Glossary of SDSS Terminology

This Glossary is a work in progress, and more terms will be added during the lifetime of SDSS-V. If you have come across SDSS Terminology that is not explained yet in the Glossary below, then please let us know via the SDSS Helpdesk at


Adaptive Moment
A method of measuring object shapes (i.e., fitting those shapes to ellipses). These moments are close to optimal for measuring the shapes of faint galaxies, and have been used for weak lensing studies
Alfred P. Sloan Foundation
A philanthropic nonprofit institution ( established in 1934 by Alfred Pritchard Sloan, Jr., then President and Chief Executive Officer of the General Motors Corporation. An early award from the Sloan Foundation was recognized by naming the survey the Sloan Digital Sky Survey. The Sloan Foundation has continued to be a major supporter of all four phases of the SDSS.
Apache Point Observatory (APO)
Apache Point Observatory, located in Sunspot, New Mexico. This is the location of the 2.5m SDSS telescope as well as the Photometric Telescope (PT), as well as other non-SDSS telescopes, such as the ARC 3.5m telescope. See the APO homepage.
High resolution (R~22,500) H-band spectrographs fed by 300 fibers. Named for the SDSS-III Apache Point Observatory Galactic Evolution Experiment, for which they were developed.
In the FPS context, an assignment refers to “assigning” a target (specifically a carton-to-target entry) to a robot and spectrograph.
Astra is the software that automates all data analysis for the Milky Way Mapper. Astra takes reduced spectra (from the data reduction pipeline) as inputs, and includes numerous pipelines for analysing different types of stellar spectra.
AQMES (Apache Point Observatory Quasar Multi Epoch Survey)
BHM program acquiring few-epoch (2-12) BOSS spectra for known SDSS AGN/QSOs, probing restframe timescales from ~weeks to ~decades.


Black Hole Mapper (BHM)
One of the two SDSS-V MOS programs, focused primarily on programs collecting optical BOSS spectra of QSOs and AGN during dark time.
The telescope control computer keeps track of a specific point in the telescope focal plane that is called the boresight. The boresight is not fixed in the array but is at one of two places for the two strips that compose a stripe. It is the boresight that will track great circles. Technically, the boresight tracks a path that is a great circle in J2000 coordinates as viewed from the solar system barycenter.
Medium resolution (R~2000) optical spectrographs fed by 500 fibers. Named for the SDSS-III Baryon Oscillation Spectroscopic Survey for which they were upgraded from the original SDSS spectrographs.


