Precision alignment and integration of DESI's focal plane using a laser tracker

The recently commissioned Dark Energy Spectroscopic Instrument (DESI) will 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 delivers light to 5000 fiber optic positioners. The fibers in turn feed ten broad-band spectrographs. We describe the use of a Faro Laser Tracker with custom hardware and software tools for alignment during integration of DESI’s focal plane. The focal plane is approximately one meter in diameter and consists primarily of ten radially symmetrical focal plane segments (“petals”) which were individually installed into the telescope. The nominal clearance between petals is 600 microns, and an alignment accuracy of 100 microns and 0.01 degrees was targeted. Alignment of the petals to their targeted locations on the telescope was accomplished by adjusting a purpose-built alignment structure with 14 degrees of freedom using feedback from the laser tracker, which measured the locations of retroreflectors attached to both the petal and the telescope and whose positions relative to key features were precisely known. These measurements were used to infer the locations of aligning features in both structures, which were in turn used to calculate the adjustments necessary to bring the system into alignment. Once alignment was achieved to within acceptable tolerances, each petal was installed while monitoring building movement due to wind and thermal variations

ProtoDESI: First On-Sky Technology Demonstration for the Dark Energy Spectroscopic Instrument

The Dark Energy Spectroscopic Instrument (DESI) is under construction to measure the expansion history of the universe using the baryon acoustic oscillations technique. The spectra of 35 million galaxies and quasars over 14,000 square degrees will be measured during a 5-year survey. A new prime focus corrector for the Mayall telescope at Kitt Peak National Observatory will deliver light to 5,000 individually targeted fiber-fed robotic positioners. The fibers in turn feed ten broadband multi-object spectrographs. We describe the ProtoDESI experiment, that was installed and commissioned on the 4-m Mayall telescope from August 14 to September 30, 2016. ProtoDESI was an on-sky technology demonstration with the goal to reduce technical risks associated with aligning optical fibers with targets using robotic fiber positioners and maintaining the stability required to operate DESI. The ProtoDESI prime focus instrument, consisting of three fiber positioners, illuminated fiducials, and a guide camera, was installed behind the existing Mosaic corrector on the Mayall telescope. A Fiber View Camera was mounted in the Cassegrain cage of the telescope and provided feedback metrology for positioning the fibers. ProtoDESI also provided a platform for early integration of hardware with the DESI Instrument Control System that controls the subsystems, provides communication with the Telescope Control System, and collects instrument telemetry data. Lacking a spectrograph, ProtoDESI monitored the output of the fibers using a Fiber Photometry Camera mounted on the prime focus instrument. ProtoDESI was successful in acquiring targets with the robotically positioned fibers and demonstrated that the DESI guiding requirements can be met.Comment: Accepted versio

Design and production of the DESI fibre cables

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

Probing Dark Energy via Weak Gravitational Lensing with the Supernova Acceleration Probe (SNAP)

SNAP is a candidate for the Joint Dark Energy Mission (JDEM) that seeks to place constraints on the dark energy using two distinct methods. The first, Type Ia SN, is discussed in a separate white paper. The second method is weak gravitational lensing, which relies on the coherent distortions in the shapes of background galaxies by foreground mass structures. The excellent spatial resolution and photometric accuracy afforded by a 2-meter space-based observatory are crucial for achieving the high surface density of resolved galaxies, the tight control of systematic errors in the telescope's Point Spread Function (PSF), and the exquisite redshift accuracy and depth required by this project. These are achieved by the elimination of atmospheric distortion and much of the thermal and gravity loads on the telescope. The SN and WL methods for probing dark energy are highly complementary and the error contours from the two methods are largely orthogonal. The nominal SNAP weak lensing survey covers 1000 square degrees per year of operation in six optical and three near infrared filters (NIR) spanning the range 350 nm to 1.7 {micro}m. This survey will reach a depth of 26.6 AB magnitude in each of the nine filters and allow for approximately 100 resolved galaxies per square arcminute, {approx} 3 times that available from the best ground-based surveys. Photometric redshifts will be measured with statistical accuracy that enables scientific applications for even the faint, high redshift end of the sample. Ongoing work aims to meet the requirements on systematics in galaxy shape measurement, photometric redshift biases, and theoretical predictions

