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Glossary of SDSS-III Terminology

This page contains a comprehensive glossary of SDSS-related terminology. Terms are given in alphabetical order, with definitions and links to extended descriptions where available. This version of the page is specific to SDSS-III. It is based heavily on the equivalent glossary used in SDSS-I/II, but defines some new terms, and has removed some terms that are not relevant for SDSS-III. If you come across a term you don't understand, or think we should include, please contact us.



Adaptive Moments
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. See the Adaptive Moments section of the algorithms pages.
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 three phases of the SDSS.
Ancillary Target (BOSS)
Spectroscopic target selection in BOSS is dominated by two categories: LRGs, and quasars. However, a small fraction of the fibers are devoted to a variety of additional projects, ranging from studies of variable stars to host galaxies of supernovae. These additional targets are referred to as BOSS Ancillary Targets. A full list of the target flags may be found in the descriptions of ANCILLARY_TARGET1 and ANCILLARY_TARGET2.
A neural network technique for selecting quasars for spectroscopy in the BOSS survey. It is one of the techniques used in the BONUS quasar sample. The algorithm is described in Yèche et al. 2010.
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.
The Apache Point Observatory Galactic Evolution Experiment, one of four surveys included in SDSS-III. This is using a high-resolution (R~22,500) H-band spectrograph fed by 300 fibers. The spectrograph saw first light in 2011, and the survey will observe roughly 100,000 stars, mostly in the disk of the Milky Way, to obtain detailed chemical compositions and kinematics. The first data from APOGEE was released publicly in Data Release 10.
Astrophysical Research Consortium. Incorporated in the State of Washington. Members include the University of Washington; New Mexico State University; The University of Chicago, Princeton University; Johns Hopkins University; the University of Virginia; the University of Colorado; the Institute for Advanced Study. Body that owns and operates APO.
Asinh Magnitude
Magnitudes within SDSS-I/II are expressed as inverse hyperbolic sine (or "asinh") magnitudes, sometimes referred to informally as luptitudes. The transformation from linear flux measurements to asinh magnitudes is designed to be virtually identical to the standard astronomical magnitude at high signal-to-noise ratio, but to switch over to linear behavior at low S/N and even at negative values of flux, where the logarithm in the Pogson magnitude fails. Details can be found in the Photometry section of the Algorithms pages, or the Lupton et al. (1999) AJ paper.
asTrans file
FITS binary table with astrometric transformations for every field in a single imaging run. It transforms frame (row,col) coordinates to great circle (μ,ν) coordinates for a given inclination. Available in the SAS.
The data acquisition and analysis system used to collect data from the survey telescope cameras.
The data processing pipeline that maps CCD pixel coordinates to celestial coordinates. The detailed workings of this pipeline are described in the Astrometry section of the algorithms pages, or the Pier et al. (2003) AJ paper.
Astrometric Chip
The SDSS camera contains 24 2048x400 pixel CCDs (in addition to the 30 2048x2048 photometric CCDs) which were used for astrometry and focus monitoring. Because they are smaller chips, the effective exposure time is only 11 seconds, allowing the survey to observe brighter stars without saturation. These bright star positions were necessary to match to objects present in astrometric catalogs used by the Astrom pipeline. They are no longer used for this purpose; soon after SDSS operations started, the UCAC catalog of astrometric standards faint enough not to be saturated in the main SDSS chips became available. For more details on these chips, please see the appropriate section of the camera page or the AJ imaging camera paper (Gunn et al. 1998).
The detailed methodology utilized by the SDSS to do astrometric (positional) calibration is described in the Astrometry section of the algorithms pages.
A software environment in which several of the SDSS pipelines are run, including the image processing pipeline, photo.
Atlas Image
For each detected object, the atlas image comprises the pixels that were detected as part of the object in any filter. These are available through the SAS, as a single file for each field.

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Binned Frame
Each file is a FITS image for one filter, 512 x 372 pixels, with WCS information. These are the corrected frames with detected objects removed and binned 4 pixels by 4 pixels. All pixels that are in atlas images are replaced by the background level before binning, with suitable noise added. All of the header parameters from the original image are inherited as well. Available in the SAS.
Black Book
See Project Book.
BONUS quasars
The principal science goal of the BOSS quasar sample is to measure the clustering signal in the Lyman-α forest, which does not require that the quasar sample be uniformly selected across the sky. While the CORE quasars are uniformly selected, an additional 20 quasars per square degree, the BONUS quasars, are selected using a combination of techniques and available data that is not required to be uniformly selected. See also CORE quasars.
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.
The Baryon Oscillation Sky Survey, one of the four component surveys of SDSS-III. It is obtaining redshifts of 1.5 million galaxies, and spectra of 150,000 z > 2.2 quasars, to measure the baryon oscillation signal in the correlation function as a geometrical probe of cosmology. It also obtained imaging over roughly 3100 deg2 of the Southern Galactic Cap beyond that in SDSS-I/II. It uses substantially improved spectrographs over those used in SDSS-I and SDSS-II, with more fibers per plate (1000 vs. 640), smaller fiber aperture (2", not 3"), improved throughput, and somewhat wider wavelength coverage.
BRIGHT detections
A technical term in the photometric reductions. The photometric pipeline performs a first-pass detection that searches for very significant objects, which are typically bright stars and galaxy, in preparation for the full object detection algorithm. These objects are measured and included in the final catalog, but never as survey primary or survey secondary objects. All such objects are redetected and remeasured in the second-pass of the pipeline, and those (non-BRIGHT) detections are the ones to actually use. BRIGHT detections have the BRIGHT bit set in their photometric flags.

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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 Camcol is the output of one camera column of CCDs (each with a different filter) as part of a Run. Therefore, 1 Camcol = 1/6 of a Run. It is also a portion of a scanline.
The instrument used for imaging in SDSS. It consists of 30 photometric and 24 astrometric CCDs (see the camera description). The camera is a mosaic of 54 CCD detectors in the focal plane, 30 large devices arranged in 6 columns of 5 each and 24 smaller devices around the periphery. See this sketch of the camera focal plane or this real life image. The camera is described in detail in the imaging camera paper (Gunn et al. 1998). The last imaging data in SDSS was released in DR8; the camera has now been decommissioned.
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.
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 spectrosopic caveats.
Central Meridian
The meridian which passes through the center of the survey area in the Northern Galactic Cap, 12h 20m, defines the central meridian of an imaging scan. The great circle perpendicular to it passing through the survey center at δ = 32.8° is the survey equator.
A product of the deblending process in the image processing. When two objects are near each other on the sky, their images may appear merged. The deblender tries to split this merged image; the resulting sub-images are called children. The initial merged image is called a parent. For more details, see the Deblending section of the algorithms pages.
Classification (photometric)
The method by which we assign a type (star or galaxy) to each object in the SDSS images. The details of this procedure are described in the Classification section of the algorithms pages; in brief, it is based on the difference between PSF and model magnitudes.
Cloud Camera
A camera at the mountain that takes continuous pictures of the sky at 10 μm, a wavelength at which clouds emit. It is a sensitive measure of the photometricity of the sky as a function of time. It is described here.
Cloud Camera Status

The cloud camera status is built from an analysis of the 10 μm cloud cameras used at the APO site. The full history is stored in two files in the photoop product:

  • opfiles/opCloudcam.par (when was the cloudcam on)
  • opfiles/opCloudveto.par (when was it cloudy)

The software generating these files from the raw IRCAM data is in pro/ircam.

