Tractor Catalog Format

Name Type Units Description
BRICKID int32   Brick ID [1,662174]
BRICKNAME char   Name of brick, encoding the brick sky position
OBJID int64   Catalog object number within this brick; a unique identifier hash is BRICKID,OBJID; OBJID spans [0,N-1] and is contiguously enumerated within each brick
BRICK_PRIMARY char   "T" if the object is within the brick boundary
BLOB int64   Blend family; objects with the same [BRICKID,BLOB] identifier were modeled (deblended) together; contiguously numbered from 0
TYPE char   Morphological model: PSF=stellar, EXP=exponential, DEV=deVauc, COMP=composite
RA float64 deg Right ascension at equinox J2000
RA_IVAR float32 1/deg² Inverse variance of RA, excluding astrometric calibration errors
DEC float64 deg Declination at equinox J2000
DEC_IVAR float32 1/deg² Inverse variance of DEC (no cos term!), excluding astrometric calibration errors
BX float32 pix X position (0-indexed) of coordinates in brick image stack
BY float32 pix Y position (0-indexed) of coordinates in brick image stack
BX0 float32 pix Initialized X position (0-indexed) of coordinates in brick image stack
BY0 float32 pix Initialized Y position (0-indexed) of coordinates in brick image stack
LEFT_BLOB char   "T" if an object center has been optimized to be outside the fitting pixel area; otherwise "F"
DECAM_FLUX float32[6] nanomaggies DECam model flux in ugrizY
DECAM_FLUX_IVAR float32[6] 1/nanomaggies² Inverse variance oF DECAM_FLUX
DECAM_APFLUX float32[8,6] nanomaggies DECam aperture fluxes on the co-added images in apertures of radius [0.5,0.75,1.0,1.5,2.0,3.5,5.0,7.0] arcsec in ugrizY
DECAM_APFLUX_RESID float32[8,6] nanomaggies DECam aperture fluxes on the co-added residual images
DECAM_APFLUX_IVAR float32[8,6] 1/nanomaggies² Inverse variance oF DECAM_APFLUX
DECAM_MW_TRANSMISSION float32[6]   Galactic transmission in ugrizY filters in linear units [0,1]
DECAM_NOBS int32[6]   Number of images that contribute to the central pixel in each filter for this object (not profile-weighted)
DECAM_RCHI2 float32[6]   Profile-weighted χ² of model fit normalized by the number of pixels
DECAM_FRACFLUX float32[6]   Profile-weight fraction of the flux from other sources divided by the total flux (typically [0,1])
DECAM_FRACMASKED float32[6]   Profile-weighted fraction of pixels masked from all observations of this object, strictly between [0,1]
DECAM_FRACIN float32[6]   Fraction of a source's flux within the blob, near unity for real sources
OUT_OF_BOUNDS bool[6]   "T" for objects whose center is on the brick; less strong of a cut than BRICK_PRIMARY
DECAM_ANYMASK int32[6]   Bitwise mask set if the central pixel from any image satisfy each condition
DECAM_ALLMASK int32[6]   Bitwise mask set if the central pixel from all images satisfy each condition
WISE_FLUX float32[4] nanomaggies WISE model flux in W1,W2,W3,W4
WISE_FLUX_IVAR float32[4] 1/nanomaggies² Inverse variance of WISE_FLUX
WISE_MW_TRANSMISSION float32[4]   Galactic transmission in W1,W2,W3,W4 filters in linear units [0,1]
WISE_NOBS int32[4]   Number of images that contribute to the central pixel in each filter for this object (not profile-weighted)
WISE_FRACFLUX float32[4]   Profile-weight fraction of the flux from other sources divided by the total flux (typically [0,1])
WISE_RCHI2 float32[4]   Profile-weighted χ² of model fit normalized by the number of pixels
DCHISQ float32[4]   Difference in χ² between successfully more-complex model fits
FRACDEV float32   Fraction of model in deVauc [0,1]
FRACDEV_IVAR float32   Inverse variance of FRACDEV
SHAPEEXP_R float32 arcsec Half-light radius of exponential model (>0)
SHAPEEXP_R_IVAR float32 1/arcsec² Inverse variance of R_EXP
SHAPEEXP_E1 float32   Ellipticity component 1
SHAPEEXP_E1_IVAR float32   Inverse variance of SHAPEEXP_E1
SHAPEEXP_E2 float32   Ellipticity component 2
SHAPEEXP_E2_IVAR float32   Inverse variance of SHAPEEXP_E2
SHAPEDEV_R float32 arcsec Half-light radius of deVaucouleurs model (>0)
SHAPEDEV_R_IVAR float32 1/arcsec² Inverse variance of R_DEV
SHAPEDEV_E1 float32   Ellipticity component 1
SHAPEDEV_E1_IVAR float32   Inverse variance of SHAPEDEV_E1
SHAPEDEV_E2 float32   Ellipticity component 2
SHAPEDEV_E2_IVAR float32   Inverse variance of SHAPEDEV_E2
EBV float32 mag Galactic extinction E(B-V) reddening from SFD98, used to compute DECAM_MW_TRANSMISSION and WISE_MW_TRANSMISSION

