3 research outputs found
Focal Plate Structure Alignment of 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
Oscillation (BAO) technique. The spectra of 35 million galaxies and quasars
over will be measured during the life of the experiment.
A new prime focus corrector for the KPNO Mayall telescope will deliver light to
5000 robotically positioned optic fibres. The fibres in turn feed ten broadband
spectrographs. Proper alignment of focal plate structure, mainly consisting of
a focal plate ring (FPR) and ten focal plate petals (FPP), is crucial in
ensuring minimal loss of light in the focal plane. A coordinate measurement
machine (CMM) metrology-based approach to alignment requires comprehensive
characterisation of critical dimensions of the petals and the ring, all of
which were 100% inspected. The metrology data not only served for quality
assurance (QA), but also, with careful modelling of geometric transformations,
informed the initial choice of integration accessories such as gauge blocks,
pads, and shims. The integrated focal plate structure was inspected again on a
CMM, and each petal was adjusted according to the updated focal plate metrology
data until all datums were extremely close to nominal positions and optical
throughput nearly reached the theoretically best possible value. This paper
presents our metrology and alignment methodology and complete results for
twelve official DESI petals. The as-aligned, total RMS optical throughput for
6168 positioner holes of twelve production petals was indirectly measured to be
, well above the 99.5% project requirement. The successful
alignment fully demonstrated the wealth of data, reproducibility, and
micron-level precision made available by our CMM metrology-based approach.Comment: 17 pages, 10 figures, 3 table
DESI Mock Challenge: Halo and galaxy catalogs with the bias assignment method
We present a novel approach to the construction of mock galaxy catalogues for
large-scale structure analysis based on the distribution of dark matter halos
obtained with effective bias models at the field level. We aim to produce mock
galaxy catalogues capable of generating accurate covariance matrices for a
number of cosmological probes that are expected to be measured in current and
forthcoming galaxy redshift surveys (e.g. two- and three-point statistics). We
use the bias assignment method (BAM) to model the statistics of halo
distribution through a learning algorithm using a few detailed -body
simulations, and approximated gravity solvers based on Lagrangian perturbation
theory. Using specific models of halo occupation distributions, we generate
galaxy mocks with the expected number density and central-satellite fraction of
emission-line galaxies, which are a key target of the DESI experiment. BAM
generates mock catalogues with per cent accuracy in a number of summary
statistics, such as the abundance, the two- and three-point statistics of halo
distributions, both in real and redshift space. In particular, the mock galaxy
catalogues display accuracy in the multipoles of the power
spectrum up to scales of . We show that covariance
matrices of two- and three-point statistics obtained with BAM display a similar
structure to the reference simulation. BAM offers an efficient way to produce
mock halo catalogues with accurate two- and three-point statistics, and is able
to generate a variety of multi-tracer catalogues with precise covariance
matrices of several cosmological probes. We discuss future developments of the
algorithm towards mock production in DESI and other galaxy-redshift surveys.
(Abridged)Comment: Accepted for publication at A&
Integration and testing of the DESI multi-object spectrograph: performance tests and results for the first unit out of ten
International audienceThe 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 deg² 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. A consortium of Aix-Marseille University (AMU) and CNRS laboratories (LAM, OHP and CPPM) together with LPNHE (CNRS, Universities Pierre et Marie Curie and Paris-Diderot) and the WINLIGHT Systems company based in Pertuis (France), are in charge of integrating and validating the performance requirements of the full spectrographs. This includes the cryostats, shutters and other mechanisms. The first spectrograph of the series of ten has been fully tested and the performance requirements verified for the following items: focus, image quality, straylight, stability, detector properties and throughput. We present the experimental setup, the test procedures and the results