arrays, which are fabricated by means of freeform diamond machining. Using this technique, the inherent accuracy of the diamond machining equipment is exploited to achieve the required relative alignment accuracy of the facets, as well as obtain the necessary optical surface quality for each individual facet, thus facilitating the integration and subsequent testing of these complex systems. The CfAI have pioneered the use of such arrays in the IFU for the Gemini Near-InfraRed Spectrograph (GNIRS IFU), which was installed at Gemini South in April, 2004. The requirements for the next generation of IFUs, however, demand a considerable improvement in the optical performance of these components, e.g. alignment accuracy of the facets, surface form accuracy and roughness. In our paper we briefly discuss the optical designs of KMOS and JWST NIRSpec IFU, and summarise the requirements on the optical components. We then present details of the diamond machining techniques employed to fabricate these highquality components and discuss the latest results from our prototyping activities, which demonstrate our capability of producing optical components that meet the demanding specifications.
KMOS is a near-infrared multi-object integral field spectrometer which has been selected as one of a suite of second-generation instruments to be constructed for the ESO VLT in Chile. The instrument will be built by a consortium of UK and German institutes working in partnership with ESO and is currently at the end of its preliminary design phase. We present the design status of KMOS and discuss the most novel technical aspects and the compliance with the technical specification.
Optical designs of fore-optics and Advanced Image Slicer (AIS) systems made for the second generation VLT instruments KMOS1 and MUSE2,3 conceptual design studies are presented. KMOS is an infrared multi-integral-field spectrograph with 24 fields, each 2.8" x 2.8" with a 0.2" resolution, patrolling a 7' field. The described optics of KMOS are the fore-optics, from the images given by the pickoff system to the slicers, and the slicer systems themselves. The study also includes a derotator design in case the instrument would have been too heavy to be attached to the telescope. MUSE is an integral field spectrograph for the 0.465 µm to 1 µm bandwidth with a 1' x 1' field and a resolution of 0.2". Two optical designs were proposed, one mostly transmissive which is now the baseline, the other mostly using reflective optics. The later is described in this paper. It includes a derotator, an atmospheric dispersion corrector, a transmissive removable magnifier, a transmissive field splitter that cut the field in 24 subfields, the relay optics of each subfield to each slicer and the slicer systems. While MUSE is for the visible and would then in principle need transmissive optics, the use of reflective optics is justified because its minimum wavelength is 0.465 µm; modern reflective coatings give transmission larger than 98% for these wavelengths. We discuss the development of the manufacturing of AIS to extend its application to the visible from its actual use in the IR.
A prototype cryogenic 'pick-off' arm for selecting a small field from the focal plane of a large telescope has been built and tested against a set of scientific requirements representative of those for proposed multi-integral-field spectrographs. In this paper, we present the design of the arm and the results of the cryogenic testing. Since the proposed instruments will require tens of arms, perhaps hundreds, we have also considered the industrialisation of the manufacture and assembly of the arms. We briefly discuss this aspect of the design and the possibilities for future instrumentation on Extremely Large Telescopes.
We describe the design of a 2nd generation instrument for the ESO VLT which will deliver a unique multiple deployable integral field capability in the near-infrared (1-2.5μm). The science drivers for the instrument are presented and linked to the functional specification. The baseline instrument concept is described with emphasis on technological innovations. Detailed discussions of specific technologies, and ongoing prototype studies, are described in separate papers.
We present results from cryogenic tests of a Volume-Phase Holographic
(VPH) grating at 200 K measured at near-infrared wavelengths. The aims
of these tests were to see whether the diffraction efficiency and
angular dispersion of a VPH grating are significantly different at a low temperature from those at a room temperature, and to see how many
cooling and heating cycles the grating can withstand. We have completed 5 cycles between room temperature and 200 K, and find that the performance is nearly independent of temperature, at least over the temperature range which we are investigating. In future, we will not only try more cycles between these temperatures but also perform
measurements at a much lower temperature (e.g., ~80 K).
We describe a proposed 2nd generation instrument (KMOS) for the ESO VLT which will deliver a unique multiple integral field capability in the near-infrared (1-2.5 μm). The science drivers for such an instrument are presented and linked to the functional specification. The overall instrument concept is discussed in the context of two proposed solutions for delivering a deployable integral field capability. Detailed discussions of these two approaches, and ongoing prototype studies, are described in separate papers.
We present the results of a detailed technical study of the use of image slicers for multiple integral field spectroscopy at infrared wavelengths. Our solution uses independently controlled robotic arms to relay selected portions of the focal plane to fixed positions where they are dissected using a set of advanced image slicers. We discuss the technical requirements of this approach and describe a feasibility study to examine the risks and technical challenges.
