The measurement (or catalog) parameter file

In addition to the configuration file detailed above, SExtractor requires a file containing the list of measurements ("catalog parameters") that will be listed in the output catalog for every detection. This allows the software to compute only the measurements that are needed. The name of this catalog parameter file is traditionally suffixed with .param, and must be specified using the PARAMETERS_NAME config parameter. The full set of parameters can be queried with the command

$ sex -dp

Format

The format of the catalog parameter list is ASCII, and there must be a single keyword per line. Presently two kinds of keywords are recognized by SExtractor: scalars and vectors. Scalars, like X_IMAGE, produce single numbers in the output catalog. Vectors, like MAG_APER(4) or VIGNET(15,15), produce arrays of numbers. The ordering of measurements in the output catalog is identical to that of the keywords in the parameter list. Comments are allowed, they must begin with a #.

Variants

For many catalog parameters, especially those related to flux, position, or shape, several variants of the same measurement are available:

Fluxes and magnitudes

Fluxes may be expressed in counts (ADUs) or as Pogson [4] magnitudes. Flux measurements in ADUs are prefixed with FLUX_, for example: FLUX_AUTO, FLUX_ISO, etc. Magnitudes are prefixed with MAG_ e.g., MAG_AUTO, MAG_ISO, ... The MAG_ZEROPOINT configuration parameter sets the magnitude zero-point of magnitudes:

(1)\[\begin{split} {\tt MAG} = \left\{\begin{array}{ll} \mathrm{MAG\_ZEROPOINT} - 2.5 \log_{10} {\tt FLUX}\ &\mbox{if } {\tt FLUX} > 0\\ 99.0\ &\mbox{otherwise}. \end{array}\right.\end{split}\]

Flux and magnitude uncertainties

Flux uncertainties (error estimates) follow a scheme similar to that of fluxes. They are prefixed with FLUXERR_, as in FLUXERR_AUTO or FLUXERR_ISO. Magnitude uncertainties start with MAGERR_, for instance: MAGERR_AUTO, MAGERR_ISO,... They are computed using

(2)\[\begin{split} {\tt MAGERR} = \left\{\begin{array}{ll} \frac{2.5}{\ln 10}({\tt FLUXERR}/{\tt FLUX})\ &\mbox{if } {\tt FLUX} > 0\\ 99.0\ &\mbox{otherwise}. \end{array}\right.\end{split}\]

Pixel values and Surface brightnesses

Pixel values (averaged or not) \(p\) are expressed in counts (ADUs). There is no specific prefix (THRESHOLD, BACKGROUND, FLUX_MAX, etc.). Surface brightnesses are given in magnitudes per square arcsecond, and prefixed with MU_ e.g., MU_THRESHOLD, MU_MAX, ... The conversion to surface brightness relies on the MAG_ZEROPOINT and the PIXEL_SCALE configuration parameters:

(3)\[\begin{split} {\tt MU} = \left\{\begin{array}{ll} \mathrm{MAG\_ZEROPOINT} - 2.5 \log_{10} (p \times {\rm PIXEL\_SCALE}\,^2)\ &\mbox{if } p > 0\\ 99.0\ &\mbox{otherwise}. \end{array}\right.\end{split}\]

Setting PIXEL_SCALE to 0 instructs SExtractor to compute the pixel scale from the local Jacobian of the astrometric deprojection, based on the celestial WCS info [5] in the FITS image header, if available.

Positions and shapes

Positions, distances and position angles are computed in pixel coordinates. They may be expressed in image pixels, world coordinates, or in celestial coordinates, depending on the suffix:

_IMAGE

Measurements are given in pixel coordinates, in units of pixels. For example: Y_IMAGE, ERRAWIN_IMAGE, THETA_IMAGE etc. Following the FITS convention, in SExtractor the center of the first image pixel has coordinates (1.0,1.0). Position angles are counted from the x axis (axis 1), positive towards the y axis (axis 2)

_WORLD

Measurements are given in so-called “world coordinates”, converted from pixel coordinates using the local Jacobian of the transformation between both systems. This requires WCS metadata [6] to be present in the FITS image header(s). Position angles are counted from the first world axis, positive towards the second world axis.

_SKY, _J2000, _B1950

Measurements are given in celestial (equatorial) coordinates, converted from pixel coordinates using the local Jacobian of the transformation between both systems. Positions and distances are in units of degrees. This requires celestial WCS metadata [5] to be present in the FITS image header(s). _SKY measurements are given in the native world coordinate system. _J2000 and _B1950 measurements are automatically converted from the native WCS, taking into account the change of reference frame. In all cases, positions angles are counted East-of-North.

_FOCAL

Measurements are given in “focal plane coordinates”, which are actually projected coordinates, in degrees. This requires WCS metadata [6] to be present in the FITS image header(s). The computation of focal plane coordinates from pixel coordinates is similar to that of _SKY coordinates except that they are not de-projected and remain Cartesian. The main purpose of focal plane coordinates is to provide a common system for all the chips in a mosaic camera.