In individual files in the reductions the CLOUDCAM flag gets set with the following possible values:

  • -1 : UNKNOWN (cloud camera not on)
  • 0 : CLOUDY (cloud camera indicates clouds)
  • 1 : CLEAR (cloud camera indicates clear)
A sample of galaxies targeted for spectroscopic followup in the BOSS survey. It is designed to select luminous galaxies in the redshift range 0.4 < z < 0.8, with a roughly uniform space density of 3 × 10-4 (h/Mpc3) to z ~ 0.6, with a tail to z ~ 0.8. "CMASS" stands for "Constant (stellar) MASS." See also luminous red galaxy and LOWZ.
Coordinate Measuring Machine, a device used to measure the positions of holes in the fiber mount plates for the spectrographic survey.
Cmodel magnitude
Composite model magnitude. See magnitude, cmodel.
Color Image Pipeline
A pipeline to produce 3-color JPG files of zoomed-down SDSS images; these are available from the CAS.
A convex is the intersection of one or more circles, with a depth (the number of circles involved). If we have two intersection circles, A and B, then both (A) and (B) are a convex of depth 1, their intersection (A)(B) is also a convex, but of depth 2. We call these simple convexes "wedges."
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.
CORE quasars
In the BOSS survey, 20 quasar targets per square degree are selected for spectroscopy using a spatially uniform technique, called "CORE," allowing science requiring a uniform selection of quasars to be done. The CORE algorithm starting in the second year of BOSS is the "extreme deconvolution" technique, XDQSO See also BONUS quasars.
Corrected Frame
Each corrected frame is a FITS image for one filter, 2048 columns by 1489 rows, with row number increasing in the scan direction. These are the imaging frames with flat-field, bias, cosmic-ray, and pixel-defect corrections applied, sky subtracted, and photometric calibration applied. A raw image contains 1361 rows, and a corrected frame has the first 128 rows of the following corrected frame appended to it. The pixels subtend 0".396 square on the sky. Header information using the world coordinate system (WCS) allows standard astronomical FITS tools to convert pixel position to right ascension and declination. Available in the SAS.
Corrected Survey Coordinates
A variant on Survey Coordinates, for which the range of η is -180 to 180 and the range of λ is -90 to 90. See the algorithms page for SDSS survey coordinates.
CSV (comma-separated variable/value)
Comma Separated Variable (or Comma Separated Value) file. A file format used commonly for input to and output from the CAS.
The coordinates on the unit sphere utilized by the HTM code; stored in the CAS.

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The Data Archive Server, which provided access, for SDSS-I/II, to the imaging and spectroscopic products of the survey as flat files. Its functionality is replaced with the Science Archive Server (SAS).
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.
Deblending is the process by which overlapping objects in images are separated. The frames pipeline attempts to determine whether each object actually consists of more than one object projected on the sky and, if so, to deblend such a parent object into its constituent children, self-consistently across the bands (thus, all children have measurements in all bands). For details on how this works, and what flags this procedure may set, see the Deblending section of the algorithms pages.
de Vaucouleurs

Also known as the r1/4 law, it describes the radial light profile of a typical elliptical galaxy. Defined as

I(r) = I0 exp(-7.67 [r/re]1/4).

An elliptical version of this profile is fit to every detected object, yielding the deV parameters. See the model magnitude section of the photometry algorithm page for details.

Data Release 1 through 10, the formal periodic releases of SDSS data to the public. A small portion of the data was released as part of the Early Data Release (EDR). The data releases are cumulative: later releases include more recent data and re-processed older data. DR7 is the final release of SDSS-II data. DR8 is the first release of SDSS-III data, and includes most (although not all) SDSS-I/II data as well. Each Data Release is associated with a journal article that describes changes from the previous release.

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Early Data Release. The first public release of SDSS data occurred in June of 2001. The EDR is described in the EDR paper.

Measures how elliptical an object in the imaging data is. In the SDSS, we have numerous methods to measure this:

eta (η)
Latitude in the survey coordinate system. Eta is the angle between the survey equator and the great circle passing through the point perpendicular to the survey meridian, positive to the north. Constant latitude curves are great circles. See the description of SDSS survey coordinates.
Exponential Model

The radial light distribution of a disk galaxy can often be fit by an exponential profile:

I(r) = I0 exp(-1.68 r/re)

The number 1.68 is chosen so that the model radius is a half-light radius. An elliptical version of this profile is fit to every detected object, yielding the exp parameters. See the model magnitude section of the photometry algorithm page for details.

We observe all but the nearest stars through a foreground of dust extinction from our own Milky Way. The CAS tabulates extinction corrections in magnitudes at the position of each object, reddening, following Schlegel, Finkbeiner & Davis (1998). For the APOGEE survey, which focuses on the most dust-obscured regions of the Galaxy disk and bulge, see the discussion of extinction in the Infrared Spectra Targets pages, and references therin.
Extra Plates
Reobservations of main survey plates, useful for variability studies or checks. See the scope page for more information.

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Family Objects
These are objects that are generated when photometric objects are neither primary nor secondary survey objects but a composite object that has been deblended or the part of an object that has been deblended incorrectly (like the spiral arms of a galaxy). These objects are kept to track how the deblender is working.
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. Each fiber is surrounded by a large sheath which prevents any pair of fibers from being placed closer than 55 arcsec on the same plate (62 arcsec for BOSS). The APOGEE spectrograph employs 300 infrared fibers that each subtend 2 arcsec on the sky; no two fibers can be placed closer than 71.5 arcsec on a given plate. When two targets are closer to each other than this separation, the highest-priority target is observed. The other target may be observed if there is an overlapping plate covering this region.
The magnitude measured by the frames pipeline to simulate the flux that would fall into a 3" fiber in typical seeing. Similarly, fiber2Mag simulates the 2" fiber magnitude. See Magnitude, fiber.
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.
The fieldID parameter is an integer uniquely identifying a detection in the photo catalog. 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 exactly as for objid, but setting the object id within the field to 0. For IDL users, the function sdss_fieldid() in photoop will convert RUN, CAMCOL, FIELD, RERUN into a properly formatted fieldID.
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.
A bitmask used in the database to specify various properties of an object in the imaging data. There are many flags in the SDSS catalogs, indicating, for example, whether the object was deblended or saturated. There are status flags set by PSP, and by survey operations, on a frame-by-frame basis, describing the quality of the PSF, and the quality of the data overall. Each object is given status flags that sort out overlaps. The spectra have flags at two levels as well: pixel-by-pixel flags, and warning flags accompanying the redshifts and classification to indicate trouble. There is extensive documentation of the image processing flags at the flags recommendation page. The spectroscopic data are accompanied by their own set of flags. APOGEE also employs unique data and targeting flags. A full list of values for each flag is given in the Bitmasks pages.
Fermi National Accelerator Laboratory, one of the participating institutions in SDSS-I/II. Their homepage is
The area on the sky covered by the SDSS imaging and spectroscopy (both optical and infrared). The footprint is described on the scope page.
fpAtlas file
A FITS binary table containing the atlas images for all objects detected in all five filters in a single field. Requires special software to decode into individual FITS images for each object. The FITS images are available in the SAS.
fpBin file
See binned frame.
fpC file
An uncalibrated version of a corrected frame. Note that while in DR7 and previous the fpC files were the only version of corrected frames released, in DR8 and later the fpC files are discontinued in favor of the fully calibrated corrected frames.
fpFieldStat file
A FITS binary table containing a statistical summary of the results of the frames pipeline for one field for a single frames pipeline run. This information is also found in the Field table in the CAS database. Available in the SAS.
fpM file
See mask frame.
fpObjc file
These are FITS binary tables containing catalogs of detected objects output by the frames pipeline. These are uncalibrated, and thus present positions in pixel coordinates, and counts, not fluxes. Available in the SAS.
The data stream from a single CCD in a scanline is cut into a series of frames which measure 2048 × 1489 pixels and overlap 10% with the adjacent frame. The frames in the 5 filters for the same part of the sky are called a field.
Frames Pipeline
This is the core of the image processing of SDSS image data. In this pipeline, the images are bias-subtracted and flat-fielded, and bad columns, cosmic rays, and bleed trails are interpolated over. This yields corrected frames. Then objects are found by running a PSF-matched filter over the image, and matched between the five frames making up a field. A deblender is run to resolve overlaps, and the properties of each object are measured and written to the fpObjc files. Atlas images are written to the fpAtlas files.
Fundamental Standard
The photometry of the primary standard stars is ultimately tied to the SED of the star BD 17 +4708, which is the fundamental standard for the SDSS photometric system. See the Smith et al. (2002) AJ paper for more details.

