Tractor Catalog Format

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 (zero-row) 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,N-1] 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 Tycho-2 (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 4-character 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 (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	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 more-complex model fits: PSF, SIMPle, DEV, EXP, COMP. The difference is versus no source.
EBV	float32	mag	Galactic extinction E(B-V) 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 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	uint8[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
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 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
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
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.

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
10	1024	longthin	\(\gt 5\sigma\) connected components with major axis \(\gt 200\) pixels and major/minor axis \(\gt 0.1\). To mask, e.g, satellite trails.

Goodness-of-Fits

The DCHISQ values represent the χ² sum of all pixels in the source's blob for various models. This 5-element vector contains the χ² difference between the best-fit 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 slightly-extended but low signal-to-noise objects. The DCHISQ values are the χ² difference versus no source in this location---that 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 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.

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 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(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*}