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 describe the DESI corrector optics, a series of six fused silica and borosilicate lenses. The lens diameters range from 0.8 to 1.1 meters, and their weights 84 to 237 kg. Most lens surfaces are spherical, and two are challenging 10th-order polynomial aspheres. The lenses have been successfully polished and treated with an antireflection coating at multiple subcontractors, and are now being integrated into the DESI corrector barrel assembly at University College London.
We describe the final performance of the lenses in terms of their various parameters, including surface figure, homogeneity, and others, and compare their final performance against the demanding DESI corrector requirements. Also we describe the reoptimization of the lens spacing in their corrector barrel after their final measurements are known. Finally we assess the performance of the corrector as a whole, compared to early budgeted estimates.
The Dark Energy Spectroscopic Instrument (DESI), currently under construction, will be used 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 broadband AR coating (360 nm to 980nm) that was applied to the lenses of the camera system for DESI using ion assisted deposition techniques in a 3 m coating chamber. The camera has 6 lenses ranging in diameter from 0.8 m to 1.14 m, weighing from 84 kg to 237 kg and made from fused silica or BK7. The size and shape of the surfaces provided challenges in design, uniformity control, handling, tooling and process control. Single surface average transmission and minimum transmission met requirements. The varied optical surfaces and angle of incidence considerations meant the uniformity of the coating was of prime concern. The surface radius of curvature (ROC) for the 12 surfaces ranged from nearly flat to a ROC of 611 mm and a sag of 140 mm. One lens surface has an angle of incidence variation from normal incidence to 40°. Creating a design with a larger than required bandwidth to compensate for the non-uniformity and angle variation created the ability to reduce the required coating uniformity across the lens and a single design to be used for all common substrate surfaces. While a perfectly uniform coating is often the goal it is usually not practicable or cost effective for highly curved surfaces. The coating chamber geometry allowed multiple radial positions of the deposition sources as well as substrate height variability. Using these two variables we were able to avoid using any masking to achieve the uniformity required to meet radial and angle performance goals. Very broadband AR coatings usually have several very thin and optically important layers. The DESI coating design has layers approaching 3 nm in thickness. Having sensitive thin layers in the design meant controlling layer thickness and azimuthal variation were critical to manufacturing repeatability. Through use of strategically placed quartz crystal monitors combined with stable deposition plumes, the manufacturing variability was reduced to acceptable levels. Low deposition rates and higher rotation rates also provided some stability to azimuthal variation.
The Keck Planet Finder (KPF) is a fiber-fed, high-resolution, high-stability spectrometer in development for the W.M. Keck Observatory. The instrument recently passed its preliminary design review and is currently in the detailed design phase. KPF is designed to characterize exoplanets using Doppler spectroscopy with a single measurement precision of 0.5 m s−1 or better; however, its resolution and stability will enable a wide variety of other astrophysical pursuits. KPF will have a 200 mm collimated beam diameter and a resolving power greater than 80,000. The design includes a green channel (445 nm to 600 nm) and red channel (600 nm to 870 nm). A novel design aspect of KPF is the use of a Zerodur optical bench, and Zerodur optics with integral mounts, to provide stability against thermal expansion and contraction effects.
The Dark Energy Spectroscopic Instrument (DESI) is under construction to measure the expansion history of the Universe, using the Baryon Acoustic Oscillation technique and the growth of structure using redshift-space distortions (RSD). The spectra of 40 million galaxies 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 will describe modeling and mitigation of stray light within the front end of DESI, consisting of the Mayall telescope and the corrector assembly. This includes the creation of a stray light model, quantitative analysis of the unwanted light at the corrector focal surface, identification of the main scattering sources, and a description of mitigation strategies to remove the sources.
The Dark Energy Spectroscopic Instrument, to be located at the prime focus of the Mayall telescope, includes a wide field corrector, a 5000 fiber positioner system, and a fiber view camera. The mapping of the sky to the focal plane, needed to position the fibers accurately, is described in detail. A major challenge is dealing with the large amount of distortion introduced by the optics (of order 10% scale change), including time-dependent non-axisymmetric distortions introduced by the atmospheric dispersion compensator. Solutions are presented to measure or mitigate these effects.
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 square degrees will be measured during the life of the experiment. A new prime focus corrector for the Kitt Peak National Observatory Mayall telescope will deliver light to 5000 fiber optic positioners. The fibers in turn feed ten broad-band spectrographs. We will describe the status of the DESI corrector optics, a series of 0.8 to 1.1-meter fused silica and borosilicate lenses currently being fabricated to demanding requirements. We will describe the specs for lenses that are finished or underway, including surface figure, homogeneity, and other parameters; the current schedule for lens production; and a comparison against DESI corrector requirements.