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Galaxy Zoo
An international program whereby laypeople from all over the world contribute visual classifications of galaxies, based on SDSS images. These classifications are made available in DR10. More details may be found at their website.
Great Circle Coordinates

One of the main coordinate systems utilized by the SDSS. In this system, μ and ν are spherical coordinates (corresponding to ra and dec) in a system whose equator is along the center of the stripe in question. The stripes of the main survey in the Northern Galactic Cap are great circles which all cross at the survey nodes of (RA, Dec) = (95, 0) an (RA, Dec) = (270, 0) degrees.

The scans are defined by their inclination with respect to the equator, and are indexed by integers n such that the inclination of a stripe is -25 + 2.5n.

The stripes of the main survey in the South Galactic cap are great circles with the same nodes, where the indexing is such that the inclination relative to the equator is 2.5(82 - n). The celestial Equator therefore corresponds to stripe number 10 in the Northern Cap and 82 in the Southern Cap. Stripes 1-44 were observed in the North, and stripes 61 to 90 in the South. While the imaging scans of the SEGUE survey are also great circles, they have an assortment of different nodes and are numbered with stripe numbers > 200.

See also the description of SDSS survey coordinates.

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A computer program which ran at Apache Point Observatory, which used information from the photometric telescope and the cloud camera to determine the photometricity of a given night. It was written by David Hogg and colleagues, and is described in a paper.
The Hierarchical Triangular Mesh (HTM) is a partitioning scheme to divide the surface of the unit sphere into spherical triangles. It is a hierarchical scheme and the subdivisions have roughly equal areas. HTM is used to index the coordinates in the object databases for faster querying speeds. For more details, and downloadable software, see the HTM Website.

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The spectroscopic pipeline (also sometimes referred to as Spectro-2D and specBS), which reduces the raw 2D spectral frames to extracted 1D calibrated spectra (flux density versus wavelength) and then to redshifts and classifications.

The operational goals of the extraction pipeline are to:

  1. interpolate over bad pixels
  2. mask all bad pixels and pixels contaminated by strong sky emission.
  3. bias and dark subtract the raw 2D data frames
  4. trim the frame
  5. flat-field using calibration frames taken at the same telescope pointing position before and after an exposure on the sky.
  6. optimally extract 1D spectra from this 2D frame.
  7. apply wavelength calibration, rebin to a common wavelength solution, and sky subtract.
  8. coadd the three or more individual exposures for each object.
  9. put the red and blue halves of the spectrum together.
  10. flux calibrate the spectrum.

Following extraction, the idlspec2d pipeline classifies objects and measures redshifts from the Legacy, SEGUE and BOSS surveys based on the calibrated 1-dimensional spectra. It fits spectra to templates in real space, as described in the redshift fitting algorithms documentation. This aspect of the idlspec2d has sometimes been referred to as "specBS".

For SDSS-I/II, there was another pipeline, spec1d, which cross-correlated spectra to templates in Fourier space; that pipeline is not being run in SDSS-III.

Institute for the Physics and Mathematics of the Universe, a branch of the University of Tokyo, one of the participating institutions in SDSS-III.
The IS_BEST flag is set for each SDSS/BOSS spectroscopic observation (labeled by its MJD) of each plate. It is "1" if we consider that MJD to be the best observation of that plate, "0" otherwise.
The IS_PRIMARY flag is set for each SDSS/BOSS spectroscopic observation (labeled by its MJD) of each plate. It is "1" if we consider that MJD to be the best observation of that plate, and for it to be an acceptable observation from a science point of view (with PLATEQUALITY either 'marginal' or 'good'). It is "0" either if there is a better observation or if all observations are labeled 'bad'. A clean set of plates can be selected by demanding that IS_PRIMARY be equal to one. Note, however, that just because a plate is marked 'bad' does not always mean its spectra are useless.
The IS_TILE flag is set for each SDSS/BOSS spectroscopic observation (labeled by its MJD) of each plate. It is "1" if we consider that PLATE-MJD to be the best instance of the TILE it is associated with in the SDSS Legacy survey. Tiles are unique RA/Dec centers. It is "0" otherwise, or if the plate is not associated with Legacy. To get well-defined targeting and window functions, one wants to use only plates that have IS_TILE of 1. A plate can be IS_TILE only if it is also IS_PRIMARY.

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Johns Hopkins University, one of the participating institutions in SDSS. The SDSS JHU homepage is

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lambda (λ)
One of the coordinates of the survey coordinate system. The stripe longitude λ is measured from the survey central meridian positive to the east along the great circles perpendicular to that meridian. Constant longitude curves are circles centered on the survey poles. See the description of SDSS survey coordinates.
One of the three surveys that comprise the second phase (SDSS-II) of the Sloan Digital Sky Survey (SDSS). It completes the SDSS-I survey of the extragalactic universe. SDSS-I plus Legacy obtained images and redshifts of a million galaxies and quasars over a contiguous 7500 deg2 in the Northern Galactic Cap, and three stripes in the Southern Galactic Cap.
Legacy primary
Spectra that are unique observations of a location on the sky, within the full set of Legacy plates, are labeled as legacyPrimary in the CAS (called speclegacy in the flatfiles on SAS). There are several criteria used to determine whether the legacyPrimary flag should be set to 1 for an object in the SpecObjAll table, described in the spectroscopic catalog documentation. This flag is a special version of the science primary flag, which uses all plates.
A sample of galaxies targeted for spectroscopic followup in the BOSS survey. It is designed to select red galaxies in the redshift range 0.15 < z < 0.4, with a roughly uniform space density of 3 × 10-4 [h/Mpc]3). It is similar to the LRG sample used in the SDSS-I/II Legacy survey, but extends to lower luminosities. See also CMASS and luminous red galaxy.
Luminous Red Galaxy
SDSS luminous red galaxies (LRGs) are selected on the basis of color and magnitude to yield a sample of luminous intrinsically red galaxies that extends fainter and farther than the SDSS main galaxy sample. See Eisenstein et al. (2001) for detailed discussions of sample selection, efficiency, use, and caveats.
An informal name for the asinh magnitude.

