---
title: 'DESI: Dark Energy Spectroscopic Instrument Survey'
url: https://www.emergentmind.com/topics/dark-energy-spectroscopic-instrument-survey
type: topic
---

# DESI: Dark Energy Spectroscopic Instrument Survey

The Dark Energy Spectroscopic Instrument Survey (DESI) is a Stage-IV, massively multiplexed spectroscopic campaign conducted on the 4-meter Mayall Telescope at Kitt Peak. The survey is designed to map the large-scale structure of the Universe via precise measurements of baryon acoustic oscillations (BAO) and redshift-space distortions (RSD) across 14,000 deg² and a redshift range 0 < z < 3.5, targeting ∼50 million extragalactic and Galactic sources over five years. DESI aims to provide percent-level constraints on dark energy, cosmic expansion history, neutrino mass, and primordial density fluctuations, leveraging a robust instrumentation platform and a suite of ultra-efficient pipelines for survey operations, targeting, astrometry, spectral extraction, redshift estimation, and exposure control [1611.00036][1611.00037][2306.06309][2306.06308][2208.08518][2209.14482][2405.19288][2101.11736][2407.08026][2307.06238].

## 1. Survey Design and Targeting Principles

The DESI Survey’s core design divides observing time into “dark” and “bright” programs dependent on moon phase and seeing. Primary dark-time targets are emission-line galaxies (ELGs), luminous red galaxies (LRGs), and quasi-stellar objects (QSOs), optimized for BAO and RSD science. Bright time is dedicated to the Bright Galaxy Survey (BGS) and Milky Way Survey (MWS). Secondary programs (encoded in SCND_TARGET bitmasks) utilize fibers for bespoke targets such as supernova hosts, clusters, and high-z QSOs [2208.08518].

Survey targeting utilizes the “desitarget” pipeline, which processes all imaging-based targets and assigns unique TARGETIDs via a hierarchical 64-bit signed integer scheme:
\[
\mathrm{TARGETID} = \mathrm{OBJID} + \mathrm{BRICKID}\cdot2^{28} + \mathrm{RELEASE}\cdot2^{46} + \mathrm{MOCK}\cdot2^{58}+\mathrm{SKY}\cdot2^{59}+\mathrm{GAIADR}\cdot2^{60}
\]
Positive TARGETIDs cover all real and simulated objects, while negative TARGETIDs are used for on-the-fly sky assignment to broken fibers [2208.08518].

Multiple bitmasks within targeting files encode selection flags:
- DESI_TARGET, BGS_TARGET, MWS_TARGET, SCND_TARGET: each 64-bit mask flags target type/class.
- OBSCONDITIONS: encodes DARK/BRIGHT/BACKUP status per target [2208.08518].

Survey targeting is phase-dependent, with separate masking for CMX (commissioning), SV1/SV2/SV3 (survey validation), and main survey [2208.08518][2306.06308][2306.06307].

## 2. Instrumentation: Focal Plane, Spectrograph, and Metrology

DESI is built around a six-element fused-silica corrector, providing a 3.2° diameter focal plane (0.8 m diameter), yielding a plate scale of ~14–18″/mm. The focal plane hosts 5,000 robotic “theta–phi” positioners, each capable of placing a 107 μm fiber with RMS lateral accuracy ~8–11 μm (2-D), verified through laboratory metrology and on-sky dither astrometry [1611.00037][2307.06238][1710.08875].

Fiber positioning leverages a closed-loop system with back-illumination and the Fiber View Camera (FVC), which images fiber tips and fiducials through the corrector. Astrometric transformations from sky to focal plane are characterized with distortion models using spin-weighted Zernike polynomials and adaptive compensation for thermal expansion, flexure, and atmospheric dispersion [2307.06238][2407.08026].

Each fiber feeds one of ten identical three-arm spectrographs, with dichroic splits at 553 nm and 752 nm, covering 360–980 nm. The spectral resolution varies:
- Blue: R ≈ 2,000 at 360 nm, rising to ~3,000 at 5800 Å
- Red: R ≈ 3,000–4,000
- NIR: R ≈ 4,000–5,500 to 980 nm
This throughput enables reliable [O II] doublet and LRG continuum measurements for targeted redshifts [1611.00037][2306.06308].