Time domain spectroscopy is a critical component of SDSS V. In the FPS context, a cadence specifies the timing between observations (“epochs”) of a field. Cadences typically specify a minimum and maximum time between observations, the maximum length that a series of exposures (one “epoch”) can span, the number of exposures per “epoch”, and observing parameters per epoch such as maximum moon illumination.
The process by which the photometric and spectroscopic observations are calibrated. The goal of calibration is to take the digital camera readouts and convert them to measured quantities like fluxes. The details of these procedures can be found on the algorithms pages.
A set of astronomical targets with a shared set of selection criteria and observing requests (ie, cadence requirements). This set of targets is collectively known as a ‘carton’. The target to carton associations, as well as the set observing requirements, are stored and tracked in the targeting database. The name “carton” was a suggestion from Adam Leroy, who was asked by Jennifer Johnson about a better name for Milky Way Mapper targets from a selection function than “subprograms.
CAS (Catalog Archive Server)
The Catalog Archive Server is a searchable database that contains the measured parameters from all objects in the imaging survey and the spectroscopic survey.
A batch (asynchronous) interface to the CAS for querying the database and storing results. CasJobs is designed to handle especially long or otherwise demanding queries.
Catalog Database / Catalog DB
An SQL database schema containing all parent catalogs from which SDSS-V FPS targets are selected. Examples of parent catalogs stored in Catalog DB are the Gaia, 2MASS, TESS, PanSTARRs and LegacySurvey photometric catalogs, as well as the DR17 SDSS photometric and spectroscopic catalogs. catalogdb also contains the results of the catalog cross-match, including a catalog table with the list of unique astronomical sources (each identified with a Catalog ID), and relational tables associating them with each one of the parent catalogs.
Catalog ID
A unique identifier for every astronomical object stored in catalogDB as a result of catalog cross-matching.
A descriptor that identifies the highest level purpose of observing a spectroscopic target: valid categories are science, standard, or sky.
Also known as “known problems.” SDSS maintains a list of issues or imperfections of the data and/or its processing. We have a list of imaging caveats, (optical) spectroscopic caveats, and infrared spectroscopic caveats.
COINS (Committee On INclusiveness in SDSS)
COINS was formed to address issues having to do with diversity, inclusion, and equality in the survey. Our broad goals are to assess the SDSS project and collaboration’s climate and demographics, to recommend to the collaboration management new policies or practices with regard to increasing inclusiveness, and to assist in the implementation of these new activities where necessary. We publicize our findings to the collaboration and ensure that resources that promote inclusivity are widely accessible. We aim to provide the collaboration tools to improve the climate of equity and inclusion. Some of the committee’s materials can be found in their
In the FPS context, a configuration refers to a specific set of positions for all 500 robots, along with the target assignments for each robot. Configurations are created from designs, and a new configuration is created every time a design is loaded on the FPS.
The SDSS uses several different coordinate systems. We use standard astronomical right ascension (RA) and declination (Dec), J2000. There is also the survey coordinate system, with coordinates λ and η. This is just a rotation of the usual RA, Dec system. There is a variant on this known as corrected survey coordinates with different ranges for λ and η. There is the great circle coordinate system, which is actually a separate coordinate system for each stripe. The coordinates of this system are μ and ν. For APOGEE infrared spectra in Data Release 10 and later, we also use galactic coordinates: galactic longitude (L) and galactic latitude (B). (Note that our convention is to use capital letters to symbolize galactic coordinates to avoid confusion between the lowercase letter l and the number 1). The algorithms pages have details on all these coordinate systems.
The process, and product, of linking astronomical objects across parent catalogs. This is performed by either following catalog identifier links provided by the parent catalogs (i.e, associating Legacy Survey DR8 sources with Gaia DR2 objects using the relationship provided in the Legacy Survey catalog), or by performing a spatial crossmatch to associate objects within a specified matching radius (typically 1 arcsecond) across two catalogs.
Comma Separated Variable/Value (CSV)
Comma Separated Variable (or Comma Separated Value) file. A file format used commonly for input to and output from the CAS.


Data Model
The description of the structure and organization of the data in a database. The data model tells you all the table names, their contents, and how they are related to, and linked with, one another. The actual implementation of a data model is called the database schema. For the flat files available in the SAS, the data models are available here.
Data Release
A specific and coherent set of targeting and scientific data to be released to the public along with web and published documentation
In the FPS context, a design is a set of target assignments intended to be observed together in a configuration of the FPS. A design is associated with a field to determine where on the sky it is to be observed.


eROSITA Final Equatorial Depth Survey; BHM program acquiring single-epoch BOSS spectra over a wide area (140 deg2) eROSITA performance verification field near RA,Dec=9H ,+2 deg. Dominated by extra-galactic X-ray emitting sources (AGN, clusters of galaxies), but with a significant number of Galactic stars, and compact objects
In the FPS context, an epoch refers to a number of observations of a field intended to be a single point in time. Epochs can consist of observations on a single night, and in the case of monitoring stars that vary from night to night they must be no longer than a single night. In other cases, epochs can stretch over many days for objects that vary more slowly such as distant quasars.
The eROSITA red-sequence Matched-filter Probabilistic Percolation galaxy cluster finding algorithm – an algorithm for finding clusers of red sequence galaxies associated with eROSITA X-ray detections


