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/ o Data Productso Sky Coverageo Data Accesso Algorithms

● SDSS collaboration DR4 Catalog Archive Server (CAS) http://cas.sdss.org/collabDR4/en/o 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

Good

BadTarget Chunk

(“resolved” of overlaps)

Runs comprising Stripe 30, N and S strips

5

SDSS DR4 Sky Coverage

Imaging Area 6670 sq. deg.

Spectroscopic Area 5320 sq. deg.

See sky coverage page for the lists of target/best stripes, runs, fields, …

Use “Foot” (footprint) server to convert RA/Dec to run/rerun/camcol/field

6

SDSS Images● “Corrected frames”: fpC-$run-$filter$camcol-$field.fit

o E.g., fpC-002126-g6-0428.fito 0.396 arcsec/pixelo 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 similar tables/views in CAS● tsObj-$run-$camcol-$rerun-$field.fit binary fits tables

o 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 containo Run, rerun, camcol, field, ido Position: RA/Dec, etc.o Object classifications and photometric processing flagso Target selection flagso Various calibrated SDSS magnitudes o Surface brightness, concentration index, radial profileso Adaptive momentso 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) o exptime=53.907456, aa=zeropoint,

kk=extinction coefficiento tsField file or Field table

● Conventional magnitude = -2.5 * log10(f/f0)

● asinh magnitude = -(2.5/ln(10)) * [asinh((f/f0)/2b)+ln(b)]

o Lupton, Gunn, & Szalay 1999, AJ, 118o b is a softening parameter, set to be about 1 sigma of sky noise

in each filter (see DR4 web site)o 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

o Corrections are computed following Schlegel, Finkbeiner & Davis (1998)

o 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

o Given as the average surface brightness in a series of annuli o Units are maggies/sq. arcsec, where 1 maggie of flux has an AB

magnitude of 0o 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 o 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)

o 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

o Main galaxies, Cut I LRGs: rPSF - rmodel > 0.24 o 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 flago Main galaxies, quasars, high-z quasars, luminous red galaxies

(LRGs) , …o E.g., to select main sample galaxies: (primTarget & 64) > 0

● secTarget: secondary target selection flago Used to indicate calibration stars, skies, guide stars, etc. o Southern Survey targets indicated with highest bit set in

secTarget and primTarget

17

primTarget flags Stoughton et al. (2002)

18

secTarget flags Stoughton et al. (2002)

19

Photometric Processing Flags● Flags objc_flags, objc_flags2 in 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

o Primary objects to avoid duplicateso Reject saturated objectso Reject objects with deblending problemso Reject objects with interpolation problemso Other cuts …

20

Spectra● 2D spPlate-$plate-$mjd.fit fits images and binary tables

o 640 spectra together in single fits file for each plateo Flux-calibrated spectra, inverse variance, quality masks, and

other supporting informationo No measured parameters (e.g., redshifts)

● 1D spSpec-$plate-$mjd-$fiber.fit image and binary tableso 1 fits file for each fiber of each plateo Calibrated spectrum, continuum-subtracted spectrum, noise,

quality masko Spectral classification, redshift, redshift error and confidenceo Binary tables with line measurements, line indices, emission-line

and cross-correlation redshift information, …● spDiag-$mjd-$plate.par ascii file

o Summary information for all 640 spectra on each plateo Object id, target flagso 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), …

o Get from DAS (tsObjFromMap option) or via http (e.g. with wget)

23

24

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 at http://www-sdss.fnal.gov/targetlink/southernEqSurvey/

● See Ivan Baldry’s page and catalogs at http://mrhanky.pha.jhu.edu/~baldry/sdss-southern/

25

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

26

SDSS Documentation Redux

● PLEASE make use of extensive SDSS documentation available on the web

● SDSS DR4 web site http://www.sdss.org/dr4/ o Data Productso Sky Coverageo Data Accesso Algorithms

● SDSS collaboration DR4 Catalog Archive Server (CAS) http://cas.sdss.org/collabDR4/en/o Schema Browser