Mask Values

The DECAM_ANYMASK and DECAM_ALLMASK bit masks are defined as follows from the CP Data Quality bits.

Bit Value Name Description
0 1 detector bad pixel/no data detailed at
1 2 saturated detailed at
2 4 interpolated detailed at
4 16 single exposure cosmic ray detailed at
6 64 bleed trail detailed at
7 128 multi-exposure transient detailed at
8 256 edge detailed at
9 512 edge2 detailed at


The DCHISQ values represent the penalized χ² of all the pixels compared to various models. This 4-element vector contains the χ² difference between the best-fit point source, deVauc model, exponential model, and a composite model. The number of degrees of freedom to include as a penalty to these χ² values are 2 for a point source (ra,dec), 5 for the deVauc or exp model, and 9 for the composite model.

The DECAM_RCHI2 values are interpreted as the reduced χ² pixel-weighted by the model fit, computed as the following sum over pixels in the blob for each object:

\begin{equation*} \chi^2 = \frac{\sum \left[ \left(\mathrm{image} - \mathrm{model}\right)^2 \times \mathrm{model} \times \mathrm{inverse\, variance}\right]}{\sum \left[ \mathrm{model} \right]} \end{equation*}

The above sum is over all images contributing to a particular filter. The above can be negative-valued for sources that have a flux measured as negative in some bands where they are not detected.

SDSS_TREATED_AS_POINTSOURCE indicated whether an object was initialized as an SDSS point source (if "T") or galaxy (if "F"). This is based upon the SDSS morphological classifications, where SDSS_OBJTYPE=6 indicates a point source and =3 indicates a galaxy. However, SDSS_TREATED_AS_POINTSOURCE is also set to "T" for sources satisfying any of the following conditions: the effective radius is measured as S/N less than 3, sources with very large flux, the effective radius reported is the largest allowed, or the effective radius S/N is larger than expected given the measured flux of the objects. These are almost the same conditions as described in Lang et al 2014 (, and are further described there.

Galactic Extinction Coefficients

The Galactic extinction values are derived from the SFD98 maps, but with updated coefficients to convert E(B-V) to the extinction in each filter. These are reported in linear units of transmission, with 1 representing a fully transparent region of the Milky Way and 0 representing a fully opaque region. The value can slightly exceed unity owing to noise in the SFD98 maps, although it is never below 0.

Extinction coefficients for the SDSS filters have been changed to the values recommended by Schlafly & Finkbeiner 2011 ( ; Table 4) using the Fizpatrick 1999 extinction curve at R_V = 3.1 and their improved overall calibration of the SFD98 maps. These coefficients are A / E(B-V) = 4.239, 3.303, 2.285, 1.698, 1.263 in ugriz, which are different from those used in SDSS-I,II,III, but are the values used for SDSS-IV/eBOSS target selection.

