tractor/<AAA>/tractor<brick>.fits
FITS binary table containing Tractor photometry. Note there is a known issue regarding the fact that some bricks contain pixels but zero sources, hence have empty (zerorow) catalog files.
Name 
Type 
Units 
Description 

BRICKID 
int32 
Brick ID [1,662174] 

BRICKNAME 
char 
Name of brick, encoding the brick sky position, eg "1126p222" near RA=112.6, Dec=+22.2 

OBJID 
int32 
Catalog object number within this brick; a unique identifier hash is BRICKID,OBJID; OBJID spans [0,N1] and is contiguously enumerated within each brick 

BRICK_PRIMARY 
boolean 
True if the object is within the brick boundary 

BLOB 
int32 
Blend family; objects with the same [BRICKID,BLOB] identifier were modeled (deblended) together; contiguously numbered from 0 

NINBLOB 
int32 
Number of sources in this BLOB (blend family); isolated objects have value 1. 

TYCHO2INBLOB 
boolean 
Is there a Tycho2 (very bright) star in this blob? 

TYPE 
char[4] 
Morphological model: "PSF"=stellar, "SIMP"="simple galaxy" = 0.45" round EXP galaxy, "DEV"=deVauc, "EXP"=exponential, "COMP"=composite. Note that in some FITS readers, a trailing space may be appended for "PSF ", "DEV " and "EXP " since the column data type is a 4character string 

RA 
float64 
deg 
Right ascension at equinox J2000 
RA_IVAR 
float32 
1/deg² 
Inverse variance of RA (no cosine term!), excluding astrometric calibration errors 
DEC 
float64 
deg 
Declination at equinox J2000 
DEC_IVAR 
float32 
1/deg² 
Inverse variance of DEC, excluding astrometric calibration errors 
BX 
float32 
pix 
X position (0indexed) of coordinates in brick image stack 
BY 
float32 
pix 
Y position (0indexed) of coordinates in brick image stack 
BX0 
float32 
pix 
Initialized X position (0indexed) of coordinates in brick image stack 
BY0 
float32 
pix 
Initialized Y position (0indexed) of coordinates in brick image stack 
LEFT_BLOB 
boolean 
True if an object center has been optimized to be outside the fitting pixel area 

OUT_OF_BOUNDS 
boolean 
True for objects whose center is on the brick; less strong of a cut than BRICK_PRIMARY 

DCHISQ 
float32[5] 
Difference in χ² between successively morecomplex model fits: PSF, SIMPle, DEV, EXP, COMP. The difference is versus no source. 

EBV 
float32 
mag 
Galactic extinction E(BV) reddening from SFD98, used to compute DECAM_MW_TRANSMISSION and WISE_MW_TRANSMISSION 
DECAM_FLUX 
float32[6] 
nanomaggy 
DECam model flux in ugrizY 
DECAM_FLUX_IVAR 
float32[6] 
1/nanomaggy² 
Inverse variance oF DECAM_FLUX 
DECAM_APFLUX 
float32[8,6] 
nanomaggy 
DECam aperture fluxes on the coadded 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] 
nanomaggy 
DECam aperture fluxes on the coadded residual images 
DECAM_APFLUX_IVAR 
float32[8,6] 
1/nanomaggy² 
Inverse variance oF DECAM_APFLUX 
DECAM_MW_TRANSMISSION 
float32[6] 
Galactic transmission in ugrizY filters in linear units [0,1] 

DECAM_NOBS 
uint8[6] 
Number of images that contribute to the central pixel in each filter for this object (not profileweighted) 

DECAM_RCHI2 
float32[6] 
Profileweighted χ² of model fit normalized by the number of pixels 

DECAM_FRACFLUX 
float32[6] 
Profileweight fraction of the flux from other sources divided by the total flux (typically [0,1]) 

DECAM_FRACMASKED 
float32[6] 
Profileweighted 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 