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A maggie is a linear measure of flux density; one maggie has an AB magnitude of 0, or a flux density of 3631 Jansky in any band. In SDSS-III, all brightnesses are made available in units of maggies, as well as AB magnitudes.
Magnitude, cmodel
Composite model magnitude. The magnitude obtained from the best-fitting linear combination of the best-fitting de Vaucouleurs and exponential model for an object's light profile (cf. magnitude, model). See cmodel magnitude description in Photometry section of the Algorithms pages.
Magnitude, fiber
The flux contained within the aperture of a spectroscopic fiber (3" in diameter) is calculated by the frames pipeline in each band and stored in fiberMag. There is also a 2" fiber magnitude (fiber2Mag), matching the size of the BOSS fibers. Details can be found in the Photometry section of the Algorithms pages.
Magnitude, model

Just as the PSF magnitudes are optimal measures of the fluxes of stars, the optimal measure of the flux of a galaxy would use a matched galaxy model. With this in mind, the code fits two models of arbitrary ellipticity to the two-dimensional image of each object in each band:

  1. a pure de Vaucouleurs profile, and
  2. a pure exponential profile,

both convolved with the local PSF. The best-fit model in the r-band is fit to the other four bands; the results are stored as the model magnitudes. Details can be found in the Photometry section of the Algorithms pages.

Magnitude, Petrosian
Stored as petroMag. For galaxy photometry, measuring flux is more difficult than for stars, because galaxies do not all have the same radial surface brightness profile, and have no sharp edges. In order to avoid biases, we wish to measure a constant fraction of the total light, independent of the position and distance of the object. To satisfy these requirements, the SDSS has adopted a modified form of the Petrosian (1976) system, measuring galaxy fluxes within a circular aperture whose radius is defined by the shape of the azimuthally averaged light profile. Details can be found in the Photometry section of the Algorithms pages and the Strauss et al. (2002) AJ paper on galaxy target selection. Model magnitudes share most of the advantages of Petrosian magnitudes, and have higher S/N; they are therefore used instead of Petrosian magnitudes for target selection in BOSS.
Magnitude, Pogson

The Pogson magnitude is the standard astronomical magnitude system, where one increment in magnitude is an increase in brightness by the fifth root of 100. A star of 1st magnitude is therefore 100 times as bright as a star of 6th magnitude. That is, for two objects

M1 - M2 = -2.5 log(F1/F2)

where M1 and M2 are the magnitudes of two objects, and F1 and F2 are their luminous fluxes.

Magnitude, PSF
Stored as psfMag. For isolated stars, which are well-described by the point spread function (PSF), the optimal measure of the total flux is determined by fitting a PSF model to the object. Details can be found in the Photometry section of the Algorithms pages.
Main Sample
The Main Galaxy Sample is a complete tiled spectroscopic sample of galaxies observed as part of the Legacy survey in SDSS-I/II, consisting of objects brighter than Petrosian r < 17.77. The main galaxy sample target selection algorithm is detailed in Strauss et al. (2002).
Software tool used to track the detailed geometry of the sky coverage of the SDSS large-scale structure samples, both imaging and spectroscopy (especially the Legacy and BOSS surveys). It is based on spherical polygons, analogous to those used by the CAS region tools and footprint server. Molly Swanson has put together a useful webpage describing it.
The Multi-object APO Radial Velocity Exoplanet Large-area Survey, one of the four surveys making up SDSS-III, uses a sixty-object fiber-fed spectrograph to repeatedly observe bright stars to detect the radial velocity variations caused by orbiting planets. The spectrograph uses an innovative fixed-delay interferometer.
Regions of data that are excluded from analysis. For example, some wavelength regions in the spectra may be masked out, some pixels in the photometric images may suffer from cosmic rays and so are masked out, and some parts of the sky (e.g. around bright stars) may be masked.
Mask Frame
A binary FITS table corresponding to a corrected image frame, showing mask bits associated with each pixel. The mask values are described in Table 8 of the EDR paper. Available in the SAS.
Modified Julian Date, used to indicate the date that a given piece of SDSS data (image or spectrum) was taken.
The model magnitude. See magnitude, model.
mu (μ)
One of the coordinates in the SDSS great circle coordinate system. Mu corresponds to RA, or longitude. See the description of SDSS survey coordinates.

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A linear unit of flux density equal to 10-9 maggies. A star of brightness 1 nanomaggie has a magnitude of 22.5 in any band, or a flux density of 3.631 × 10-6 Jansky.
The conversion factor between counts and nanomaggies. It depends on the airmass and extinction terms of the calibration of the given location in the given field of imaging data, as well as the flat-field for the filter at the epoch of the run in question. It can be found in the photoObj files or tables for each object.
nu (ν)
One of the coordinates in the SDSS great circle coordinate system. Nu corresponds to Dec, or latitude. See the description of SDSS survey coordinates.

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Enumerates detected photometric objects within a given imaging field. Thus, multiple fields may have objects with the same object number. In CAS, this identification number is named obj in the photoObjAll and related tables. In the flatfiles in SAS, this identification number is named id.

A number identifying an object in the image catalog used by the CAS. It is a bit-encoded integer of run, rerun, camcol, field, object. Note that when the data are reprocessed (rerun), the object identification changes, so that the ObjID value in DR8 and DR9 differ from that of previous data releases. For spectroscopic objects, there are two possible choices for the matching photometric measurement: bestObjID corresponds to the "flux-based" match, which is the object that contributes the most light at the location of the fiber; origObjID corresponds to the "position-based" match, which is the object whose center is closest to the fiber. In a fraction of a percent of cases, these two identifications differ. We also provide targetObjID, which is the object identification number corresponding to the object that was originally targeted (virtually always in a previous rerun, which we do not include in the DR8 and DR9 releases).

The bits are assigned in objid as follows:

  • 63: 0, unassigned
  • 59-62: skyVersion resolved sky version (for DR8 only use 2; 0-15 available)
  • 48-58: rerun, number of pipeline rerun
  • 32-47: run, run number
  • 29-31: camcol, camera column (1-6)
  • 28: 0, unassigned
  • 16-27: field, field number within run
  • 0-15: id, object number within field

For IDL users, the function "sdss_objid()" in the photoop package distributed with DR9 will convert RUN, CAMCOL, FIELD, ID, RERUN into a properly formatted objID. The function "unwrap_objid()" will return the RUN, CAMCOL, FIELD, ID, RERUN corresponding to a given objID.

For Python users, the sdsspy package provides "sdsspy.util.objid()" and "sdsspy.util.photoid_extract()" to convert RUN, CAMCOL, FIELD, ID, RERUN into objID and back, respectively.

In the CAS, there is a database function fObjIDFromSDSS() which will convert RUN, CAMCOL, FIELD, ID, RERUN into a properly formatted objID. The function fSDSSFromObjID() will return the RUN, CAMCOL, FIELD, ID, RERUN corresponding to a given objID.

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.