Seeing the Nature of the Accelerating Physics: It's a SNAP

For true insight into the nature of dark energy, measurements of the precision and accuracy of the Supernova/Acceleration Probe (SNAP) are required. Precursor or scaled-down experiments are unavoidably limited, even for distinguishing the cosmological constant. They can pave the way for, but should not delay, SNAP by developing calibration, refinement, and systematics control (and they will also provide important, exciting astrophysics)

Conceptual Design of the Modular Detector and Readout System for the CMB-S4 survey experiment

We present the conceptual design of the modular detector and readout system for the Cosmic Microwave Background Stage 4 (CMB-S4) ground-based survey experiment. CMB-S4 will map the cosmic microwave background (CMB) and the millimeter-wave sky to unprecedented sensitivity, using 500,000 superconducting detectors observing from Chile and Antarctica to map over 60 percent of the sky. The fundamental building block of the detector and readout system is a detector module package operated at 100 mK, which is connected to a readout and amplification chain that carries signals out to room temperature. It uses arrays of feedhorn-coupled orthomode transducers (OMT) that collect optical power from the sky onto dc-voltage-biased transition-edge sensor (TES) bolometers. The resulting current signal in the TESs is then amplified by a two-stage cryogenic Superconducting Quantum Interference Device (SQUID) system with a time-division multiplexer to reduce wire count, and matching room-temperature electronics to condition and transmit signals to the data acquisition system. Sensitivity and systematics requirements are being developed for the detector and readout system over a wide range of observing bands (20 to 300 GHz) and optical powers to accomplish CMB-S4's science goals. While the design incorporates the successes of previous generations of CMB instruments, CMB-S4 requires an order of magnitude more detectors than any prior experiment. This requires fabrication of complex superconducting circuits on over 10 square meters of silicon, as well as significant amounts of precision wiring, assembly and cryogenic testing.Comment: 25 pages, 15 figures, presented at and published in the proceedings of SPIE Astronomical Telescopes and Instrumentation 202

Broadband anti-reflection coating for the meter class Dark Energy Spectroscopic Instrument lenses

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 Robotic Multiobject Focal Plane System of the Dark Energy Spectroscopic Instrument (DESI)

A system of 5020 robotic fiber positioners was installed in 2019 on the Mayall Telescope, at Kitt Peak National Observatory. The robots automatically retarget their optical fibers every 10-20 minutes, each to a precision of several microns, with a reconfiguration time of fewer than 2 minutes. Over the next 5 yr, they will enable the newly constructed Dark Energy Spectroscopic Instrument (DESI) to measure the spectra of 35 million galaxies and quasars. DESI will produce the largest 3D map of the universe to date and measure the expansion history of the cosmos. In addition to the 5020 robotic positioners and optical fibers, DESI’s Focal Plane System includes six guide cameras, four wave front cameras, 123 fiducial point sources, and a metrology camera mounted at the primary mirror. The system also includes associated structural, thermal, and electrical systems. In all, it contains over 675,000 individual parts. We discuss the design, construction, quality control, and integration of all these components. We include a summary of the key requirements, the review and acceptance process, on-sky validations of requirements, and lessons learned for future multiobject, fiber-fed spectrographs

Astro2020 APC White Paper: The MegaMapper: a z > 2 spectroscopic instrument for the study of Inflation and Dark Energy

MegaMapper is a proposed ground-based experiment to measure Inflation parameters and Dark Energy from galaxy redshifts at

Snowmass 2021 CMB-S4 White Paper

This Snowmass 2021 White Paper describes the Cosmic Microwave Background Stage 4 project CMB-S4, which is designed to cross critical thresholds in our understanding of the origin and evolution of the Universe, from the highest energies at the dawn of time through the growth of structure to the present day. We provide an overview of the science case, the technical design, and project plan