Fabry-Perot Interferometer (FPI)
A precise calibration lamp that generates a regularly spaced pattern of interferometer fringes. This pattern allows us to track minute changes in APOGEE instruments over time and is especially useful for tracking radial velocity drifts in APOGEE.
Faculty And Student Team (FAST) Initiative
An SDSS research and education initiative in which teams of faculty and students from minority-serving institutions partner with SDSS member institutions to receive training and complete research projects. The program is described on the FAST Initiative page, which includes a link for faculty and students to apply to create a new FAST team.
Focal Plane Systems (FPS)
SDSS-V instruments that feed the APOGEE and BOSS spectrographs using robotically positioned optical fibers at the Sloan and DuPont telescopes. Each FPS unit carries 500 fiber positioner robots, off-axis guide, acquisition, and focus (GFA) cameras, a metrology fiber backlighting system, glycol cooling, and a suite of environmental sensors (temperature, humidity, and cooling system pressure and flow). An outboard instrument electronics box provides system control, power, network, and sensor monitoring. A telescope-mounted fiber viewing camera (FVC) is used with the back-lit metrology fibers to precisely position the science fibers.
The SDSS spectrograph uses optical fibers to direct the light at the focal plane from individual objects to the slithead. Each object is assigned a corresponding fiberID. The fibers for SDSS-I/II (and for SEGUE-2 in SDSS-III) were 3 arcsec in diameter in the source plane; they are 2 arcsec in diameter for BOSS and APOGEE. The APOGEE spectrographs employ 300 infrared fibers. See Wilson et al. 2019 for a complete description of the APOGEE spectrographs.
Field (APOGEE)
A field has a different meaning in the context of plate-based APOGEE infrared spectra than it does for imaging or FPS spectroscopy. In the context of APOGEE plate observations, a field is a location on the sky, defined by central coordinates and a radius. Fields can be identified by a string Field Name (e.g. 090+08) or an integer LocationID (e.g. 4102).
Field (Imaging)
A field is a part of a camcol that is processed by the Photo pipeline at one time. Fields are 2048 × 1489 pixels; a field consists of the frames in the 5 filters for the same part of the sky. Fields overlap each other by 128 rows; primaries are decided when runs are resolved (basically, using objects between rows 64 and 1425 as primaries). See the resolve algorithm documentation for details.
Field (FPS)
In the FPS context, a field is specified by an RA, Dec, and Position angle. It is associated with a cadence, specifying how often the field should be observed. Fields are typically associated with a number of designs, but always at least one.
Field Center (APOGEE)
The coordinates of the centers of each APOGEE field, expressed in galactic coordinates (L and B).
The SDSS imaging camera takes images in five filters: u,g,r,i,z which span the window from 3000 Angstroms to 1 micron. Please read the camera page for details. This term is used to describe both the physical filter and to tag parameters as measured through that filter.
The Flexible Image Transport System, a standard method of storing astronomical data (both images and tables). FITS files are used extensively in the SAS. The FITS format has a home page.


Galactic Genesis (GG)
MWM program targeting broad sample of uniformly selected red stars (aiming for giants, but keeping selection function simple so sweeping up bright red dwarfs too).
A file transfer protocol (Joel might be best for this one?).


A hierarchical pixelization method to divide a sphere into equal-area (but variable side size) elements, initially developed for the WMAP and Planck missions. The centers of the pixels are selected to be located in rings of equal latitude. HEALpixel refers both to the method used to select the pixel elements, and to the elements themselves. The method can produce different pixelizations of a sphere, defined by the nside parameter, which sets the area of each HEALpixel. See Górski et al. 2005.


Instrument used to observe a spectroscopic MOS target: either APOGEE or BOSS.



A software package that generates a collision-free path for SDSS-V robots.


Las Campanas Observatory (LCO)
The site of the Irénée du Pont Telescope, which is used for souther APOGEE + BOSS observations, as well as the LVM-I site.
Local Volume Mapper (LVM)
An SDSS-V survey of the Milky Way (at ~pc resolution), and LMC/SMC (at ~10pc resolution) using a new IFU telescope based in the southern hemisphere out of LCO.
Local Volume Mapper Instrument (LVMI)
A new optical Integral-Field Unit telescope, leveraging 1800 fibers on a 16cm telescope to provide 37″ spaxels on the sky. LVMI will cover a wavelength range of 3600-10000 Å, with spectral resolution R~4000.
Coarse categorization of the expected sky brightness for which an observation is planned: either Bright or Dark.