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Par (Parameter) File
Also known as a Yanny parameter file. This is a simple ascii file format developed to store lists of parameters for the SDSS, such as the plates and run lists in the scope page. See the formal description.
A product of the deblending process. When two objects are near each other on the sky, their images may appear merged. The deblender tries to split this merged image; the initial merged image is called a parent, while the resulting sub-images are called children. For more details, see the Deblending section of the algorithms pages.
Participating Institution
One of the institutions involved in the survey. These institutions have contributed hardware, software, manpower, or financial support to the survey, and thus have pre-public access to data.
The Petrosian magnitude. See Magnitude, Petrosian.
The Petrosian radius. A measure of the angular size of an image, most meaningful for galaxies. Units are seconds of arc. The Petrosian radius (and related measures of size called petroR50 and petroR90) are derived from the surface brightness profile of the galaxy, as described in Algorithms.
Photometric Pipeline (Photo)
A series of linked pipelines (Serial Stamp Collecting pipeline, SSC; Postage Stamp Pipeline, PSP, and Frames) which analyze the raw image data, including bias subtraction, sky and PSF determination, flat-fielding, and finding and measuring the properties of objects. The astrometric and photometric calibration is carried out with the astrometric pipeline and ubercalibration. We provide in the imaging section an outline of the photometric pipeline and its data products.
Photometric Telescope
Also abbreviated PT. A 0.5-meter telescope that was used in SDSS-I/II for monitoring the transmission of the atmosphere during the course of the survey and photometrically calibrating the 2.5m image data. In SDSS-III, photometric calibration has been carried out instead with the Ubercalibration process, which uses overlaps between scanlines to tie the survey together photometrically.
The fits binary tables containing photometrically and astrometrically calibrated quantities from the imaging pipeline. Available in the SAS.
A table in the CAS that includes only the most frequently accessed PhotoObj columns. GalaxyTag and StarTag are views of PhotoTag that contain only Primary extended images and point-like images, respectively. None of these tables and views contain spectroscopic information.
The photometric redshift. The measured colors of galaxies and quasars are observed to depend on their redshift, as the spectral energy distribution of the object moves through the filters. This allows the redshift to be estimated from the measured colors, in the absence of a spectrum. Photometric redshifts have larger random and systematic errors than spectroscopic redshifts, but they can be obtained for fainter and thus many more galaxies and quasars. They have been estimated using two different techniques for galaxies in SDSS image data, and are available in the CAS.
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 IDs.
Plate ID
A unique integer assigned to each spectroscopic plate in SDSS, SEGUE, BOSS, or APOGEE.
Plate Mapper
An instrument that maps which plugged fiber corresponds to which target object. The 640 fibers (for SDSS-I/II, and SEGUE-2), 1000 fibers (BOSS), or 300 fibers (APOGEE) of the spectrograph are placed by hand without regard to which fiber corresponds to which position. An automated fiber mapper resolves this object-to-fiber match-up by scanning lasers across the terminal ends of the fibers and observing where they "light up" on the focal plane.
The PLATEQUALITY string is set for each SDSS/BOSS spectroscopic observation (labeled by its MJD) of each plate. The conditions for setting this quality indicator are described in the documentation for the optical spectroscopic catalogs.
Point Spread Function (PSF)
The detailed shape of the response of the atmosphere, telescope plus instrument to a star. It varies with position, filter, and time, due to changes in the atmosphere and the optics of the telescope. In the image processing pipeline, it is measured by the PSP, and used extensively in determining each objects' properties.
SDSS uses a number of coordinate systems to describe positions on the sky. These are spherical polar coordinates, where great circles are lines of longitude that converge at the poles. Specifying the location of the poles on the sky is part of the definition of the coordinate system.
Postage Stamp pipeline (PSP)
This pipeline determines for an imaging run the background sky, the flat-field, and the spatially varying Point Spread Function (PSF) in each CCD, all of which will be used by the Frames pipeline.
Primary (object)
Generically, the "main" observation given multiple observations of an object, either in spectroscopy or photometry. For spectroscopy, see science primary, legacy primary, and SEGUE primary. For imaging, see survey primary and run primary.
Primary Standard
One of the 158 stars on the SDSS photometric system that was observed by the Photometric Telescope (PT) to measure the extinction of a given night, and which is used to calibrate the magnitudes of the secondary standards. The primary standard system is described in the Smith et al. (2002) AJ paper.

An azimuthally-averaged radial surface brightness profile. In the imaging catalogs, it is given as the average surface brightness in a series of annuli. This quantity is in units of nanomaggies per square arcsec. The number of annuli for which there is a measurable signal is listed as nprof, the mean surface brightness is listed as profMean, and the error is listed as profErr. This error includes both photon noise, and the small-scale 'bumpiness' in the counts as a function of azimuthal angle.

When converting the profMean values to a local surface brightness, it is not the best approach to assign the mean surface brightness to some radius within the annulus and then linearly interpolate between radial bins. Do not use smoothing splines, as they will not go through the points in the cumulative profile and thus (obviously) will not conserve flux. What frames pipeline does, e.g., in determining the Petrosian ratio, is to fit a taut spline to the cumulative profile and then differentiate that spline fit, after transforming both the radii and cumulative profiles with asinh functions. We recommend doing the same here.

The annuli used are provided in the magnitude algorithms page.

A spectroscopic program name, assigned to each plate depending on what it was designed for. Within each survey (legacy, segue1, etc.) there can be multiple possible programname values. See the basics of spectroscopic data, the SDSS/BOSS target flag page for more details and links to the various program names and their meanings.
Project Book
This document provides a description of the original science goals of the Sloan Digital Sky Survey, as well as the hardware and software designs, and was the 1996 proposal to NASA. It is not updated, so information within it may no longer be current; in particular, it does not describe the SEGUE and Supernova aspects of SDSS-II, and doesn't have anything to say about the science goals of SDSS-III. Nevertheless, it provides an excellent overview of the initial survey. The science content of this proposal is available online.
psField file
A FITS binary table with preliminary photometric calibration, as well as final point-spread-function fit, for a single field in an imaging run. Available in the SAS.
The PSF magnitude. See Magnitude, PSF.

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QA hole
A spectroscopic target selected for quality assurance with a drilled hole in the respective plate. Most QA targets are repeat objects on adjacent plates.

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See extinction.
In CAS geometry software, a region is the union of convex areas. These are concepts useful in understanding the detailed geometry of the survey on the sky. In the language of the mangle software, a region mathematically equivalent to a set of spherical polygons.
A reprocessing of an imaging run. The underlying imaging data are the same, but the software version and/or calibration are different.
The resolve algorithm assigns the primary observation of each object in the imaging data in regions of overlap. The primary observation is typically drawn from the best SDSS imaging that covers each region of the sky. See the resolve algorithm documentation for details.
A Run is a length of a strip observed in a single continuous image observing scan, bounded by lines of μ and ν. A strip covers a great circle region from pole to pole; this cannot be observed in one pass. The fraction of a strip observed at one time (limited by observing conditions) is a Run. Runs can (and usually do) overlap at the ends. Like strips, it takes a pair of runs to fill in a length of a stripe. This is why you may read about data taken from "Runs 752/756" or some similar terminology. Each individual run contains 6 camcols spanning the same range of ν, but not delimited by η. These run pairs might not have the same starting and ending ν coordinates. See this imaging basics for details.
Run primary
The unique detection of an object within a single run. Adjacent fields within a run and camcol overlap by 128 pixels. In the raw data, these overlap regions are literally identical, and are only included to mitigate edge effects in the object detection. The photometric pipeline detects objects within the overlap regions, but the resolve algorithm only retains a single detection as the "run primary" detection. Except in very rare occurrences, only run primary detections can be survey primary.
Reprocessing number for the spectroscopy. For a given plate and MJD, the underlying raw data is unchanged for different run2d values, but the software versions are different. In DR8 and DR9, run2d indicates a specific version of both the spectroscopic extraction and the redshift-fitting.

