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Introductory Overview of SDSS Data Products

Introductory Overview of SDSS Data Products. Huan Lin Experimental Astrophysics Group Fermilab. SDSS Documentation. PLEASE make use of extensive SDSS documentation available on the web SDSS DR4 web site http://www .sdss.org/dr4/ Data Products Sky Coverage Data Access Algorithms

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Introductory Overview of SDSS Data Products

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  1. Introductory Overview of SDSS Data Products Huan Lin Experimental Astrophysics Group Fermilab

  2. SDSS Documentation • PLEASE make use of extensive SDSS documentation available on the web • SDSS DR4 web site http://www.sdss.org/dr4/ • Data Products • Sky Coverage • Data Access • Algorithms • SDSS collaboration DR4 Catalog Archive Server (CAS) http://cas.sdss.org/collabDR4/en/ • Schema Browser

  3. A Little SDSS Nomenclature • Run: a scan made by the imaging camera • Runs make up a strip, 2 interleaved N and S strips make up each 2.5 deg wide survey stripe • Rerun: number assigned to each (re)processing of the same run • Camcol: 1 of the 6 camera columns comprising each run • Field: 2048 pixel x 1361 pixel areas dividing each run • Target: reduction used for spectroscopic target selection • Best: best available reduction

  4. Runs comprising Stripe 30, N and S strips Good Target Chunk (“resolved” of overlaps) Bad

  5. SDSS DR4 Sky Coverage See sky coverage page for the lists of target/best stripes, runs, fields, … Imaging Area 6670 sq. deg. Use “Foot” (footprint) server to convert RA/Dec to run/rerun/camcol/field Spectroscopic Area 5320 sq. deg.

  6. SDSS Images • “Corrected frames”: fpC-$run-$filter$camcol-$field.fit • E.g., fpC-002126-g6-0428.fit • 0.396 arcsec/pixel • 2048 x 1489 pixels (note adjacent fpC images overlap; e.g., along field direction primary area is only 1361pixels long) • Retrieve from Data Archive Server (DAS), or via http with, say, wget • Convert counts to calibrated magnitudes as on later slide • A “sofbias” = 1000 counts is included in the background of the fpC images (subtract it off first to get real sky) • See DR4 web site for astrometric calibration beyond the WCS given in the header

  7. Image-Related Products • Atlas images fpAtlas*.fit: “postage-stamp” images of individual objects • PSF files psField*.fit: PSF information • Image masks fpM*.fit: binary masks indicating saturation, interpolation, detected objects, etc. • Code to read fpAtlas, psField, and fpM files are available on the DR4 web site • Bad region masks mask*.csv: ascii files indicating regions affected by saturation/bright stars, satellite trails, bad seeing, and survey holes

  8. Field Information • Field table in CAS • tsField-$run-$camcol-$rerun-$field.fit binary fits tables • Field quality (good, acceptable, bad, …) • Photometric calibration zeropoints and extinction coefficients • PSF and seeing information • Sky (atmospheric extinction corrected) • Astrometric transformation • Plus other stuff …

  9. Object Lists • PhotoObj and similartables/views in CAS • tsObj-$run-$camcol-$rerun-$field.fit binary fits tables • Readable using standard fits I/O routines; see web site for example using SM • Field quality, seeing, sky are also in fits header • Fits tables contain • Run, rerun, camcol, field, id • Position: RA/Dec, etc. • Object classifications and photometric processing flags • Target selection flags • Various calibrated SDSS magnitudes • Surface brightness, concentration index, radial profiles • Adaptive moments • Lots of other quantities … see data model

  10. Primary Survey Objects • Objects in the SDSS can be duplicated because of geometric overlaps between adjacent runs, fields, etc., and because of details of how photo defines detected objects • In tsObj files, to avoid duplicate objects, select objects with the their status flag set as PRIMARY • In the CAS, similarly select on the status flag or use views like PhotoPrimary, Galaxy, or Star, which are comprised of only primary survey objects

  11. SDSS asinh Magnitudes • Count rate f/f0 = counts/exptime * 100.4*(aa + kk * airmass) • exptime=53.907456, aa=zeropoint, kk=extinction coefficient • tsField file or Field table • Conventional magnitude = -2.5 * log10(f/f0) • asinh magnitude = -(2.5/ln(10)) * [asinh((f/f0)/2b)+ln(b)] • Lupton, Gunn, & Szalay 1999, AJ, 118 • b is a softening parameter, set to be about 1 sigma of sky noise in each filter (see DR4 web site) • Difference between asinh and conventional mag < 1% for objects brighter than asinh mag m(f/f0 = 10b) = 22.12, 22.60, 22.29, 21.85, 20.32 for ugriz

  12. SDSS Magnitudes • PSF: Fit of a PSF model to the object • Petrosian: Magnitude w/i the Petrosian radius rP, where the ratio of the local surface brightness in an annulus at rP to the mean surface brightness within rP is 0.2 (aperture set by r band) • Model: The better of the fits to a PSF-convolved deVaucouleurs (deV) or exponential (exp) galaxy profile (aperture set by r band) • cmodel: The best linear combination of the deVaucouleurs and exponential fits in each band; the flux is Fcomposite= fracDeV FdeV + (1 - fracDeV) Fexp • Fiber: Magnitude inside the 3”-diameter spectroscopic fiber aperture (images convolved to 2” seeing first)