Milky Way Mapper (MWM)
One of the two SDSS-V MOS programs, focused on studies of stellar astrophysics and Galactic structure, using the APOGEE and BOSS spectrographs in bright time, and in dark time (mostly by co-observing in BHM-led fields).
Modified Julian Date (MJD)
Modified Julian Date, used to indicate the date that a given piece of SDSS data (image or spectrum) was taken.
MOS (Multiple-Object Spectroscopy)
Taking spectra of multiple objects at the same time. Both BHM and MWM are referred to as MOS surveys.



Open Fiber Programs
Cartons of targets to meet science goals solicited from the collaboration, to complement the core survey science goals, selected to take advantage of robots that have not been assigned after an initial robostrategy run due to insufficient target density.


SDSS data processing uses a collection of pipelines to reduce the raw exposures taken by the telescopes at each observatory. Each pipeline consists of a series of computational steps, generating 2D spectra (flux vs wavelength) and 1D best-fit astrophysical parameters (such as the redshift), and analysis pipelines that further model the reduced spectra to compute astrophysical parameters of scientific interest.
Each spectroscopic exposure employs a large, thin, circular metal plate that positions optical fibers via holes drilled at the locations of the images in the telescope focal plane. These fibers then feed into the spectrographs. Each plate has a unique serial number, which is called plate in views such as SpecObj in the CAS. In APOGEE data in DR10 and later, note that while “plate” is commonly used interchangeably with Design, multiple plates may exist for the same design (i.e. for the same set of stars). For example, two plates may have identical targets but be drilled for observations at different hour angles, temperatures, or epochs, thereby making them unique plates with different Plate numbers.
Plate Number
A unique integer assigned to each spectroscopic plate in SDSS, SEGUE, BOSS, or APOGEE.
A unique bit-encoded 64-bit ID used in CAS to identify plates used for single-fiber observations with the SDSS and BOSS spectrographs. It is generated from the plate number and mjd. It is completely independent of any photometric enumeration system. It needs to be cast as unsigned 64-bit, though in many files we waste a few bytes and write it as an string to avoid FITS compliance issues. The bits are assigned in PlateID as follows:
– 50-63 plate id number (14 bits)
– 38-49 0
– 24-37 MJD (date) of plugging minus 50000 (14 bits)
– 10-23 “run2d” rerun number of pipeline reduction (14 bits)
– 0-9 all 0 (10 bits)
Note that even though the effective “run2d” value for many plates are the same as in DR7, this definition of plateID is different than that in DR7. run2d can be an integer, like 26, or a string of the form ‘vN_M_P’, where N, M and P are integers, with the restriction 5≤N≤6, 0≤M≤99, and 0≤P≤99. This is understood to be the run2d value for a spectrum. In the latter case, the 14 bits corresponding to run2d are filled with a number corresponding to the digits of N, M, and P (with no leading zeros) concatenated. Due to an error, this is subtly different than the procedure used for SpecObjID.
A grouping of several ‘cartons’ that share some astronomical commonality (e.g., mwm_yso for Young Stellar Objects, or bhm_rm for BHM Reverberation Mapping cartons). Program information is tracked along the carton in targetdb and used for scheduling.


Quasi-stellar Objects (QSOs)
Quasi-stellar objects are galaxies hosting a super-massive black hole at its center accreting matter. This accretion disk radiates strongly making quasars visible to very high-redshifts. The eBOSS survey observed quasars for clustering from redshifts of 0.8 to 2.2 and for the Lyman-alpha forest at redshifts from 2.2 to 4.0.


Reverberation Mapping (RM)
BHM program obtaining heavily multi-epoch BOSS spectra to study QSO variability. 174 epoch targets, on plates to be observed every few nights.
The software package that performs survey planning for the FPS instruments. It determines the field’s locations, cadences, and planned hour angle distributions, and assigns fibers to science and calibration targets within each design of each field.