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The Science Archive Server, which serves flat-files of SDSS-III data. It replaces the Data Archive Server (DAS), which was used in SDSS-I/II.
A subdivision of a strip. Each strip is covered by 6 "scanlines". Scanlines are defined in great circle coordinates. The great circle coordinate system is different for each strip, and is defined by setting the equator of the coordinate system to be the center line of constant η for the stripe to which the strip belongs. A scanline is then bounded on the top and bottom by lines of constant ν, with no east or west boundaries. Scanlines touch but don't overlap, and thus are a unique mapping on the sky for that stripe only. Scanlines for different stripes do overlap. This is because the scanlines come from the camera columns, or camcols, which have a fixed physical width, while the spherical coordinates converge at the poles. Note: The term scanline has been used interchangeably (and improperly) with camcol.
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.
Science Primary
Spectra that are unique observations of a location on the sky are labeled as sciencePrimary in the CAS (called specprimary in the flatfiles on SAS). The SpecObj view of the SpecObjAll table includes only sciencePrimary observations. There are several criteria used to determine whether the sciencePrimary flag should be set to 1 for an object in the SpecObjAll table, described in the spectroscopic catalog documentation. Special versions of science primary are also defined, which are restricted to Legacy plates (Legacy primary) and SEGUE-1 and SEGUE-2 plates (SEGUE primary).
The "score" is a numerical rating for the quality of an SDSS imaging field. It ranges from 0 to 1. Its meaning is described in detailed in the resolve documentation, and is based on the imaging quality information.
The Sloan Digital Sky Survey. The survey has proceeded in three phases. SDSS-I was in operation from 2000 through 2005. SDSS-II continued for the following three years, and SDSS-III began in July 2008 and will continue through 2014.

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 will continue 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 proposed fourth phase of the Sloan Digital Sky Survey. It is planned to start in September 2014, and will continue for six years. It has three components: a emission-line galaxy and quasar redshift survey to study baryon oscillations (eBOSS), a spatially-resolved spectral survey of 10,000 nearby galaxies (MANGA), and an extension of the APOGEE survey to much more of the sky, including the Southern hemisphere.
A sector is basically an intersection of TileRegions, useful as a region over which the completeness of the spectroscopic survey can be assumed to be uniform. It is a plate wedge modified by intersections with overlapping tile boundaries. If the TilingBoundary regions are complex (multiple convexes) or if they are holes (isMask=1), then the resulting sector is also complex (a region of multiple convexes). As such, a sector is just a single convex. Tiling boundaries do not add any depth to the sectors; they just truncate them to fit in the boundary.
This is a piece of DR7 terminology. In DR7, a segment is a piece of a given frames pipeline reduction (run/rerun/camcol), covering a piece of a scanline, bounded on the east and west by lines of constant μ. Because segments are defined before the primary area of a stripe, segments can actually go beyond the η limits of a stave. Indeed, near the very end of a stripe (near the poles), a segment may fall completely outside a stave. The entire concept of a "segment" is obsolete with the DR8 and DR9 versions of resolve.

The Sloan Extension for Galactic Understanding and Exploration mines the stellar content of the Milky Way in order to create a detailed three-dimensional map of the Galaxy. SEGUE obtained 3500 square degrees of new imaging data, including scans at low Galactic latitude. SEGUE obtained spectra of 240,000 stars in the disk and spheroid, revealing the age, composition and phase-space distribution of stars within the various Galactic components. These stellar excavations provide essential clues for understanding the structure, formation and evolution of our Galaxy.

SEGUE is one of the three surveys that comprise the second phase (SDSS-II) of the Sloan Digital Sky Survey (SDSS).

SEGUE Primary
Spectra that are unique observations of a location on the sky, within the full set of SEGUE-1 and SEGUE-2 plates, are labeled as seguePrimary in the CAS (called specsegue in the flatfiles on SAS). There are several criteria used to determine whether the seguePrimary flag should be set to 1 for an object in the SpecObjAll table, described in the spectroscopic catalog documentation. This flag is a special version of the science primary flag, which uses all plates.
An extension of the SEGUE survey, that continued during the first year of SDSS-III. It obtained spectra of another 250,000 stars. It used the same spectrograph as that used in SDSS-I and SDSS-II.

An open category of targets used in SDSS-I and SDSS-II whose selection criteria explore different regions of parameter space. These include:

  • objects lying outside the stellar locus in color space (SERENDIP_RED, SERENDIP_BLUE, SERENDIP_DISTANT)
  • objects coincident with FIRST sources but fainter than the equivalent in quasar target selection; also not restricted to point sources (SERENDIP_FIRST)
  • hand-selected targets (SERENDIP_MANUAL)
Serial Stamp Collecting Pipeline (SSC)
The first stage of the Photometric pipeline. It collects for further analysis the atlas images of stars to be used in astrometric calibration and PSF determination.
Sky Version
The SkyVersion is a numerical designator that distinguishes what set of runs and reruns are being used to define the catalog. In the DR9 data, the skyVersion is 2, which means rerun 301 is used. In the DR7 data, skyVersion=0 referred to the targeting version of the photometry used for Legacy, while skyVersion=1 referred to the final, best version of the photometric reductions and calibrations for DR7.
The public outreach website for distribution of SDSS data. Includes tools to get images, spectra, and catalog info, as well as educational and fun materials.
Small Circle
A section of a sphere which does not contain a diameter of the sphere. Lines of constant latitude are small circles.
A pared-down version of the software that analyzes the raw spectroscopic data from SDSS-I/II, as well as SEGUE-2 and BOSS. It runs on the mountain, and gives feedback to the observers about the signal-to-noise ratio of each spectroscopic exposure and any possible problems in the data.
Southern Survey
See Southern Survey and Stripe 82
Southern Survey and Stripe 82
In SDSS-I and II, three stripes were observed (both imaging and spectroscopy). Stripe 82, the equatorial stripe (stretching roughly from 21 hours to 4 hours right ascension, and from -1.25 degrees to +1.25 degrees declination) was imaged repeatedly, especially as part of the Supernova survey. These data have been used for variability studies, as well as a deep coaddition. Stripe 82 was also the target of a wide variety of ancillary spectroscopic observations, as described in the DR4 paper. The BOSS survey also carried out extensive spectroscopic observations on Stripe 82. BOSS has carried out contiguous imaging of 3100 deg2 in the Southern Galactic Cap.
See idlspec2d.
Special Plates
Special plates obtain spectra beyond the SDSS-I/II main survey targets, according to a variety of schemes for selecting targets. Many of the special plates were targeted along Stripe 82. A list of the types of special programs, and a description of the targeting schemes, is given in the special target algorithms pages.

