Dark Energy Spectroscopic Instrument (DESI)


The Dark Energy Spectroscopic Instrument (DESI) was built by the Department of Energy (DOE) to conduct a Stage-IV dark energy experiment (Instrument Overview Paper). Over the course of the survey (2021-2026), the DESI collaboration will obtain spectra of about 40 million galaxies and quasars as well as 10 million stars (DESI collaboration 2016a,b).

For more complete information about the DESI data, please visit https://data.desi.lbl.gov/doc/. General information about the instrument, experiment and science results can be found at https://www.desi.lbl.gov.

The NOIRLab Astro Data Lab will serve a selected subset of data from each DESI public data release, starting with the Early Data Release (EDR). The main spectroscopic and photometric catalogs are hosted at Data Lab and available for queries, and the spectra are searchable and retrievable from SPARCL as described in the Data Access section. These value-added services are provided for convenience to the astronomy community, with a reminder that any use of DESI data must be accompanied by the official DESI acknowledgments.

Scientific Goals

The DESI collaboration seeks to understand the nature of dark energy and its evolution. In addition, the DESI data enable a suite of other scientific discoveries and research thanks to the legacy value of the immense spectroscopic dataset. The DESI collaboration maintains an ADS library of DESI papers.

DESI Spectra

The DESI collaboration produces one-dimensional (1-D) spectra that have been sky-subtracted, flux-calibrated, and modeled with a best-fitting template to identify the spectral type (spectype) and measure the redshift (z) with the Redrock pipeline (Bailey et al., in preparation). There are a few categories of spectra at different stages of data combination. Namely:

  • Per exposure (spectra from a single exposure at the telescope)
  • Per night (coadded spectra obtained on multiple exposures during the same night)
  • Per tile (coadded spectra obtained for the same tile but possibly from multiple nights)
  • Per healpix (coadded spectra per position on the sky grouped by HEALPix (with n=64); which can be from multiple nights and/or multiple tiles)

Furthermore, the spectra are treated separately per SURVEY:

  • cmx = commissioning
  • sv1 = first phase of Survey Validation (SV)
  • sv2 = operations testing ("dress rehearsal" for sv3)
  • sv3 = one-percent survey (covering 1% of area; final phase of SV)
  • special = dedicated secondary targets or test targets
  • main = main DESI survey (starting with DR1)

And per PROGRAM:

  • dark = dark time for the main targets: Luminous Red Galaxies (LRG), Emission-Line Galaxies (ELG), and Quasi-Stellar Objects / Quasars (QSO)
  • bright = bright time for the Bright Galaxy Survey (BGS) and the Milky Way Survey (MWS)
  • backup = when observing conditions were worse thn the bright conditions
  • other = none of the above

The SPARCL database only includes DESI spectra that have been coadded per healpix. All other spectra are available in a file format directly through the public release at NERSC. See the DESI Data Access page for details.

The following figure illustrates a DESI tile (divided into 10 petals) and a DESI rosette (divided into HEALPix for illustration purposes). The left-hand panel shows the sky positions of the fibers from a single tile (pointing) color-coded by PETAL_LOC values as labeled. The right-hand panel shows the fiber sky positions from Rosette 1, which was obtained by observing the same area of the sky with multiple dithered tiles, color-coded per HEALPIX value (figure credit: S. Juneau / DESI collaboration / NSF’s NOIRLab Astro Data Lab).


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Early Data Release (EDR)

The Early Data Release of DESI contains spectra and catalogs from commissioning and Survey Validation (SV) campaigns (SV Overview paper, EDR paper [in preparation]). The data will become available after the public release by the DESI collaboration.

DESI-EDR Summary
Number of healpix coadded spectra(1) 2,044,588
Number of useful(2) spectra 1,769,157
Number of useful spectra of unique targets 1,712,004
Which include: GALAXY 1,125,635
QSO 90,241
STAR 496,128
Spectral coverage(3) 360-982.4 nm
Spectral resolution R 2000 (at 360 nm) - 5500 (at 980 nm)
Wavelength system Vacuum
Photometric bands (Legacy Surveys DR9) g, r, z, W1, W2, W3, W4
DESI EDR approximate area ~200-250 square degrees
Footnotes: (1) Number of spectra for fibers placed on targets (OBJTYPE="TGT"), which excludes sky fibers and known faulty fibers; (2) Useful spectra defined as having ZWARN=0, which means no errors due to instrument or data and no flags from spectral fitting issues; (3) Original spectra are split on three spectrograph arms: blue (B), red (R), infrared (Z). The version available in the SPARCL database has been coadded across cameras into a single array spanning the full BRZ spectral range.

The catalog tables listed below include a redshift catalog per healpix (zpix; recommended for most analyses) and a redshift catalog per tile (ztile; can be useful for instrument-dependent analyses). We consider the zpix catalog as the primary spectroscopic catalog (analogous to specObjAll for SDSS) but some of the information such as photometry and observing conditions need to be obtained by joining with other catalog tables.

The desi_edr.zpix table has been crossmatched against our default reference datasets within a 1.5 arcsec radius, nearest neighbor only. These tables will appear with x1p5 in their name in our table browser. Example: desi_edr.x1p5__zpix__gaia_dr3__gaia_source.

Table Name Description
exposure Summary quantities for every DESI exposure
fiberassign Quantities obtained when a DESI target is assigned to a fiber
frame Summary quantities for each petal of the DESI instrument in a given exposure; in normal operation there are ten frames for every exposure
photometry Photometric quantities from LS DR9 for every TARGETID
potential For a given tile, this table lists all targets that could have received a fiber assignment
target The quantities obtained when photometric objects are analyzed in the target selection process
tile Summary quantities for every DESI tile (pointing on the sky)
zpix Redshift and other quantities for spectra that have been grouped and coadded by HEALPixel
ztile Redshift and other quantities for spectra that have been grouped and coadded by tile
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