## 3. Survey Operations, Tiling, and Exposure Control

DESI employs a “depth-first,” overlapping-tile strategy: 9,929 “dark” tiles (7 passes) and 5,676 “bright” tiles (4 passes) over the 14,246 deg² footprint, yielding mean coverage of ~5.2 passes per dark tile and ~3.2 for bright tiles [2306.06309].

Nightly observing is orchestrated via automated field selection, based on real-time survey speed S = (S/N)^2, airmass, tile priority, moon avoidance, and weather-driven masks. Exposure time is dynamically set by the Exposure Time Calculator (ETC), absorbing inputs from guiders (PSF, transparency) and the Sky Continuum Monitor, which measures sky background in real time using discrete “sky fibers.” The ETC ensures a pre-defined SNR threshold for all field types, adjusting for airmass:
\[
t_{\rm exp}(X) = t_0 X^{1.25}
\]
Uniform effective exposure time (EFFTIME_ETC) is maintained across conditions, maximizing survey speed and scientific homogeneity. Observing overheads are ≲2 min per tile, driven by fiber repositioning, plate acquisition, and readout [2306.06309][2101.11736][2306.06308].

Performance metrics from early survey operations indicate the survey is running ∼7% faster than forecast, with observed mean survey speed S ≈ 1.15× nominal [2306.06309].

## 4. Data Reduction, Spectral Extraction, and Redshift Determination

Raw CCD data (30 amplifiers, ten spectrographs × three arms) are processed through a hierarchical pipeline:
- CCD calibration: bias, overscan, gain, cosmic-ray masking
- PSF mapping, wavelength calibration (Legendre polynomials per pixel row)
- Spectroperfectionism: forward-modeling fiber-trace extraction, yielding a decorrelated resolution matrix R and uncorrelated fluxes
- Flat-fielding: per-fiber throughput correction using dome flats
- Sky modeling: least-squares fit of a deconvolved sky spectrum on dedicated sky fibers, reconvolved to each science fiber
- Spectrophotometric calibration: F-star standards, color matching, and aperture corrections
- Redrock: redshift and classification via χ²-matching of observed spectra to PCA or archetype-based templates, supplemented for QSOs by QuasarNET and Mg II line search [2209.14482][2405.19288].

Recently, an archetype-based redshift fitter refines Redrock performance for galaxy spectra, reducing catastrophic failures by 10–40%, improving purity by 0.1–0.3% for LRGs/ELGs, and decreasing sky-fiber false positives by 5–40%, via bounded-variable least squares over physical templates and per-camera Legendre polynomials. Computational cost increases by a factor 2–3, yet remains tractable at scale [2405.19288].

All coaddition and model-fitting steps maintain strictly uncorrelated noise properties by avoiding wavelength resampling and rigorously tracking the resolution matrix R per spectrum [2209.14482].

## 5. Validation Campaigns, Data Model, and Catalog Products

Survey Validation (SV) campaigns—SV1, SV2, and SV3 (One-Percent Survey)—established reproducible target selection, exposure strategies, completeness mapping, and empirical calibration for main operations, covering 140 deg² in the “One-Percent” pilot. SV data informed the final selection cuts and operational workflows, ensuring dark-energy science requirements for sample sizes, completeness, and n(z) are met [2306.06307][2306.06308].

Data products are organized into FITS and HDF5 formats and include:
- Raw and calibrated exposures
- Single-tile and HEALPix coadded spectra
- Redrock redshift catalogs with ZWARN, DELTACHI2, and ZCAT_PRIMARY flags
- Value-added catalogs (VACs) including visual-inspection redshifts, photometry cross-matches, and large-scale structure science weights (“completeness” w_comp; FKP weights) [2306.06308].

Quality cuts typically impose ZWARN = 0 (no pipeline failures) and DELTACHI2 thresholds (e.g. >15 for LRG, >40 for BGS, [O II]-Δχ² composite for ELG) [2306.06308].