Note

Conversion to _FOCAL coordinates is available only for a limited subset of measurements.

Important

The WCS library currently implemented in SExtractor is a customized version of an early implementation (v1.1.1) by Calabretta. Several projections from later versions and alternative astrometric descriptions such as AST or that of original DSS plates are not supported at this time.

Measurement parameter list

Below is an exhaustive list of all the measurement parameters known to SExtractor. Please refer to the next sections for a detailed description of their meaning.

Table 1 SExtractor measurement parameters

Name	Unit	Description
NUMBER	...	Running object number
ID_PARENT	...	Parent ID (before deblending)
EXT_NUMBER	...	FITS extension number
FLAGS	...	Source extraction flags
FLAGS_WEIGHT	...	Weighting flags
IMAFLAGS_ISO	...	External flags combined within the isophotal footprint
NIMAFLAGS_ISO	...	Number of combined external flags
FLUX_ISO	count	Isophotal flux
FLUXERR_ISO	count	RMS error estimate for the isophotal flux
MAG_ISO	magnitude	Isophotal magnitude
MAGERR_ISO	magnitude	RMS error estimate for the isophotal magnitude
FLUX_ISOCOR	count	Corrected isophotal flux
FLUXERR_ISOCOR	count	RMS error estimate for the corrected isophotal flux
MAG_ISOCOR	magnitude	Corrected isophotal magnitude
MAGERR_ISOCOR	magnitude	RMS error estimate for the corrected isophotal magnitude
FLUX_APER	count	Flux(es) within fixed circular aperture(s)
FLUXERR_APER	count	RMS error estimate(s) for the flux(es) within fixed circular aperture(s)
MAG_APER	magnitude	Circular aperture magnitude(s)
MAGERR_APER	magnitude	RMS error estimate(s) for circular aperture magnitude(s)
FLUX_AUTO	count	Kron-like automated aperture flux
FLUXERR_AUTO	count	RMS error estimate for Kron-like automated aperture flux
MAG_AUTO	magnitude	Kron-like automated aperture magnitude
MAGERR_AUTO	magnitude	RMS error estimate for Kron-like automated aperture magnitude
KRON_RADIUS	...	Kron radius in units of A or B
FLUX_PETRO	count	Petrosian-like aperture flux
FLUXERR_PETRO	count	RMS error estimate for Petrosian-like aperture flux
MAG_PETRO	magnitude	Petrosian-like aperture magnitude
MAGERR_PETRO	magnitude	RMS error estimate for Petrosian-like aperture magnitude
PETRO_RADIUS	...	Petrosian radius in units of A or B
BACKGROUND	count	Background level at the position of the centroid
X_IMAGE	pixel	Pixel x coordinate of the isophotal image centroid
Y_IMAGE	pixel	Pixel y coordinate of the isophotal image centroid
X_FOCAL	degree	Focal plane x coordinate of isophotal image centroid
Y_FOCAL	degree	Focal plane y coordinate of isophotal image centroid
X_WORLD	...	World x coordinate of the isophotal image centroid
Y_WORLD	...	World y coordinate of the isophotal image centroid
ALPHA_SKY	degree	Native right ascension of the isophotal image centroid
DELTA_SKY	degree	Native declination of the isophotal image centroid
ALPHA_J2000	degree	J2000 right ascension of the isophotal image centroid
DELTA_J2000	degree	J2000 declination of the isophotal image centroid
ALPHA_B1950	degree	B1950 right ascension of the isophotal image centroid
DELTA_B1950	degree	B1950 declination of the isophotal image centroid
ERRX2_IMAGE	pixel2	Estimated variance of isophotal image centroid x coordinate
ERRY2_IMAGE	pixel2	Estimated variance of isophotal image centroid y coordinate
ERRXY_IMAGE	pixel2	Estimated covariance of isophotal image centroid x and y coordinates
ERRA_IMAGE	pixel	Major axis of the isophotal image centroid error ellipse
ERRB_IMAGE	pixel	Minor axis of the isophotal image centroid error ellipse
ERRTHETA_IMAGE	degree	Position angle of the isophotal image centroid ellipse
ERRCXX_IMAGE	pixel-2	Isophotal image centroid Cxx error ellipse parameter
ERRCYY_IMAGE	pixel-2	Isophotal image centroid Cyy error ellipse parameter
ERRCXY_IMAGE	pixel-2	Isophotal image centroid Cxy error ellipse parameter
XPEAK_IMAGE	pixel	Pixel x coordinate of the brightest pixel
YPEAK_IMAGE	pixel	Pixel y coordinate of the brightest pixel
XPEAK_FOCAL	degree	Focal plane x coordinate of the brightest pixel
