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 [28] 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:
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
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:
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 [29] 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 [30] 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 [29] 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 [30] 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.