Integral Field Spectroscopy (IFS) is a powerful tool for astronomy, of particular importance to large aperture telescopes. We have designed and constructed a prototype integral field unit (IFU) for multiple-IFS which may be deployed to any desired position in a 30' diameter field of view and will deliver a good image quality simultaneously at visible (0.45 - 1.0 μm) and near infrared (1.0 - 1.8 μm) wavelength ranges. The design and construction of the multiple-IFU for the prime focus of an 8-meter telescope is discussed in this paper. The IFU uses optical fibers whose flexibility is an important advantage for a multiple-IFU. Simple and compact optics is essential for the design of the IFU. Key design issues, such as the fore-optics, microlens array and fiber bundle, are described in detail. Finally the achievable performance of the IFU is estimated.
Conventional adaptive optics systems using a single wavefront corrector suffer from a limited field of view. Multi-conjugate adaptive optics use two or more corrector to improve off-axis correction. We describe an experimental system which simulates dual-layer turbulence, and present results using a single corrector showing anisoplanatic effects. Future experiments using a second corrector are also discussed.
Gemini have funded a design study to investigate the technologies needed in a versatile multi-object spectrograph for IR astronomy. We report on our investigations into wide- field spectroscopy using multiple integral-field units (MIFUs) to match particular areas of interest to the available detector(s). Such technologies enable integral field spectroscopy of several targets over a much wider field than can be covered with a single IFU. A brief overview of the scientific rationale for a multipel0IFU capability matched to multi-conjugate adaptive optics, and with its wider uncorrected field, on Gemini is given. A proposed method of deploying MIFUs is then described along with the optical consequences of the method.
In order to enhance the spectroscopic capabilities of the William Herschel Telescope (WHT) we have recently completed an integral field unit comprising 1000 elements. Integral field units maximize the efficiency of a spectrograph by re- formatting a 2D field in order to match the entrance slit of the camera. Such techniques enable high-resolution spectral data to be obtained over the whole field simultaneously, and are particularly suited for use with adaptive optics systems. TEIFU is an optical fiber system employing microlens arrays for input and output coupling. The field is divided into two halves, permitting object and background to be derived during the same exposure. In addition, the fields can be optically re-positioned to form a larger, single field for greater object coverage. Thus the observer can choose between different observing modes to emphasize background subtraction or contiguous field. The fore-optics can be changed to alter the image scale and to interface to the NAOMI adaptive optics system which is currently under construction. TEIFU in its present configuration as tested on the WHT, gives a spatial sampling of 0.25 arcsec with a total field of 7.8 by 7.0 arcsec, but a 0.125 arcsec sampling option may be provided. We are also considering an option to upgrade TEIFU for near IR operation. This paper will outline system design, operation and preliminary results.
EMIR is a near-IR, multi-slit camera-spectrograph under development for the 10m GTC on La Palma. It will deliver up to 45 independent R equals 3500-4000 spectra of sources over a field of view of 6 feet by 3 feet, and allow NIR imaging over a 6 foot by 6 foot FOV, with spatial sampling of 0.175 inch/pixel. The prime science goal of the instrument is to open K-band, wide field multi-object spectroscopy on 10m class telescopes. Science applications range from the study of star-forming galaxies beyond z equals 2, to observations of substellar objects and dust-enshrouded star formation regions. Main technological challenges include the large optics, the mechanical and thermal stability and the need to implement a mask exchange mechanism that does not require warming up the spectrograph. EMIR is begin developed by the Instituto de Astrofisica de Canarias, the Instituto Nacional de Tecnica Aeroespacial, the Universidad Complutense de Madrid, the Observatoire Midi-Pyrennees, and the University of Durham. Currently in its Preliminary Design phase, EMIR is expected to start science operation in 2004.
We have proposed a new Nasmyth instrument for the William Herschel Telescope which exploits the potential of wide field-of-view correction of atmospheric turbulence to produce a versatile, high spatial resolution, high efficiency, multi-object spectrograph and imager optimized for the 0.7-1.6 micron region. Using a low-altitude Rayleigh beacon guide star to correct the boundary layer turbulence which dominates the atmospheric seeing at La Palma on more than 25 percent of nights, MOSAIC combines the angular resolution gains of adaptive optics with the observing efficiency gains of multiple-object spectroscopy. Additional operating modes could include a narrow-band tunable filter and a fiber feed to a bench-mou8nted high resolution echelle spectrography. The instruments would provide a unique capability on 4-meter telescopes, opening up a wide variety of new scientific capabilities ranging from spectroscopic studies of crowded star fields to resolved studies of the kinematics of distant galaxies.
Design concept of the fiber multi-object spectrograph (FMOS) for Subaru Telescope together with innovative ideas of optical and structural components is presented. Main features are; i) wide field coverage of 30 arcmin in diameter, ii) 400 target multiplicity, iii) 0.9 to 1.8 micrometers near-IR wavelengths, and iv) OH-airglow suppression capability. The instrument is proposed to be built under the Japan-UK-Australia international collaboration scheme.