A unique bit-encoded 64-bit ID used for optical spectroscopic objects. (APOGEE targets are assigned an "apogee_id" ID). It is generated from plateid, mjd, and fiberid. 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 SpecObjID as follows:

  • 50-63: plate id number (14 bits)
  • 38-49: fiber id number (12 bits)
  • 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 specObjID 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 (N-5)*10000+M*100+P.

In the CAS, there is a database function fSpecIdFromSDSS() which will convert PLATE, MJD, FIBER into a properly formatted specObjID.

See idlspec2d.
See idlspec2d.
The instrument used to obtain spectra of objects. SDSS-I/II used two identical spectrographs, each receiving as input 320 of the fibers from a plate. This spectrograph was used for SEGUE-2, and then was substantially upgraded for BOSS, to receive 500 fibers each, and to increase throughput. The MARVELS and APOGEE projects each have their own specialized spectrographs. Details about all the instruments can be found on the instruments page.
The procedure for absolute flux calibration of spectra. Please see the Spectrophotometry section of the algorithms pages for an extensive description.
spPlate file
A FITS image containing the wavelength- and flux-calibrated combined spectrum over all exposures (potentially spanning multiple nights) for a given mapped plate. Output as part of spec2d, it does not contain redshift information. Available in the SAS.
The CAS table containing stellar line indices measured by the SEGUE Stellar Parameter Pipeline (SSPP). These quantities are indexed on the 'specObjID' key index parameter for queries joining to other tables such as specobjall and photoobjall.
The CAS table containing the outputs of the SEGUE Stellar Parameter Pipeline (SSPP), including metallicities, gravity, and effective temperature. These outputs are indexed on the 'specObjID' key index parameter for queries joining to other tables such as specobjall and photoobjall.
The Structured Query Language, a standard means of asking for data from databases. For more, see our SQL help page.
The SEGUE Stellar Parameter Pipeline, is the union of a set of techniques gathered together in one set of pipeline for measuring common stellar atmospheric parameters (namely metallicity, effective temperature and surface gravity) for stars based on medium resolution (R ~ 2000) spectra of the stars, combined with accurate stellar photometry. This pipeline yields Teff, [Fe/H] ([M/H]), and log g measurements for all stars of sufficient S/N (generally better than 10 or 15 for bluer stars). The results are available in the CAS in the sppParams table.
A unique region of sky, bounded by two lines of constant η. A stave is a portion of a stripe that is tapered near the poles so that it does not overlap with the neighboring stave. This term is used analogously to the meaning of stave in barrelmaking.
Stokes Parameters

These quantities are related to object ellipticities. Define the flux-weighted second moments of the object as:

Mxx = <x2/r2> , Myy = <y2/r2> , Mxy = <xy/r2>

In the case that the object's isophotes are self-similar ellipses, one can show that:

Q = Mxx-Myy = [(a-b)/(a+b)] × cos 2 φ
U = Mxy = [(a-b)/(a+b)] × sin 2 φ

where a and b are the semimajor and semiminor axes and φ is the position angle. Q and U are Q and U in the table PhotoObj and are referred to as "Stokes parameters." They can be used to reconstruct the axis ratio and position angle, measured relative to row and column of the CCDs. This is equivalent to the normal definition of position angle (east of north), for the scans on the equator. The performance of the Stokes parameters are not ideal at low signal-to-noise ratio, in which case the adaptive moments will be more useful.

A strip is a scan along a line of constant survey latitude η. The center of a stripe is set at a given η; centers of strips have a boresight offset added. Because the columns of CCDs have gaps between them, it is necessary to take two offset but overlapping observations to fill in a stripe. These two scans are called strips, one North and one South. Note that while strips are centered on a given η, they are not bound by η lines. Thus they are rectangular regions and can overlap at the poles.
Stripes are the sum of two strips, defined in survey coordinates (λ, η). A "stripe" is defined by a line of constant η, bounded on the north and south by the edges of the two strips that make up the stripe, and bounded on the east and west by lines of constant λ. Because both strips and stripes are defined in "observed" space, they are rectangular areas which overlap as one approaches the poles. NOTE: You may see the term stripe used to mean an area bounded by η lines, which would be a unique part of the sky. That is a common use of the term, as some of the target selection documentation uses it that way. The proper term for the unique, η-bound portion of a stripe is a stave.
Stripe 82
See Southern Survey.
Stripe 82 coadd
An SDSS data product that sums (i.e., co-adds) individual pixels from the photometric sections of runs on Stripe 82. There are two coadds: one for the North strip and one for the South strip. These co-added runs were processed via the photometric pipeline to produce catalog values for objects that go much deeper than the individual runs. Thus "Stripe 82 coadd" can also refer to the catalog values as well as to the pixels. The coadd is available through the DR7 CAS.
Stripe 82 database
The Stripe 82 database, accessible via the DR7 CAS, includes the Stripe 82 coadds as well as all runs ever obtained on Stripe 82. Runs obtained earlier than 2004 were in photometric conditions and were part of the Southern Survey. More recent runs were for the Supernova Survey and have a faster cadence, but were often obtained in non-photometric conditions, or with Moon, or with poor seeing. All of the runs have been calibrated in a uniform way, although the non-photometric runs will of course have larger errors. The DR9 and DR10 releases do not separately distribute this data.
One of the three surveys that comprised the second phase (SDSS-II) of the Sloan Digital Sky Survey (SDSS). It featured repeated imaging of Stripe 82 to find these remnants of gigantic explosions from dying stars, together with detailed spectroscopic and photometric followup to get redshifts and lightcurves. The survey discovered almost 500 confirmed Type 1a supernovae with redshifts up to about 0.4. Their light curves allow determination of their distances, making them useful to map the rate of expansion of the universe, and thus quantify the properties of cosmological dark energy.
Surface Brightness

The frames pipeline also reports the radii containing 50% and 90% of the Petrosian flux for each band, petroR50 and petroR90 respectively. The usual characterization of surface-brightness in the target selection pipeline of the SDSS is the mean surface brightness within petroR50.

It turns out that the ratio of petroR50 to petroR90, the so-called "inverse concentration index", is correlated with morphology (Shimasaku et al. 2001, Strateva et al. 2001). Galaxies with a de Vaucouleurs profile have an inverse concentration index of around 0.3; exponential galaxies have an inverse concentration index of around 0.43. Thus, this parameter can be used as a simple morphological classifier.

An important caveat when using these quantities is that they are not corrected for seeing. This causes the surface brightness to be underestimated, and the inverse concentration index to be overestimated, for objects of size comparable to the PSF.

