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	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