## tractor/<AAA>/tractor-<brick>.fits

FITS binary table containing Tractor photometry. Before using these catalogs, note that there are known issues regarding their content and derivation.

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 | int16 | 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 |

CPU_SOURCE | float32 | seconds | CPU time used for fitting this source |

CPU_BLOB | float32 | seconds | CPU time used for fitting this blob of sources (all sources in this brick with the same blob number) |

BLOB_WIDTH | int16 | size of this blob of pixels in brick coordinates, bounding box width | |

BLOB_HEIGHT | int16 | size of this blob of pixels in brick coordinates, bounding box height | |

BLOB_NPIX | int32 | size of this blob of pixels in brick coordinates, number of brick pixels | |

BLOB_NIMAGES | int16 | number of images overlapping this blob | |

BLOB_TOTALPIX | int32 | total number of pixels from all the images overlapping this blob | |

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 | 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] | 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 | 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 | |

WISE_LC_FLUX | float32[5,2] | nanomaggies | analog of WISE_FLUX, for each of up to five unWISE coadd epochs; W1 and W2 only |

WISE_LC_FLUX_IVAR | float32[5,2] | 1/nanomaggies² | analog of WISE_FLUX_IVAR, for each of up to five unWISE coadd epochs; W1 and W2 only |

WISE_LC_NOBS | int16[5,2] | analog of WISE_NOBS, for each of up to five unWISE coadd epochs; W1 and W2 only | |

WISE_LC_FRACFLUX | float32[5,2] | analog of WISE_FRACFLUX, for each of up to five unWISE coadd epochs; W1 and W2 only | |

WISE_LC_RCHI2 | float32[5,2] | analog of WISE_RCHI2, for each of up to five unWISE coadd epochs; W1 and W2 only | |

WISE_LC_MJD | float32[5,2] | mean MJD in W1 and W2, for up to five unWISE coadd epochs; 0 means epoch unavailable | |

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/nanomaggies² | 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/nanomaggies² | 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 | See the CP Data Quality bit description. |

1 | 2 | saturated | See the CP Data Quality bit description. |

2 | 4 | interpolated | See the CP Data Quality bit description. |

4 | 16 | single exposure cosmic ray | See the CP Data Quality bit description. |

6 | 64 | bleed trail | See the CP Data Quality bit description. |

7 | 128 | multi-exposure transient | See the CP Data Quality bit description. |

8 | 256 | edge | See the CP Data Quality bit description. |

9 | 512 | edge2 | See the CP Data Quality bit description. |

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:

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 (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. (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:

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 \(r, b/a, \phi\):