The Dark Energy Spectroscopic Instrument (DESI), which is currently under construction, is designed 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 fibre optic positioners. The fibres in turn feed ten broad-band spectrographs. The prime focus corrector for DESI consists of six lenses that range in diameter from 0.80 - 1.14 meters and from 83 - 237 kg in weight. The alignment of the large lenses of the optical corrector poses a significant challenge as in order to meet the fibre throughput requirements they have to be aligned to within a tolerance of ~50 micrometres. This paper details the design for the cells that will hold the lenses and the alignment and assembly procedure for the mounting of the lenses into the cells and into the complete barrel assembly. This is based on the experience obtained from the alignment of the Dark Energy Camera (DECam) instrument which was successfully assembled and aligned by the same team and we include in the paper the lessons learnt and design modifications that will be implemented on the DESI system.
The Dark Energy Spectroscopic Instrument (DESI), currently under construction, is designed 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. This paper describes the overall design and construction status of the prime focus corrector. The size and complexity of the system poses significant design and production challenges. The optics of the corrector consists of six lenses, ranging from 0.8 - 1.14m in diameter, two of which can be rotated to act as an atmospheric dispersion corrector. These lenses are mounted in custom cells that themselves are mounted in a barrel assembly the alignment of which can be actively controlled by a hexapod system to micrometer precision. The whole assembly will be mounted at the prime focus of the Mayall 4m telescope at Kitt Peak observatory and will be one of the largest lens systems ever built for an optical telescope. Construction of the corrector began in 2014 and is well advanced. The system is due to be delivered to the telescope for installation in early 2018.
An imaging spectrometer for observations of the Martian corona and the Martian thermosphere is presented. The corona extends over 10 Martian radii and its measurement requires observations over a very wide field. The spectrometer covers the wavelength region 120-170 nm where this band includes coronal spectral lines of hydrogen Lyman alpha and oxygen, and thermospheric spectral lines from atomic oxygen and carbon and the 4th positive band of CO. Stellar occultation observations will provide atmospheric density measurements. These scientific requirements are fulfilled by an Offner-type spectrometer with a 110 degree instantaneous field of view and no moving mechanisms. Both the spectral and imaging resolution vary across the field, from higher resolution across the planet body, to lower resolution required at the diffuse outer parts of the corona. This Offner-type design has not been previously used in the FUV.
In the frame of the ICON (Ionospheric Connection Explorer) mission of NASA led by UC Berkeley, CSL and SSL Berkeley have designed in cooperation a new Far UV spectro-imager. The instrument is based on a Czerny-Turner spectrograph coupled with two back imagers. The whole field of view covers [± 12° vertical, ± 9° horizontal]. The instrument is surmounted by a rotating mirror to adjust the horizontal field of view pointing by ± 30°. To meet the scientific imaging and spectral requirements the instrument has been optimized. The optimization philosophy and related analysis are presented in the present paper. PSF, distortion map and spectral properties are described. A tolerance study and alignment cases were performed to prove the instrument can be built and aligned. Finally straylight and out of band properties are discussed.
The Dark Energy Spectroscopic instrument (DESI) is a 5000 fiber multi-object spectrometer system under development
for installation on the National Optical Astronomy Observatory (NOAO) Kitt Peak 4m telescope (the Mayall telescope).
DESI is designed to perform a 14,000° (square) galaxy and Quasi-Stellar Object (QSO) redshift survey to improve
estimates of the dark energy equation of state. The survey design imposes numerous constraints on a prime focus
corrector design, including field of view, geometrical blur, stability, fiber injection efficiency, zenith angle, mass and
cost. The DESI baseline wide-field optical design described herein provides a 3.2° diameter field of view with six 0.8-
1.14m diameter lenses and an integral atmospheric dispersion compensator.
Understanding the earth's climate and collecting the requisite signatures over the next 10, 20, 30 years is a shared
mandate by many of the world's governments. But there remains a daunting challenge to bridge scientific missions to
'operational' systems that truly support the demands of decision makers, scientific investigators and global users'
requirements for trusted data. For this Part III paper, we will examine the required components of a coupled modeling
framework to perform, with benefit of adjoint constraints , optimal forward modeling of the climate's GHG's for both
demonstration and verification. Interrogating such forward modeling in detail will help uncover the most efficient and
sufficient set of critical climate parameters & metrics needed to systematic capture and attribute climate monitoring
environmental records. This in turn would allow globally trusted algorithms to produce climate products that the world's
governments can use to most accurately assess man's impacts on earth's climate and promote informed decisions
sustaining the earth's ability to support life. This paper is the climate modeling based extension to two earlier papers
from 2009 & 2010.
