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]

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 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]

nanomaggy

DECam aperture fluxes on the co-added 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

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]

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

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 https://legacy.noirlab.edu/noao/staff/fvaldes/CPDocPrelim/PL201_3.html

1

2

saturated

detailed at https://legacy.noirlab.edu/noao/staff/fvaldes/CPDocPrelim/PL201_3.html

2

4

interpolated

detailed at https://legacy.noirlab.edu/noao/staff/fvaldes/CPDocPrelim/PL201_3.html

4

16

single exposure cosmic ray

detailed at https://legacy.noirlab.edu/noao/staff/fvaldes/CPDocPrelim/PL201_3.html

6

64

bleed trail

detailed at https://legacy.noirlab.edu/noao/staff/fvaldes/CPDocPrelim/PL201_3.html

7

128

multi-exposure transient

detailed at https://legacy.noirlab.edu/noao/staff/fvaldes/CPDocPrelim/PL201_3.html

8

256

edge

detailed at https://legacy.noirlab.edu/noao/staff/fvaldes/CPDocPrelim/PL201_3.html

9

512

edge2

detailed at https://legacy.noirlab.edu/noao/staff/fvaldes/CPDocPrelim/PL201_3.html

Goodness-of-Fits

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 (http://arxiv.org/abs/1410.7397), 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 (http://arxiv.org/abs/1012.4804 ; 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 for the DECam \(u\), \(g\), \(r\), \(i\), \(z\), \(Y\) filters, respectively. These are slightly different than the ones in Schlafly & Finkbeiner (http://arxiv.org/abs/1012.4804).

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.

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:

\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

http://data.sdss3.org/datamodel/files/BOSS_PHOTOOBJ/RERUN/RUN/CAMCOL/photoObj.html

SDSS_CAMCOL

byte

SDSS_FIELD

int32

SDSS_ID

int32

SDSS_OBJID

int64

SDSS_PARENT

int32

SDSS_NCHILD

int32

SDSS_OBJC_TYPE

int64

SDSS_OBJC_FLAGS

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_CMODELFLUX_IVAR

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]

SDSS_RESOLVE_STATUS

int64