Extinction coefficients for the DECam filters also use the Schlafly & Finkbeiner 2011 values, with u-band computed using the same formulae and code at airmass 1.3 (Schlafly, priv. comm. decam-data list on 11/13/14). These coefficients are A / E(B-V) = 3.995, 3.214, 2.165, 1.592, 1.211, 1.064. (These are slightly different than the ones in Schlafly & Finkbeiner (

The coefficients for the four WISE filters are derived from Fitzpatrick 1999, as recommended by Schafly & Finkbeiner, considered better than either the Cardelli et al 1989 curves or the newer Fitzpatrick & Massa 2009 NIR curve not vetted beyond 2 micron). These coefficients are A / E(B-V) = 0.184, 0.113, 0.0241, 0.00910.


The ellipticity, ε, is different from the usual eccentricity, \(e \equiv \sqrt{1 - (b/a)^2}\). In gravitational lensing studies, the ellipticity is taken to be a complex number:

\begin{equation*} \epsilon = \frac{a-b}{a+b} \exp( 2i\phi ) = \epsilon_1 + i \epsilon_2 \end{equation*}

Where ϕ is the position angle with a range of 180°, due to the ellipse's symmetry. Going between \(r, \epsilon_1, \epsilon_2\) and \(r, b/a, \phi\):

\begin{align*} r & = & r \\ |\epsilon| & = & \sqrt{\epsilon_1^2 + \epsilon_2^2} \\ \frac{b}{a} & = & \frac{1 - |\epsilon|}{1 + |\epsilon|} \\ \phi & = & \frac{1}{2} \arctan \frac{\epsilon_2}{\epsilon_1} \\ |\epsilon| & = & \frac{1 - b/a}{1 + b/a} \\ \epsilon_1 & = & |\epsilon| \cos(2 \phi) \\ \epsilon_2 & = & |\epsilon| \sin(2 \phi) \\ \end{align*}

Debugging Tags to Remove in the Future

The following are from the SDSS DR13 catalogs, to be released in 2015 as the SDSS-IV/eBOSS target selection catalogs.

Name Type Units Description
SDSS_RUN int32
SDSS_CAMCOL byte    
SDSS_FIELD int32    
SDSS_ID int32    
SDSS_OBJID int64    
SDSS_PARENT int32    
SDSS_NCHILD int32    
SDSS_OBJC_TYPE int64    
SDSS_OBJC_FLAGS2 int64    
SDSS_FLAGS int64[5]    
SDSS_FLAGS2 int64[5]    
SDSS_TAI float64[5]    
SDSS_RA float64    
SDSS_DEC float64    
SDSS_PSF_FWHM float32[5]    
SDSS_MJD int64    
SDSS_THETA_DEV float32[5]    
SDSS_THETA_DEVERR float32[5]    
SDSS_AB_DEV float32[5]    
SDSS_AB_DEVERR float32[5]    
SDSS_THETA_EXP float32[5]    
SDSS_THETA_EXPERR float32[5]    
SDSS_AB_EXP float32[5]    
SDSS_AB_EXPERR float32[5]    
SDSS_FRACDEV float32[5]    
SDSS_PHI_DEV_DEG float32[5]    
SDSS_PHI_EXP_DEG float32[5]    
SDSS_PSFFLUX float32[5]    
SDSS_PSFFLUX_IVAR float32[5]    
SDSS_CMODELFLUX float32[5]    
SDSS_MODELFLUX float32[5]    
SDSS_MODELFLUX_IVAR float32[5]    
SDSS_DEVFLUX float32[5]    
SDSS_DEVFLUX_IVAR float32[5]    
SDSS_EXPFLUX float32[5]    
SDSS_EXPFLUX_IVAR float32[5]    
SDSS_EXTINCTION float32[5]    
SDSS_CALIB_STATUS int64[5]