Understanding the earth"s climate and collecting suitable signatures over the next 10, 20, 30 years is a shared objective
of many world governments. But even with significant scientific progress and demonstrations to date, there remains a
daunting challenge to bridge from scientific missions to "operational" systems that support decision makers, scientific
communities and vast numbers of users eager for verified data. In this part II paper for 2010, an expanded description of
a system of constellations reveals the capacity of supporting multiple existing missions and additional "decadal survey"
objectives, by leveraging today's capabilities in an expandable architecture. Resourcing a system of systems solution is
also challenging, but thoughts on shared cost efficiencies and common concerns will be offered specifically intended to
focus the "community" discussion on incremental solutions.
Satellite remote sensing can provide continuous surveillance to detect, characterize, and map wild fires, agricultural
fires, and land management fires. Fire management challenges require additional capability to allow rapid revisit rates,
rapid tasking, and data delivery to the field sufficient for fire management agencies in modern and developing nations
worldwide. An analysis and description of the required constellation of satellites and sensors is given with consideration
of tasking and data delivery.
Understanding the earth's climate and the how it supports life is essential to government policy makers. A new
constellation of operational earth remote sensing satellites (Triana II) is required to provide data to develop this
understanding. Comparison of several spacecraft, sensors systems, orbits, and constellations is described and one
recommended that will support many of the policy decisions facing governments around the world over the next critical
Remote-sensing hyperspectral sensors operating in the reflective bands offer the opportunity to vastly improve land
management worldwide by providing continuous coverage and continuity of satellite capability. We assess the
requirements for such sensors that will provide the needed revisit rates, coverage and imaging performance. From these
requirements we select a range of potential system-level architectures, and derive their constellations, including orbital
parameters and the number of needed satellites. We further discuss how the initial requirements drive the architecture
parameters and performance. We demonstrate that a single satellite will not meet the current needs of the environmental
sensing community, rather a constellation of multiple operational satellites is required for desirable worldwide land
The He+ ion provides a valuable tracer of solar wind dynamics and the heliospheric boundary. Mapping the heliosphere in the 30.4 nm resonance line of the He+ ion with high spectral resolution will open access to the heliopause and reveal the three-dimensional flow of the solar wind. The emission fluxes are however faint, just a few mR, which poses a serious limitation on the mapping rate at high signal-to-noise ratio. We have developed a spectrometer configuration for narrowband EUV emission that offers important advantages over previous designs: high throughput (~1cps/mR), high resolution (several thousand), no moving parts, and modest instrument size and mass. The concept combines a conventional normal-incidence Rowland mount grating and an efficient multilayer coating, with a microchannel plate detector performing two dimensional photon counting. One key innovation is the use of a large-area multi-slit at the spectrometer entrance. This multislit is a one dimensional sequence of open and opaque zones, against which pattern the accumulated spectral image can be correlated to recover the incident spectrum. The other innovation is arranging that each member of the multislit group is curved in such a way that the off-plane grating aberrations (which extend and rotate the image of each object point) do not introduce significant wavelength broadening. The curved slit arrangement yields a large well-corrected image field, and a high throughput for diffuse emission is achieved. The curved-multislit Rowland spectrometer may have a variety of other applications sensing diffuse fluxes with high spectral resolution.
Volumetric media have great potential for meeting future optical data storage demands, but homogeneous media lack internal features for tracking. A novel method of tracking inside homogeneous media is described that uses external reference tracks attached to the media. Several possible configurations for implementing the "slave-servo" concept are described and compared. An optical design for the most promising configuration is presented. This desing utilizes a diffractive optical element for dispersion compensation. Modeling describes the limits of device performance and alignment. Early prototype results are presented.
Galmatheia is a proposed far-ultraviolet (FUV) 900 - 1850 angstrom imaging spectrograph optimized for the study of diffuse emission. Its spectra will be used to study hot galactic plasmas, characterize the formation/destruction cycle and distribution of H2 in the ISM, and determine the optical properties of dust and its spatial distribution through the Galaxy. Galmatheia is a dual reflective spectrograph consisting of two elliptical diffraction gratings and a common cylindrical mirror, slit and detector. Its 5(sigma) sensitivity to C IV/O VI emission in a one-day pointing an order of magnitude fainter than any previous detection or to theoretical predictions.
Dispersive interferometric spectroscopy using all-reflective optical elements can be applied to the far UV bandpass to provide very high spectroscopic resolution in a highly compact optical configuration. Dispersive interferometric spectroscopy is therefore well suited for UV and optical space-flight missions. We describe attributes of interferometric spectroscopy and show results from a laboratory demonstration of a high-resolution FUV dispersive interferometric spectrometer.