  13. SDSS Magnitudes (cont’d) • Magnitudes are in general not corrected for Milky Way extinction • Corrections are computed following Schlegel, Finkbeiner & Davis (1998) • Given as reddening in the tsObj files or extinction in the CAS • To compute magnitudes in arbitrary circular aperture, can use the radial surface brightness profiles • Given as the average surface brightness in a series of annuli • Units are maggies/sq. arcsec, where 1 maggie of flux has an AB magnitude of 0 • See profMean in the tsObj files and the PhotoProfile table in CAS

  14. Example Magnitude Usage • Photometry of bright galaxies: e.g. for main sample galaxies, use Petrosian magnitudes (model independent, S/N remains good to r=20 or so) • Photometry of galaxies: cmodel magnitude (close to optimal S/N, better cf. Petrosian magnitudes esp. at faint magnitudes) • Colors of galaxies: model magnitudes (aperture from r band applied to all filters, unlike for cmodel magnitudes) • Photometry of distant quasars: PSF magnitude (unresolved objects) • Colors of stars: PSF magnitude

  15. Star/Galaxy Separation • Star/galaxy separation criterion used by photo • An object with psfMag - cmodelMag > 0.145 is classified as a galaxy, otherwise it’s a star (in each filter, or for summed flux in each band with a detection) • Classification given as objc_type in the tsObj files or type in the CAS (3 = galaxy, 6 = star) • Star/galaxy separation criteria used by target is different and more conservative for galaxies • Main galaxies, Cut I LRGs: rPSF - rmodel > 0.24 • Cut II LRGs: rPSF - rmodel > 0.4

  16. Target Selection Flags • Flags are used extensively to indicate type, status, quality, etc., etc. • primTarget: 32-bit primary target selection flag • Main galaxies, quasars, high-z quasars, luminous red galaxies (LRGs) , … • E.g., to select main sample galaxies: (primTarget & 64) > 0 • secTarget: secondary target selection flag • Used to indicate calibration stars, skies, guide stars, etc. • Southern Survey targets indicated with highest bit set in secTarget and primTarget

  17. primTarget flags Stoughton et al. (2002)

  18. primTarget flags Stoughton et al. (2002) secTarget flags Stoughton et al. (2002)

  19. Photometric Processing Flags • Flagsobjc_flags, objc_flags2in tsObj files, flags in CAS (also flags for each individual filter) • Important to check flags to eliminate problem objects or junk • Detailed documentation on DR4 web site • Examples given to derive clean samples of stars and galaxies • Primary objects to avoid duplicates • Reject saturated objects • Reject objects with deblending problems • Reject objects with interpolation problems • Other cuts …

  20. Spectra • 2D spPlate-$plate-$mjd.fit fits images and binary tables • 640 spectra together in single fits file for each plate • Flux-calibrated spectra, inverse variance, quality masks, and other supporting information • No measured parameters (e.g., redshifts) • 1D spSpec-$plate-$mjd-$fiber.fit image and binary tables • 1 fits file for each fiber of each plate • Calibrated spectrum, continuum-subtracted spectrum, noise, quality mask • Spectral classification, redshift, redshift error and confidence • Binary tables with line measurements, line indices, emission-line and cross-correlation redshift information, … • spDiag-$mjd-$plate.par ascii file • Summary information for all 640 spectra on each plate • Object id, target flags • Spectral classification, redshift, redshift error and confidence

  21. Spectroscopic Information • Generate vacuum wavelengths using  = 10(COEFF0 + COEFF1*i), where i denotes the (zero indexed) pixel number, and COEFF1 and COEFF2 are from file headers • Check redshift confidence z_Conf, and also status z_Status and warning z_Warnin flags (I like z_Conf > 0.7 based on eye checks of galaxy spectra) • In CAS, use the SpecObj, SpecLine, and SpecLineIndex tables/views to get spectroscopic parameters • The spectrophotometry is good (e.g., overall residual offsets vs. gri fiber magnitudes/colors of < a few percent); see documentation on DR4 web site for details

  22. Matching Spectroscopic and Photometric Data • Done in CAS joining the appropriate tables, or using SQS or IQS (now parts of CAS) first and then the DAS to retrieve the spectra and image fits files • Also available for each plate are the packaged spectral data and corresponding object lists and atlas images: spPlate, spSpec, spDiag, spObj (analogous to tsObj), … • Get from DAS (tsObjFromMap option) or via http (e.g. with wget)

  23. Southern Survey and Special Spectroscopic Programs • Mostly on Stripe 82, including u-selected galaxies, low-z galaxies, deep LRGs, faint quasars, spectra of everything, stellar programs, … • Retrieve from DAS or using PlateX table in CAS; see sdss-archive/2511, and instructions are forthcoming on DR4 web site • See the Southern Equatorial Survey plates page athttp://www-sdss.fnal.gov/targetlink/southernEqSurvey/ • See Ivan Baldry’s page and catalogs athttp://mrhanky.pha.jhu.edu/~baldry/sdss-southern/

  24. Value-Added Data Products • DR3 Quasar Catalog • Composite and unusual quasar spectra • White dwarf catalog • Moving Object Catalog • Cut & Enhance galaxy cluster catalog • Value-added galaxy catalogs: NYU, CMU-Pitt, MPA/JHU • Orion data release • Princeton spectra • SuperCosmos Science Archives

  25. SDSS Documentation Redux • PLEASE make use of extensive SDSS documentation available on the web • SDSS DR4 web site http://www.sdss.org/dr4/ • Data Products • Sky Coverage • Data Access • Algorithms • SDSS collaboration DR4 Catalog Archive Server (CAS) http://cas.sdss.org/collabDR4/en/ • Schema Browser

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