Liquid crystal spatial light modulators have recently been used to generate aberrated wavefronts. We have used two ferroelectric liquid crystal devices to simulate turbulence from two layers of the atmosphere, which will be used in future work to test multi-conjugate adaptive optics systems.
Autofib-2 is a robotic fiber system for the prime focus of the William Herschel telescope capable of placing up to 150 fibers in the 1 degree focal plane of the telescope. The fibers are fed to a purpose built spectrograph (WYFFOS) mounted on one of the Nasmyth platforms. Autofib-2 and WYFFOS are now entering a common user phase as fully commissioned instruments. We describe the novel techniques used to achieve the high precision in fiber placement delivered by this instrument and the quality control procedures devised to measure and monitor instrument stability. The characterization of the distortions of focal plane delivered by the prime focus corrector of the telescope was a vital procedure during the commissioning. We describe the methods of measuring these distortions and discuss the limitations of the instrument, telescope and astrometry.
We are currently testing an automated fibre positioner for the 4.2 m William Herschel Telescope (WHT). This instrument, known as Autofib-2, operates at the prime focus where it is able to utilize the full 1 degree field provided by the prime focus corrector (PFC). The robotic positioner is able to place 160 optical fibres in the focal plane of the WHT which feeds the light to a dedicated spectrograph (WYFFOS) located on the Nasmyth platform. This paper contains a description of the instrument which highlights the new techniques demanded by the prime focus plate scale and the scale distortions due to the PFC and its atmospheric dispersion compensator. These include robot vision to help achieve the high positioning accuracy and the use of two sky viewing probes to accurately determine the time dependent transformation from celestial coordinates to instrumental Cartesian coordinates. Also presented are the initial technical results on the performance of the instrument and the operational results of particular interest to the astronomical observer.
An astronomical AO system for use on a 4 m class telescope at visible wavelengths is described. The design of this instrument is based on the University of Durham semi common-user partial-AO system MARTINI. A brief technical resume and recent astronomical results are provided. The MARTINI system was designed on a partial, but non-modal, philosophy which is able to deliver modest image improvements normally associated with low order corrections at small D/ro. The new system will remove the limited aperture coverage of the MARTINI system and extend its operating philosophy to produce a corrected optical transfer function optimized to scientific goals. The requirements and resulting system design are described. The system is also designed to be accommodated within the general scheme of the UK national AO program which was initiated in 1993. A brief discussion of the design issues involved is provided.
Binary adaptive optics involves using a simple two-state algorithm for wavefront correction. The technique lends itself to implementation using ferroelectric liquid crystal spatial light modulators (FLC-SLMs) as half-wave and quarter-wave phase shifters. In this paper we demonstrate further the effect of half-wave and quarter-wave correction, we compare the ideal performance with conventional analogue adaptive optics and we discuss the relevant optical properties of FLC-SLMs.
The requirements of wavefront reconstruction algorithm for use in astronomical adaptive optics systems are discussed. We present the results of realistic numerical simulations of the closed-loop performance of a Shack-Hartmann wavefront sensor used in conjunction with a high-order segmented mirror suitable for adaptive correction on a 4 m telescope at visible wavelengths. Iterative over-relaxation methods are used to derive the segment piston values, and we investigate the behavior of these techniques in the presence of photon and detector noise. We also discuss the use of non variance-minimizing techniques to optimize the point- spread function for particular astronomical applications.
The Multiple Aperture Real Time Image Normalization Instrument (MARTINI) is an astronomical adaptive optics system for visible imaging and spectroscopic feedthrough at the 4.2-m William Herschel Telescope on La Palma. It consists of a six-subaperture, tip-tilt-piston, segmented mirror device and uses 4r(0) aperture-matching to provide optimum slope removal in zones large enough for operation in the visible and with reference objects fainter than V = 13 exp m. This limit is achieved by optimizing the use of reference light, by analyzing the information from a photon counting wavefront sensor using a non-flaming (i.e., irregular sampling) infinite impulse response filter for estimation and prediction of the wavefront slopes. The value of this approach is discussed along with its extension to higher-order correction schemes. Experimental evidence supporting the theoretical basis of the MARTINI system is also presented. The astronomical potential of such an approach, and the drawbacks, are outlined.
The MARTINI adaptive optics system has been engineered for regular astronomical observations on the 4.2 m William Herschel Telescope on La Palma. The design specifications for such a general purpose image-sharpening device are discussed. Examples of the imaging performance achieved during development of MARTINI are presented, together with results from visible-region astronomical imaging programs. The prospects for spectroscopic observations with this class of system are outlined.