YPEAK_FOCAL	degree	Focal plane y coordinate of the brightest pixel
XPEAK_WORLD	...	World x coordinate of the brightest pixel
YPEAK_WORLD	...	World y coordinate of the brightest pixel
ALPHAPEAK_SKY	degree	Native right ascension of the brightest pixel
DELTAPEAK_SKY	degree	Native declination of the brightest pixel
ALPHAPEAK_J2000	degree	J2000 right ascension of the brightest pixel
DELTAPEAK_J2000	degree	J2000 declination of the brightest pixel
ALPHAPEAK_B1950	degree	J2000 right ascension of the brightest pixel
DELTAPEAK_B1950	degree	J2000 declination of the brightest pixel
XMIN_IMAGE	pixel	Minimum x coordinate among detected pixels
YMIN_IMAGE	pixel	Minimum y coordinate among detected pixels
XMAX_IMAGE	pixel	Maximum x coordinate among detected pixels
YMAX_IMAGE	pixel	Maximum y coordinate among detected pixels
XWIN_IMAGE	pixel	x coordinate of windowed image centroid
YWIN_IMAGE	pixel	y coordinate of windowed image centroid
ERRX2WIN_IMAGE	pixel2	Estimated variance of windowed image centroid x coordinate
ERRY2WIN_IMAGE	pixel2	Estimated variance of windowed image centroid y coordinate
ERRXYWIN_IMAGE	pixel2	Estimated covariance of windowed image centroid x and y coordinates
ERRAWIN_IMAGE	pixel	Major axis of the windowed image centroid error ellipse
ERRBWIN_IMAGE	pixel	Minor axis of the windowed image centroid error ellipse
ERRTHETAWIN_IMAGE	degree	Position angle of the windowed image centroid ellipse
ERRCXXWIN_IMAGE	pixel-2	Windowed image centroid Cxx error ellipse parameter
ERRCYYWIN_IMAGE	pixel-2	Windowed image centroid Cyy error ellipse parameter
ERRCXYWIN_IMAGE	pixel-2	Windowed image centroid Cxy error ellipse parameter
FLAGS_WIN	...	Windowed measurement flags
X2_IMAGE	pixel2	Isophotal image 2nd order central moment in x
Y2_IMAGE	pixel2	Isophotal image 2nd order central moment in y
XY_IMAGE	pixel2	Isophotal image 2nd order central cross-moment in xy
A_IMAGE	pixel	Isophotal image major axis
B_IMAGE	pixel	Isophotal image minor axis
THETA_IMAGE	degree	Isophotal image position angle
ELONGATION	...	A_IMAGE / B_IMAGE
ELLIPTICITY	...	1 - B_IMAGE / A_IMAGE
CXX_IMAGE	pixel-2	Isophotal image Cxx ellipse parameter
CYY_IMAGE	pixel-2	Isophotal image Cyy ellipse parameter
CXY_IMAGE	pixel-2	Isophotal image Cxy ellipse parameter
ISOAREAF_IMAGE	pixel2	Isophotal area (filtered) above Detection threshold
ISOAREA_IMAGE	pixel2	Isophotal area above Analysis threshold
X2WIN_IMAGE	pixel2	Windowed image 2nd order central moment in x
Y2WIN_IMAGE	pixel2	Windowed image 2nd order central moment in y
XYWIN_IMAGE	pixel2	Windowed image 2nd order central cross-moment in xy
CXXWIN_IMAGE	pixel-2	Windowed image Cxx ellipse parameter
CYYWIN_IMAGE	pixel-2	Windowed image Cyy ellipse parameter
CXYWIN_IMAGE	pixel-2	Windowed image Cxy ellipse parameter
AWIN_IMAGE	pixel	Windowed image major axis
BWIN_IMAGE	pixel	Windowed image minor axis
THETAWIN_IMAGE	degree	Windowed image position angle
CLASS_STAR	...	Star/galaxy classifier
VECTOR_MODEL	...	Model-fitting coefficients
VECTOR_MODELERR	...	Model-fitting coefficient uncertainties
MATRIX_MODELERR	...	Model-fitting covariance matrix
CHI2_MODEL	...	Reduced modified Chi2 of the fit
FLAGS_MODEL	...	Model-fitting flags
NITER_MODEL	...	Number of model-fitting iterations
FLUX_MODEL	count	Flux from model-fitting
FLUXERR_MODEL	count	RMS error estimate for the model-fitting flux
MAG_MODEL	magnitude	Magnitude from model-fitting
MAGERR_MODEL	count	RMS error estimate for the model-fitting magnitude
FLUX_MAX_MODEL	count	Peak model flux above the background
FLUX_EFF_MODEL	count	Effective model flux above the background
FLUX_MEAN_MODEL	count	Mean effective model flux above the background
MU_MAX_MODEL	mag.arcsec-2	Peak model surface brightness above the background
MU_EFF_MODEL	mag.arcsec-2	Effective model surface brightness above the background
MU_MEAN_MODEL	mag.arcsec-2	Mean effective model surface brightness above the background
XMODEL_IMAGE	pixel	x coordinate from model-fitting
YMODEL_IMAGE	pixel	y coordinate from model-fitting
SPREAD_MODEL	...	Spread parameter from model-fitting
SPREADERR_MODEL	...	RMS error estimate on spread parameter from model-fitting