Glossary¶

Backup (program)¶

Observations taken when conditions are worse than bright.

BGS¶

Bright Galaxy Survey¶

Nearby bright galaxies observed as part of the bright program of the Main Survey. These are observed on the same tiles as the Milky Way Survey. For more information about the Bright Galaxy Survey see the papers here.

Bright (program)¶

Observations taken when the survey speed is 2.5–6\(\times\) worse than nominal dark conditions.

Camera¶

One CCD of one spectrograph. Cameras are identified by their wavelength coverage (b, r, or z) and their spectrograph number (0-9), e.g. b0, r1, z9.

Coadd¶

The combination of multiple exposures of an object into a single spectrum for convenience. The DESI spectroscopic pipeline coadds across exposures but not across cameras.

Commissioning¶

The initial period of DESI instrument testing and verification, prior to the start of Survey Validation.

cmx¶

Short for commissioning, used in file, mask, and code variable names.

Dark (program)¶

Observations taken when the survey speed is better than 2.5\(\times\) of nominal conditions.

DR1¶

Short for Data Release 1, covering the first year of DESI Main Survey data. DR1 is not yet public.

EDR¶

Short for Early Data Release, the first public DESI spectroscopic data release.

Effective Exposure Time¶

The amount of time that it would have taken to achieve this level of signal-to-noise if the observation had been taken under nominal dark conditions survey speed. Often abbreviated EFFTIME in code and data table column names.

ELG¶

Emission Line Galaxy, the most numerous DESI dark time target, primarily targeting between redshifts 0.6 and 1.6. These are observed on the same dark program tiles as LRGs and QSOs. For more information about ELGs see the papers here.

Fiber¶

An individual fiber optic from a positioner on the focal plane to a spectrograph. Fibers are numbered sequentially from 0 to 4999 on the spectrograph side such that spectrograph number spN = int(fiber/500). On the focal plane side each spectograph of fibers maps to a single petal, but the locations are purposefully randomized to dis-ambiguate systematic effects due to focal plane location versus spectrograph slit location.

Focal Plane¶

The collection of 5,000 science positioners with their fibers, 20 sky brightness monitor fibers, Guide-Focus Assemblies (GFAs), arranged in 10 petals of 500 positioners each.

Frame¶

Data from one spectrograph camera from one exposure.

LRG¶

Luminous Red Galaxies, targeted between redshifts ~0.2 and 1.0. These are observed on the same dark program tiles as ELGs and QSOs. For more information about LRGs see the papers here.

Main (survey)¶

Observations taken as part of the 5-year DESI science survey.

Milky Way Survey¶

Stellar targets observed as part of the bright program of the Main Survey. These are observed on the same tiles as the Bright Galaxy Survey. For more information about the Milky Way Survey see the papers here.

MWS¶

Short for Milky Way Survey.

Positioner¶

A two arm moveable robot holding a DESI fiber on the focal plane.

Petal¶

A wedge of 1/10 of the DESI focal plane, containing 500 positioners/fibers going to a single spectrograph. The petal number is the same as the spectrograph number to which it is attached.

Program¶

DESI Surveys are split into sub-programs (dark, bright, backup, other) depending upon the observing conditions.

QSO¶

Quasars, the most distant objects observed by DESI, mostly between redshifts ~0.6 and 3.5. These are observed on the same dark program tiles as ELGs and LRGs. For more information about DESI quasars see the papers here.

Redrock¶

The spectroscopic classification and redshift fitting pipeline for DESI spectra.

Rosette¶

One of 20 non-overlapping subfields that compose the total observation field of the One-Percent Survey (sv3). Each rosette was observed at least 12 times during the One-Percent Survey to ensure high a fiber assignment completeness of at least 85% for ELGs and over 94% for the other tracers (BGS, LRGs, and QSOs).

Secondary Targets¶

Targets that are beyond the scope of the core DESI science goals, but which can be used as low-priority filler targets for otherwise unused fibers.

Spectra¶

Flux vs. wavelength, including an error model (inverse variance), mask bits (0=good, non-zero encodes what went wrong), and a Resolution Matrix modeling the effective instrument resolution per fiber per wavelength.

Special (survey)¶

Observations of tiles designed for purposes that don’t fit within one of the other survey designations, e.g. test observations and custom observations of dedicated secondary programs.

Spectrograph¶

DESI has 10 spectrographs, each measuring 500 fibers in 3 different wavelength bands: B (~3600–5800 Å), R (~5760–7620 Å), and Z (~7520–9824 Å).

Survey¶

DESI observations are split into multiple “survey” phases: Commissioning (cmx), Survey Validation (sv1, sv2, sv3), Main, and Special.

Survey Speed¶

How long it takes to observe a reference amount of signal-to-noise, normalized to a nominal 1000 second exposure at zenith with 1.1” FWHM seeing of a non-dust-extincted (E(B - V) = 0) location on a photometric night with no moon. Sky brightness, transparency, seeing, and dust-extinction can make the survey speed slower or faster than nominal.

DESI science programs are selected based upon the current survey speed (e.g. switching to brighter targets during worse conditions), and exposure times are dynamically determined to achieve uniform depth for each tile.

Survey Validation¶

DESI Observations between December 2020 and June 2021, validating DESI target selection and survey operations procedures prior to beginning the Main Survey.

SV¶

Short for Survey Validation. DESI Survey Validation observations consist of three phases: Target Selection Validation (sv1), Operations Testing (sv2), and the One-Percent Survey (sv3).

SV1¶

Survey Validation observations to validate target selection algorithms. Target selection cuts were purposefully broader than the Main Survey, and individual tiles were observed to much greater depth than the Main Survey. Also known as Target Selection Validation.

SV2¶

Survey Validation observations as a “dress-rehearsal” for the One-Percent Survey (sv3). Also known as Operations Testing.

SV3¶

Survey Validation observations to validate survey operations procedures and final target selection updated after Target Selection Validation (sv1). Also known as the One-Percent Survey.

TARGETID¶

A unique 64-bit identifier for each astronomical object observed by DESI.

Tile¶

A single DESI pointing on the sky with assignments of which fibers should observe which targets. Tile centers are pre-determined in an overlapping pattern such that a random location within the DESI footprint has an average of ~5 overlapping tiles (for Main Survey and dark program observations), both to give greater spatial density (e.g. ELGs) and to enable repeat observations of the same targets (e.g. QSOs).

TSNR2¶

Template Signal-to-Noise Squared. A signal-to-noise metric weighted by what wavelengths matter most for determining the redshift of DESI targets, given their magnitude and redshift distributions. This depends upon target class, e.g. Lyman-alpha QSO TSNR2 more heavily weights blue wavelengths, while ELG TSNR2 more heavily weights redder wavelengths which cover the emission lines for the DESI redshifts of interest. TSNR2 depends upon the noise properties of individual spectra, but not the signal properties of the target.