34 research outputs found
Collision-free motion planning for fiber positioner robots: discretization of velocity profiles
The next generation of large-scale spectroscopic survey experiments such as
DESI, will use thousands of fiber positioner robots packed on a focal plate. In
order to maximize the observing time with this robotic system we need to move
in parallel the fiber-ends of all positioners from the previous to the next
target coordinates. Direct trajectories are not feasible due to collision risks
that could undeniably damage the robots and impact the survey operation and
performance. We have previously developed a motion planning method based on a
novel decentralized navigation function for collision-free coordination of
fiber positioners. The navigation function takes into account the configuration
of positioners as well as their envelope constraints. The motion planning
scheme has linear complexity and short motion duration (~2.5 seconds with the
maximum speed of 30 rpm for the positioner), which is independent of the number
of positioners. These two key advantages of the decentralization designate the
method as a promising solution for the collision-free motion-planning problem
in the next-generation of fiber-fed spectrographs. In a framework where a
centralized computer communicates with the positioner robots, communication
overhead can be reduced significantly by using velocity profiles consisting of
a few bits only. We present here the discretization of velocity profiles to
ensure the feasibility of a real-time coordination for a large number of
positioners. The modified motion planning method that generates piecewise
linearized position profiles guarantees collision-free trajectories for all the
robots. The velocity profiles fit few bits at the expense of higher
computational costs.Comment: SPIE Astronomical Telescopes + Instrumentation 2014 in Montr\'eal,
Quebec, Canada. arXiv admin note: substantial text overlap with
arXiv:1312.164
SDSS-V Algorithms: Fast, Collision-Free Trajectory Planning for Heavily Overlapping Robotic Fiber Positioners
Robotic fiber positioner (RFP) arrays are becoming heavily adopted in wide
field massively multiplexed spectroscopic survey instruments. RFP arrays
decrease nightly operational overheads through rapid reconfiguration between
fields and exposures. In comparison to similar instruments, SDSS-V has selected
a very dense RFP packing scheme where any point in a field is typically
accessible to three or more robots. This design provides flexibility in target
assignment. However, the task of collision-less trajectory planning is
especially challenging. We present two multi-agent distributed control
strategies that are highly efficient and computationally inexpensive for
determining collision-free paths for RFPs in heavily overlapping workspaces. We
demonstrate that a reconfiguration path between two arbitrary robot
configurations can be efficiently found if "folded" state, in which all robot
arms are retracted and aligned in a lattice-like orientation, is inserted
between the initial and final states. Although developed for SDSS-V, the
approach we describe is generic and so applicable to a wide range of RFP
designs and layouts. Robotic fiber positioner technology continues to advance
rapidly, and in the near future ultra-densely packed RFP designs may be
feasible. Our algorithms are especially capable in routing paths in very
crowded environments, where we see efficient results even in regimes
significantly more crowded than the SDSS-V RFP design.Comment: To be published in the Astronomical Journa
Snowmass2021 Cosmic Frontier: Report of the CF04 Topical Group on Dark Energy and Cosmic Acceleration in the Modern Universe
Cosmological observations in the new millennium have dramatically increased
our understanding of the Universe, but several fundamental questions remain
unanswered. This topical group report describes the best opportunities to
address these questions over the coming decades by extending observations to
the universe. The greatest opportunity to revolutionize our understanding
of cosmic acceleration both in the modern universe and the inflationary epoch
would be provided by a new Stage V Spectroscopic Facility (Spec-S5) which would
combine a large telescope aperture, wide field of view, and high multiplexing.