Survey Best
The best imaging observation of an object with multiple observations is called the survey primary object, and other observations of primary objects are survey secondary. However, occasionally there are objects detected in areas which are not detected in the primary area of any field; these are typically low signal-to-noise ratio or transient objects. These are called "survey best".
Survey Coordinates
One of the main coordinate systems used by the survey, with coordinates η and λ. This is a spherical coordinate system, where (η,λ)=(0,90.) corresponds to (ra,dec)=(275.,0.) and (η,λ)=(57.5,0.) corresponds to (ra,dec)=(0.,90.). Note also that at (η, λ)=(0.,0.), (ra,dec)=(185.,32.5). So, this is a pure rotation of the usual RA/Dec system, as opposed to the great circle system, which is defined relative to each individual stripe. For some reason, although "η" is constant along great circles, it is referred to as "survey latitude," while "λ" is referred to as "survey longitude." Also, "η runs only from -90. to 90.; the back of the sphere is covered by "λ", which runs from -180. to 180. The Survey coordinates are defined such that the "primary" area of a stripe (otherwise known as a stave) in the north is defined by a rectangle in Survey coordinates which is 2.5 degrees wide in η (coordinate width). See the algorithms page for SDSS survey coordinates.
Survey Equator
The great circle perpendicular to the central meridian, passing through the survey center at δ = 32.8° is the survey equator.
Survey Poles
The locations of the poles in the survey coordinate system. Due to the unusual nature of this system, there is an east pole and a west one, at δ = 0 , α = 18h 20m and 6h 20m.
Survey Primary
The 'main' photometric observation of an object. Because many of the survey runs overlap, an object may be observed two or more times. Only one observation is chosen as the 'primary' observation of the object, during the resolve process. Also note that during this process each area of the sky is assigned a field which is primary in that area, and all primary observations in that area must come from the primary field. Except in extremely rare occasions, survey primary detections must also be run primary.
Survey Secondary
The best imaging observation of an object with multiple observations is called the survey primary object, and other observations are stored in the PhotoObjAll table and PhotoObj view as survey secondary objects.

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A specific set of objects and their measured quantities stored in a database.
TAI is the time given in units of seconds, and is determined at APO using GPS-based NTP servers. To get the TAI time in Modified Julian Date format, use MJD = TAI/(24*3600). In practice at APO, there are several definition of MJD that can differ by of order seconds; when TAI is available it is the more reliable time source.
In DR9, an object selected as a candidate for spectroscopy (which has not necessarily ever received a fiber).
target selection
A general term used to describe how objects are selected for spectroscopic observation.
Targeting Flag and Bits
A flag (bitmask value) encoding information about how an object was targeted for spectroscopic follow-up observation.
The SDSS imaging data are taken in time-delay-and-integrate (TDI) mode at the sidereal rate almost simultaneously in five bands. The sky tracks through 5 CCD detectors in succession, each located behind a different filter. See this image to get a better idea.
The Hammer
The Hammer is the name of a piece of code developed by the University of Washington red star analysis team to provide accurate spectral types and other information for red stars (generally redder than spectral type K, extending through types M, L and T) observed in SDSS and SEGUE. The spectral type classifier output of the Hammer is provided in the sppParams table of the CAS for each star in the database. The Hammer provides the best classification information for redder stars.
Each unique source in the SDSS catalog is identified by a thingId. Each source may have been observed more than once in multiple runs, and might therefore have multiple detections listed in the catalog. Only one detection is considered primary. The resolve algorithm documentation explains these differences in full.
Tilable Target
Specific to the Legacy survey. These are spectroscopic targets which are assigned to tiles by tiling. The significance of this is that tilable targets are supposed to have as close to uniform completeness as possible and it should be possible to define well-defined samples of such targets. The bitmasks primTarget and secTarget described in the description of the target selection contain the target assignments of each object. Tilable targets for the SDSS-I/II Legacy survey are those with the primTarget flags QSO_HIZ, QSO_CAP, QSO_SKIRT, QSO_FIRST_CAP, QSO_FIRST_SKIRT, GALAXY_RED, GALAXY, GALAXY_BIG, GALAXY_BRIGHT_CORE, or STAR_BROWN_DWARF, and those with the secTarget flag HOT_STD.
A 1.49° radius circle on the sky determined by tiling and which contains the locations of up to 592 tilable targets and other science targets. For each tile one or more plates will be created. If less than the maximum of 592 tilable targets can be assigned to that tile, the spare fibers are assigned to other spectroscopic targets. The 48 remaining fibers are assigned to calibration targets, including sky fibers. These numbers are relevant for the Legacy Survey and SEGUE-2; for BOSS, there are 1000 fibers in total assigned to a tile. APOGEE and MARVELS tiles have 300 and 60 fibers, respectively.
Tile Run
A TileRun represents a single run of the tiling software. It is a logical unit of geometrical information that consists of a set of TilingBoundaries, TilingMasks, and Tiles that are associated with exactly one TileRun (i.e., each TilingBoundary, TilingMask, and Tile is associated with exactly one TileRun, with one exception that will be noted later). Sometimes this is also called a tiling "chunk."
Tiled Region
TileRegion is a term used to indicate the portion of a Tile that may have targets (i.e. within the union of the TilingBoundaries for a TileRun and outside the union of the TilingMasks for the TileRun and the global TilingMasks). These are not necessarily convex, because of the TilingMasks.
The process of designing tiles for spectroscopy.
Tiling Boundary
A TilingBoundary is a set of "rectangles" that defines the area of the sky that may be tiled in a TileRun. Only targets from within the union of the TilingBoundaries for a TileRun may be assigned to Tiles created during a TileRun. A single TilingBoundary must be contained within a single chunk.
Tiling Mask
TilingMasks are "rectangles" that should not be considered part of the tileable area during a TileRun.
Telescope Performance Monitor. Software that reports on the physical parameters of the survey telescope.
In SDSS-I/II, The tsField files are FITS binary tables containing the information about each imaging field that is exported to the Field tables in the CAS.
In SDSS-I/II, the tsObj files were FITS binary tables containing calibrated object catalogs output by the frames pipeline, one per field. In SDSS-III, they are replaced with the so-called PhotoObj files, which form the basis for the photometric data found in the PhotoObj table in CAS.
Also known as PhotoType or objc_type, which distinguishes stars (type=6) and galaxies (type=3) based on their morphology. It is quantified on the basis of the difference between the PSF and model magnitudes.

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Ubercal is the method used to photometrically calibrate the SDSS imaging survey, using overlap between scans to solve for zeropoints and extinction for each camera-column for each run. It makes heavy use of overlapping 'crossing scans', which run at an oblique angle to the standard survey stripes and yield multiple detections of the same stars in different camera columns. The method is described in detail in Padmanabhan et al. (2008).
The US Naval Observatory CCD Astrograph Catalog, a dense grid of astrometric standards over the sky off of which the SDSS imaging scans are calibrated.

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In a database, a way of looking at a subset of the data in a given Table. Views are treated just like Tables in SQL queries. For example, in the CAS, the Galaxy view shows only the subset of the PhotoObj table that are morphologically classified as galaxies.

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World Coordinate System. The FITS standard for defining astrometric calibrations in the image header.

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The name of one of the algorithms used to select quasar candidates for spectroscopy in BOSS. This algorithm currently is used for the CORE selection of quasar candidates. It uses the "extreme deconvolution" method of Bovy et al. (2011), which determines the consistency of each objects with the quasar and stellar loci in color space, given its photometry and errors.

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Yanny parameter file
See par file.

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Zhed Point
The stellar locus in SDSS color-color space is essentially one-dimensional. Various canonical positions in color-color space can be defined from bends in the stellar locus; we call these positions the Zhed Points. The constancy of these positions (after correcting for Galactic reddening) is a very useful test of the uniformity of our photometry.
A parameter set by the redshift-fitting code that indicates that a redshift or classification may be in error. The bits are described here.

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