DECAM_ANYMASK 
int16[6] 
Bitwise mask set if the central pixel from any image satisfy each condition 

DECAM_ALLMASK 
int16[6] 
Bitwise mask set if the central pixel from all images satisfy each condition 

DECAM_PSFSIZE 
float32[6] 
arcsec 
Weighted average PSF FWHM per band 
WISE_FLUX 
float32[4] 
nanomaggy 
WISE model flux in W1,W2,W3,W4 
WISE_FLUX_IVAR 
float32[4] 
1/nanomaggy² 
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 
int16[4] 
Number of images that contribute to the central pixel in each filter for this object (not profileweighted) 

WISE_FRACFLUX 
float32[4] 
Profileweight fraction of the flux from other sources divided by the total flux (typically [0,1]) 

WISE_RCHI2 
float32[4] 
Profileweighted χ² of model fit normalized by the number of pixels 

FRACDEV 
float32 
Fraction of model in deVauc [0,1] 

FRACDEV_IVAR 
float32 
Inverse variance of FRACDEV 

SHAPEEXP_R 
float32 
arcsec 
Halflight 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 
Halflight 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 

DECAM_DEPTH 
float32[6] 
1/nanomaggy² 
For a \(5\sigma\) point source detection limit, \(5/\sqrt(\mathrm{DECAM\_DEPTH})\) gives flux in nanomaggies and \(2.5[\log_{10}(5 / \sqrt(\mathrm{DECAM\_DEPTH}))  9]\) gives corresponding magnitude 
DECAM_GALDEPTH 
float32[6] 
1/nanomaggy² 
As for DECAM_DEPTH but for a galaxy (0.45" exp, round) detection sensitivity 
Mask Values
The DECAM_ANYMASK and DECAM_ALLMASK bit masks are defined as follows from the CP Data Quality bits.
GoodnessofFits
The DCHISQ values represent the χ² sum of all pixels in the source's blob for various models. This 5element vector contains the χ² difference between the bestfit point source (type="PSF"), simple galaxy model ("SIMP"), de Vaucouleurs model ("DEV"), exponential model ("EXP"), and a composite model ("COMP"), in that order. The "simple galaxy" model is an exponential galaxy with fixed shape of 0.45" and zero ellipticity (round) and is meant to capture slightlyextended but low signaltonoise objects. The DCHISQ values are the χ² difference versus no source in this locationthat is, it is the improvement from adding the given source to our model of the sky. The first element (for PSF) corresponds to a traditional notion of detection significance. Note that the DCHISQ values are negated so that positive values indicate better fits. We penalize models with negative flux in a band by subtracting rather than adding its χ² improvement in that band.
The DECAM_RCHI2 values are interpreted as the reduced χ² pixelweighted by the model fit, computed as the following sum over pixels in the blob for each object:
The above sum is over all images contributing to a particular filter. The above can be negativevalued for sources that have a flux measured as negative in some bands where they are not detected.
Galactic Extinction Coefficients
The Galactic extinction values are derived from the SFD98 maps, but with updated coefficients to convert E(BV) 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(BV) = 4.239, 3.303, 2.285, 1.698, 1.263 in ugriz, which are different from those used in SDSSI,II,III, but are the values used for SDSSIV/eBOSS target selection.
Extinction coefficients for the DECam filters also use the Schlafly & Finkbeiner (2011) values, with uband computed using the same formulae and code at airmass 1.3 (Schlafly, priv. comm. decamdata list on 11/13/14). These coefficients are \(A / E(BV)\) = 3.995, 3.214, 2.165, 1.592, 1.211, 1.064 for the DECam \(u\), \(g\), \(r\), \(i\), \(z\), \(Y\) filters, respectively. Note that these are slightly different from the coefficients in Schlafly & Finkbeiner (2011).
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(BV) = 0.184, 0.113, 0.0241, 0.00910.
Ellipticities
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:
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 \(r, b/a, \phi\):