---
title: DESI Spectroscopic Data Release
url: https://www.emergentmind.com/topics/dark-energy-spectroscopic-instrument-desi-data-release
type: topic
---

# DESI Spectroscopic Data Release

The Dark Energy Spectroscopic Instrument (DESI) data releases comprise well-documented, multi-faceted spectroscopic datasets enabling precision cosmology, stellar and galaxy evolution, and Milky Way archaeology. DESI’s modular data products and pipeline outputs facilitate a broad range of analyses, all delivered with rigorously quantified performance and public accessibility.

## 1. Data Release Scope and Survey Overview

The DESI Early Data Release (EDR) encompasses spectra acquired during a five-month Survey Validation phase (December 2020–May 2021), spanning commissioning, target validation, operational development, and the One-Percent Survey, supplemented by secondary programs and special calibration tiles. EDR contains spectra for:

| Class                        | Good Unique Spectra |
|------------------------------|--------------------|
| Milky Way Survey (Star)      | 466,447            |
| Bright Galaxy Survey (BGS)   | 428,758            |
| Luminous Red Galaxy (LRG)    | 227,318            |
| Emission Line Galaxy (ELG)   | 437,664            |
| Quasar (QSO)                 | 76,079             |
| Secondary Programs           | 137,148            |

Total: 1,712,004 good spectra; including stars (496,128), galaxies (1,125,635), and quasars (90,241) [2306.06308].

The release includes flux- and wavelength-calibrated 1D spectra, intermediate and raw products, redshift and classification catalogs, and value-added catalogues across the Milky Way, galaxy, and quasar target classes [2306.06308, 2407.06280, 2505.14787].

## 2. Spectroscopic Data Pipeline and Calibration

The DESI spectroscopic pipeline processes CCD exposures from 10 spectrographs (30 cameras), each spanning blue (3600–5930 Å, R≈2000–3200), red (5600–7720 Å, R≈3200–4100), and NIR-Z (7470–9800 Å, R≈4100–5100) arms [2209.14482].

Pipeline stages include:
- **CCD Preprocessing:** bias, gain, cosmic-ray masking.
- **Wavelength Calibration:** arc-lamp exposures fit with Legendre polynomials; barycentric correction.
- **Flux Calibration:** F-type standard stars yield throughput vectors $C(\lambda)$, applied to all target spectra. PSF corrections accommodate fiber aperture and seeing.
- **Sky Subtraction:** 10% fibers allocated to blank sky; high-res sky models fit and subtracted iteratively, including 2D spatial polynomial gradients.
- **Extraction:** 2D PSFs modeled in Gauss–Hermite basis, “spectro-perfectionism” yields decorrelated 1D spectra with resolution matrix.
- **Redshift and Classification:** PCA-based template fits via Redrock, emission-line afterburners, and custom fitting (QuasarNET, Mg II).

Quality assurance monitors PSF stability (day–night drift <0.3 pixels), wavelength accuracy (<0.03 Å per arm), and radial velocity scatter (≤0.8 km s⁻¹), with overall catalog efficiency and purity >99% for all major target classes [2209.14482].

## 3. Lyman-α Forest Data Products and Methodology

The DESI EDR Lyman-α forest catalog leverages 88,511 quasar spectra over SV and M2, corresponding to redshift 2.1 ≤ z ≤ 3.79 and ~0.5 Gpc³ survey volume [2306.06312]. Each spectrum is coadded on a linear grid (Δλ = 0.8 Å).

Pipeline workflow:
- **Masking:** cosmic rays (0.1%), CCD defects, DLA regions (CNN+GP), BALs (16% of EDR+M2), and sky/ISM lines.
- **Flux Recalibration:** calibration wiggles corrected in emission-line–free regions (CIII 1600–1850 Å); DESI requires only a single recalibration step.
- **Continuum Fitting:** iterative (5×) determination of mean flux $F̄(λ) C_q(λ)$ using both universal template and per-forest slopes; pixel variance modeled as $\sigma^2_q(λ)/[F̄ C_q]^2 = η(λ)\,\tildeσ^2_\mathrm{pip,q}(λ) + σ^2_\mathrm{LSS}(λ)$.
- **Weighting:** diagonal weights $w_q(λ) = [η(λ)\,\tildeσ^2_\mathrm{pip,q}(λ) + σ^2_\mathrm{mod}]^{-1}$ with empirically optimized modulation ($σ^2_\mathrm{mod}=7.5$ units of $〈δ^2〉$) reducing auto-correlation error bars by ~25%.

HEALPix-organized VAC FITS files contain observed wavelengths, per-forest metadata, $\delta_q(λ)$ arrays, weights, and continua, supporting direct measurement of 1D/3D Lyα forest correlations and BAO features. DESI achieves >20% error reduction compared to SDSS/eBOSS by weighting optimization alone [2306.06312].