## 6. Astrometry, Fiber Metrology, and Positioning Systematics

Astrometric calibration is achieved via integrated software modules: PlateMaker (sky-to-focal plane conversion), spotmatch (FVC centroid extraction), turbulence-correction for dome seeing, and dither analysis for throughput maximization. Transformations from (α, δ) to focal-plane (x, y) employ gnomonic projections and non-linear Zernike-based distortion models with 13 coefficients, including ADC-induced B-modes.

Positioning errors due to atmospheric turbulence are mitigated via Gaussian-process-based correlation modeling using stationary fiducials and disabled positioners as turbulence probes. This correction reduced RMS errors from 7.3 μm (raw) to 3.5 μm (corrected), yielding ~1.6% survey speed improvement by decreasing miscentering loss under typical conditions [2407.08026][2307.06238].

Final repeatable positioning accuracy is ~8 μm (1-D) and 11 μm (2-D), with negligible impact on flux calibration uniformity and spectroscopic redshift performance [2307.06238].

## 7. Cosmological Deliverables, Forecasts, and Legacy

DESI’s forecasted performance includes sub-percent precision on the BAO ruler and percent-level RSD growth-rate measurements in multi-redshift bins:
- σ_R/R < 0.28% for z < 1.1
- σ_R/R < 0.39% for 1.1 < z < 1.9
- σ_H/H < 1.05% for 1.9 < z < 3.7

Aggregate Fisher-matrix constraints anticipate:
- σ(w₀) ≈ 0.04, σ(w_a) ≈ 0.3
- σ(fσ₈(z)) ~1–2% per bin
- σ(∑m_ν) ≈ 0.02 eV
- DETF Figure-of-Merit (FoM) ≈ 150–250 (BAO+RSD)
Compared to prior surveys, DESI delivers ~6× gain in FoM, with sample sizes of 13.8 M BGS, 7.46 M LRG, 15.7 M ELG, 2.87 M QSO, and 7.2 M MWS objects [2306.06307][1611.00036][1308.0847].

The survey’s full data will deliver a 3-D map of ~35 M galaxies, ~3 M quasars, and ~7 M Milky Way stars, supporting transformative studies of cosmic acceleration, structure growth, neutrino masses, and cosmological initial conditions. Public and collaboration data releases follow robust QA procedures, with all primary science catalogs, resolution matrices, and quality flags documented and supported in the Astropy ecosystem [2306.06308][2209.14482].

## References

- "The Target-selection Pipeline for the Dark Energy Spectroscopic Instrument" [2208.08518]
- "ProtoDESI: First On-Sky Technology Demonstration for the Dark Energy Spectroscopic Instrument" [1710.08875]
- "The DESI Experiment, a whitepaper for Snowmass 2013" [1308.0847]
- "Unraveling the Universe with DESI" [1901.01581]
- "The DESI Experiment Part I: Science, Targeting, and Survey Design" [1611.00036]
- "The Spectroscopic Data Processing Pipeline for the Dark Energy Spectroscopic Instrument" [2209.14482]
- "The Early Data Release of the Dark Energy Spectroscopic Instrument" [2306.06308]
- "Validation of the Scientific Program for the Dark Energy Spectroscopic Instrument" [2306.06307]
- "The DESI Sky Continuum Monitor System" [2101.11736]
- "The DESI Experiment Part II: Instrument Design" [1611.00037]
- "Correcting Turbulence-induced Errors in Fiber Positioning for the Dark Energy Spectroscopic Instrument" [2407.08026]
- "Astrometric Calibration and Performance of the Dark Energy Spectroscopic Instrument Focal Plane" [2307.06238]
- "Archetype-Based Redshift Estimation for the Dark Energy Spectroscopic Instrument Survey" [2405.19288]
- "Survey Operations for the Dark Energy Spectroscopic Instrument" [2306.06309]

Source: https://www.emergentmind.com/topics/dark-energy-spectroscopic-instrument-survey