Such a facility could simultaneously provide a dense sample of galaxies at
lower redshifts to provide robust measurements of the growth of structure at
small scales, as well as a sample at redshifts to measure cosmic
structure at the largest scales, spanning a sufficient volume to probe
primordial non-Gaussianity from inflation, to search for features in the
inflationary power spectrum on a broad range of scales, to test dark energy
models in poorly-explored regimes, and to determine the total neutrino mass and
effective number of light relics. A number of compelling opportunities at
smaller scales should also be pursued alongside Spec-S5. The science
collaborations analyzing DESI and LSST data will need funding for a variety of
activities, including cross-survey simulations and combined analyses. The
results from these experiments can be greatly improved by smaller programs to
obtain complementary data, including follow-up studies of supernovae and
spectroscopy to improve photometric redshift measurements. The best future use
of the Vera C. Rubin Observatory should be evaluated later this decade after
the first LSST analyses have been done. Finally, investments in pathfinder
projects could enable powerful new probes of cosmology to come online in future
decades.Comment: Topical Group Report for CF04 (Dark Energy and Cosmic Acceleration in
the Modern Universe) for Snowmass 202
Overview of the instrumentation for the Dark Energy Spectroscopic Instrument
The Dark Energy Spectroscopic Instrument (DESI) embarked on an ambitious 5 yr survey in 2021 May to explore the nature of dark energy with spectroscopic measurements of 40 million galaxies and quasars. DESI will determine precise redshifts and employ the baryon acoustic oscillation method to measure distances from the nearby universe to beyond redshift z > 3.5, and employ redshift space distortions to measure the growth of structure and probe potential modifications to general relativity. We describe the significant instrumentation we developed to conduct the DESI survey. This includes: a wide-field, 3.°2 diameter prime-focus corrector; a focal plane system with 5020 fiber positioners on the 0.812 m diameter, aspheric focal surface; 10 continuous, high-efficiency fiber cable bundles that connect the focal plane to the spectrographs; and 10 identical spectrographs. Each spectrograph employs a pair of dichroics to split the light into three channels that together record the light from 360â980 nm with a spectral resolution that ranges from 2000â5000. We describe the science requirements, their connection to the technical requirements, the management of the project, and interfaces between subsystems. DESI was installed at the 4 m Mayall Telescope at Kitt Peak National Observatory and has achieved all of its performance goals. Some performance highlights include an rms positioner accuracy of better than 0.âł1 and a median signal-to-noise ratio of 7 of the [O ii] doublet at 8 Ă 10â17 erg sâ1 cmâ2 in 1000 s for galaxies at z = 1.4â1.6. We conclude with additional highlights from the on-sky validation and commissioning, key successes, and lessons learned
Probing dark energy with large-scale galaxy clustering: from instrumentation to simulation
In the standard paradigm of cosmology, everything we observe now originated from initial quantum fluctuations in a small smooth region, which were frozen in during inflation and became primordial density perturbations on large classical scales. Under gravitational collapse, the overdensities seeded the formation of stars and galaxies. Mapping the large-scale structure of the universe at the Cosmic Frontier is a promising experimental avenue which will address in the next decade several pressing open questions in cosmology and particle physics, most notably the accelerating cosmic expansion. The observed distribution of galaxies and quasars traces the underlying matter density field and contains a wealth of information from signatures of primordial conditions to the background evolution rate.
The Dark Energy Spectroscopic Instrument (DESI) is a next-generation, Stage IV dark energy experiment that will measure the expansion history of the universe through baryon acoustic oscillations and the growth of structure through redshift-space distortions with unprecedented precision. Ground-based at the Kitt Peak National Observatory, DESI features a new 8 degÂČ field-of-view corrector, 5000 robotically-actuated fibre positioners, and ten fibre-fed spectrographs. The 5-year survey beginning in 2020 will measure the spectra of 35 million galaxies and quasars up to redshift z ~ 3.5 in the 360 nm to 980 nm wavelength range, covering 14000 degÂČ of the sky. With an order of magnitude improvement over previous redshift surveys, DESI will place tight constraints on the dark energy equation of state, modified gravity, the existence of extra light species, neutrino masses, and models of inflation.
ProtoDESI was a proof of concept commissioned in 2016 to mitigate the risks associated with DESI's challenging instrument design and precision requirements. Its simplified focal plane instrument housed 3 fibre positioners and a fibre photometry camera in place of spectrographs. ProtoDESI was successful as the first on-sky technology demonstration for DESI.
For the official DESI focal plane instrument, the fibre positioning accuracy and, ultimately, the success of DESI, are grounded upon the stringent specifications of the focal plate structure (FPS) which directly holds the positioners. The FPS parts, consisting of ten focal plate petals (FPPs) and a focal plate ring, were fabricated with the required tolerances, comprehensively inspected, and aligned with appropriate shims and gauge blocks to ensure minimal loss of photons at the fibre tips. Adopting a coordinate measurement machine-based approach, we projected the fibre injection efficiency by measuring hardware features and modelling geometric transformations and fibre optics. The as-aligned, total root-mean-square optical throughput for 6168 positioner holes of 12 production FPPs (including two spares) is 99.88% ± 0.12%, well above the 99.5% project requirement.
Finally, observations of galaxy clustering cannot be properly understood alone without accompanying theoretical motivations and numerical simulations in parallel. Cosmological N-body simulations have become indispensable for designing survey strategies, developing analysis methods, and making theoretical predictions. We quantify the shifts of the acoustic scale potentially resulting from galaxy clustering bias, which constitutes an increasingly significant source of theoretical systematics in distance measurements with the standard ruler. Utilising mock catalogues based on generalised halo occupation population of high-accuracy Abacus simulations in the largest volume to date for such tests, 48hâ»ÂčGpcÂł, we find a 0.3% shift in the line-of-sight acoustic scale for one variation in the satellite galaxy population and a 0.7% shift for an extreme level of velocity bias of the central galaxies, while other models tested are consistent with zero shift at the 0.2% level after reconstruction. We note that these bias models produce sizeable and likely distinguishable changes at small scales that correlate with the shifts