## 4. Quasar Catalogs, Broad Absorption Lines, and Redshift Determination

DESI EDR quasar samples are delivered as flux-calibrated, sky-subtracted spectra, with redshift and classification catalogs. BAL quasars are catalogued (29,985 in EDR+M2, 14.3% of quasars with AI > 0 in CIV), with absorption metrics (Balnicity Index BI, Absorption Index AI) computed from PCA-fitted continua. Automated BAL identification achieves purity ≈99%, completeness ≈45% at median SNR [2309.03434].

Redshift corrections for BALs involve masking absorption troughs and refitting templates, yielding mean shifts of $〈Δv〉=+243$ km s⁻¹. Catastrophic redshift errors ($|Δv|>1000$ km s⁻¹) are reduced from 6.7% to <2% post-masking. Incorporation of accurate BAL redshifts increases usable quasar tracers by ~19% and improves Lyα forest correlation function errors by ≈12% without introducing systematics [2309.03434].

## 5. Stellar Value-Added Catalogues and Galactic Structure Applications

The Milky Way Survey value-added catalogs (VACs) for DESI EDR and DR1 provide atmospheric parameters, radial velocities, and abundances for $\sim$400,000 (EDR) and $\gtrsim$4 million (DR1) stars, with full epoch-by-epoch radial velocity measurements available for >1 million stars [2407.06280, 2505.14787].

- **Parameter Measurement:** Two independent pipelines—RVSpecFit (forward model, PHOENIX neural nets) and FERRE (χ² fitting, Kurucz grids)—yield precisions of:
    - $Δv_r$ ≃ 1 km s⁻¹
    - $Δ\log g$ ≃ 0.3 dex
    - $Δ[\mathrm{Fe}/\mathrm{H}]$ ≃ 0.15 dex (EDR); $≲0.1$ dex post-calibration (DR1)
    - $Δ [\alpha/\mathrm{Fe}] ≲ 0.1$ dex, reliable for $[\mathrm{Fe}/\mathrm{H}] \gtrsim -2$
- **Calibration:** Systematic trends in $[\mathrm{Fe}/\mathrm{H}]$ removed using temperature/gravity-dependent formula: 
    $$
    [\mathrm{Fe}/\mathrm{H}]_{\mathrm{calib}} = [\mathrm{Fe}/\mathrm{H}]_{\mathrm{DESI}} - [a + b\,x + c\,x^2]
    $$
    with $x = 10\log_{10}(T_{\rm eff}/5000\,{\rm K})$, and separate coefficients for giants/dwarfs [2505.14787].
- **Catalogue Structure:** Main FITS includes RVTAB (velocities, parameters), SPTAB (abundances), FIBERMAP (photometry, astrometry, flags), and Gaia cross-match. Recipe for high-purity samples involves RVS_WARN=0, spectral type STAR, PRIMARY=True, and S/N cuts.
- **Galactic Components:** Thin/Thick disk, stellar halo mapped; >50,000 stars at distances >10 kpc, >3,000 at >50 kpc, thousands of extremely metal-poor stars ($[\mathrm{Fe}/\mathrm{H}] < -3$), and spectroscopic members of >4 dwarf galaxies, >13 globular clusters, and >24 streams [2505.14787].
- **Scientific Impact:** Chemo-dynamic mapping, rare object populations, mass constraints from halo tracers; DESI DR1 increases faint star statistics (G = 17.5–21) by ×10 over prior surveys.

## 6. Large-Scale Structure Catalogs and Cosmological Utility

DESI releases full and clustering galaxy catalogs (BGS, LRG, ELG, QSO) for large-scale structure (LSS) studies. Completeness and FKP weighting schemes optimize clustering analyses. One-Percent Survey catalogs report completeness values in the 86–98% range across tracers and cover ~170 deg² [2306.06308]. Selection cuts combine redshift-fit confidence (Δχ² > 9), [OII] flux significance, and optimized FKP weights.

Correlation function and BAO measurements leverage the improved Lyα pipeline and quasar redshift solutions. DESI EDR+M2 Lyα forests already demonstrate 20% smaller auto-correlation errors than eBOSS DR16 in identical volume, with the full DESI survey poised to deliver sub-percent-level BAO precision [2306.06312, 2306.06311].

## 7. Access, Tools, and Future Releases

All DESI data products are publicly accessible via HTTPS (https://data.desi.lbl.gov/public), Globus/NERSC, and database/SQL interfaces (Astro Data Lab, PostgreSQL). File organization uses per-tile and HEALPix directories for efficient queries. Tutorials and APIs (specprodDB, SQLAlchemy) are provided for programmatic access [2306.06308].

Future DESI releases will scale to larger samples ($\gtrsim12$ M stars in DR2, anticipated 2027) and incorporate enhanced coadds, recalibrated pipelines, and expanded secondary target coverage, further improving survey utility for Milky Way structure and precision cosmology [2505.14787].

Source: https://www.emergentmind.com/topics/dark-energy-spectroscopic-instrument-desi-data-release