The implementation of a data model in a database.
Schema Browser
A directory in the CAS that lists all of the tables and views, and the names of the parameters stored in them, with brief descriptions.
The Sloan Digital Sky Survey. The survey has proceeded in five phases. SDSS-I was in operation from 2000 through 2005. SDSS-II continued for the following three years, with SDSS-III beginning in July 2008 and continuing through 2014, and SDSS-IV completing in 2021 with DR17. The current Phase is SDSS-V, which began taking observations from APO in late 2020, and is currently projected to complete in ?2025?
The second phase of the SDSS. It took place from July 2005 to July 2008 and has three components:
1. Legacy, completing the science goals of SDSS-I.
2. SEGUE, an imaging and spectroscopic survey of stars to study Galactic structure.
3. Supernova, repeat imaging of Stripe 82 to search for supernovae.
The third phase of the Sloan Digital Sky Survey. It started in September 2008, and continued through Summer 2014. It has four components:
1. SEGUE-2, spectroscopy of 250,000 stars for study of Galactic Structure.
2. BOSS, a spectroscopic survey of galaxies and quasars to study large-scale clustering.
3. MARVELS, a high-resolution spectroscopic survey of bright stars to look for the Doppler signature of planets, and
4. APOGEE, a high-resolution H-band spectroscopic survey of mostly giant stars, in all regions of the Galaxy, to study their chemical composition and dynamics (radial velocities).
The fourth phase of the Sloan Digital Sky Survey. It started in September 2014, and concluded in 2021. It has three components:
1. eBOSS, an emission-line and luminous-red galaxy and quasar redshift survey to study baryon oscillations (BAO).
2. MaNGA, a spatially-resolved integral field unit (IFU) spectral survey of 10,000 nearby galaxies.
3. APOGEE-2, an extension of the APOGEE survey to much more of the sky, including the Southern hemisphere using the Las Campanas Observatory (LCO) in Chile./dd>
The fifth phase of the Sloan Digital Sky Survey. It started in September 2020, and is slated to continue through ?2025?. It has three components:
1. Milky Way Mapper, Spectroscopy of >10M stars for study of Stellar Astrophysics & Galactic Structure.
2. Black Hole Mapper, a spectroscopic survey of galaxies and quasars to study the properties of galactic black holes and the physics of AGN accretion.
3. Local Volume Mapper, a integral field spectroscopic survey of the interstellar medium of the Milky Way and Magellanic Clouds.
Empty regions of sky observed with APOGEE & BOSS fibers to remove the atmospheric airglow lines and underlying sky background from observed spectra. In the SDSS Legacy survey, 5% of fibers (32 fibers per plate were devoted to sky. In BOSS, there are at least 80 sky fibers (8%) per plate, and in APOGEE, 35 sky fibers are assigned to each plate.
SPectroscopic IDentification of ERosita Sources (SPIDERS)
The SPectroscopic IDentification of ERosita Sources, one of the component surveys included in SDSS-IV. SPIDERS provides homogeneous optical spectroscopic followup of X-ray sources throughout the sky, both point-like and extended. SPIDERS will result in a sample of optical sources selected from X-ray data ~2.5 times larger than the total number of X-ray AGN with spectroscopic redshift currently known.
Structured Query Language (SQL)
The Structured Query Language, a standard means of asking for data from databases. For more, see our SQL help page.


Target Database (targetdb)
The SQL database schema containing the astronomical properties (i.e., astrometry and photometry) and observing specifications (i.e., cadence requested) of all objects that could be targeted during SDSS-V observations, as well as the meta-information about how those targets will be observed over the course of the survey (i.e., the assignments of targets to specific fields and designs). All targets are drawn from a catalog cross-match and have a catalog ID unique identifier.
TESS Input Catalog (TIC)
The TESS Input Catalog (TIC) is a compiled catalog of stellar parameters for every optically persistent, stationary object in the sky. The current DR18 version is TIC-8 and it uses the GAIA DR2 catalog as a base and merges a large number of other photometric catalogs, including 2MASS, UCAC4, APASS, SDSS, WISE, among others.
TESS Object of Interest (TOI)
A source determined to be variable and vetted by the TESS Science Office (TSO) as a promising candidate for follow-up by the TESS Follow-up Observing Program (TFOP). These objects include planet candidates, single transits of potential candidates, and other time-variable objects identified from TESS light curves


Unit of volume: 1 voxel = (100 pc)3.
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