public documents.sextractor_doc

\chapter{Flags}

A set of both {\em internal} and {\em external} \index{flags} flags is accessible

for each object. Internal \index{flags} flags are produced by the various detection

and measurement processes within {\sc SExtractor}; they tell for

instance if an object is saturated or has been truncated at the edge

of the \index{image} image. External \index{flags} flags come from \emph{flag maps}: these are images

with the same size as the one where objects are detected, where

integer numbers can be used to flag some pixels (for instance, ``bad''

or noisy pixels). Different combinations of \index{flags} flags can be applied

within the isophotal \index{area} area that defines each object, to produce a

unique value that will be written to the catalogue.

\section{Internal \index{flags} flags}

Internal \index{flags} flags contain, coded in decimal, various flag bits as a sum of powers

of 2.

\subsection{Extraction \index{flags} flags: {\tt FLAGS}}

This 16-bit flag parameter contains basic warnings about the source extraction

process, in order of increasing concern.

\begin{tabbing}

KKKK \= `TTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT \kill

\flagarg{\tt   1}{The object has \index{neighbour} \index{neighbours} neighbours, bright and close enough to

                significantly bias {\tt MAG\_AUTO} photometry\footnotemark{},

                or \index{bad pixel} \index{bad pixels} bad pixels (if more than 10\% of the integrated \index{area} area is

                affected).}

\flagarg{\tt   2}{The object was originally blended with another one.}

\flagarg{\tt   4}{At least one object pixel is saturated (or very close to).}

\flagarg{\tt   8}{The isophotal footprint of the detected object is truncated

                (too close to an \index{image} image \index{boundary} boundary).}

\flagarg{\tt  16}{Object's aperture data are incomplete or corrupted.}

\flagarg{\tt  32}{Object's isophotal data are incomplete or

                corrupted\footnotemark{}.}

\flagarg{\tt  64}{A \index{memory} memory overflow occurred during \index{deblending} deblending.}

\flagarg{\tt 128}{A \index{memory} memory overflow occurred during extraction.}

\end{tabbing}

\footnotetext[1]{This flag can be activated

only when {\tt MAG\_AUTO} magnitudes (\S\ref{chap:mag_auto})are requested.}

\footnotetext[2]{This flag is inherited from

{\sc SExtractor} V1.0, and has been kept for compatibility reasons.

With {\sc SExtractor} V2.0+, having this flag activated doesn't have

any consequence for the extracted parameters.}

For example, an object close to an \index{image} image border may have {\tt FLAGS} =

16, and perhaps {\tt FLAGS} = 8+16+32 = 56.

\subsection{Weight-map \index{flags} flags: {\tt FLAGS\_WEIGHT}}

This 16-bit flag parameter contains warnings related to the pixel weighting

process.

\begin{tabbing}

KKKK \= `TTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT \kill

\flagarg{\tt   1}{The isophotal footprint of the detected object overlaps at

                least one low weight\footnotemark{} in the measurement \index{image} image.}

\flagarg{\tt   2}{The isophotal footprint of the detected object contains or

                touches at least one low weight in the filtered detection

                image.}

\end{tabbing}

\footnotetext[1]{the term "low weight" refers to a

weight that falls below the \index{threshold} threshold set by the {\tt WEIGHT\_THRESH}

configuration parameter.}

\subsection{Weight-map \index{flags} flags: {\tt FLAGS\_WIN}}

This 16-bit flag parameter contains warnings about {\tt *\_WIN} measurements

(\S\ref{chap:winparam}).

\begin{tabbing}

KKKK \= `TTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT \kill

\flagarg{\tt   1}{Second-order \index{moments} moments measured through the Gaussian \index{window} window                      are inconsistent ($\bar{x^2}\bar{y^2}~-~\bar{xy}\le0$).}

\flagarg{\tt   2}{Second-order \index{moments} moments measured through the Gaussian \index{window} window are

                negative or zero.}

\flagarg{\tt   4}{Flux integrated within the Gaussian \index{window} window is negative or

                zero.}

\end{tabbing}

\section{External \index{flags} flags}

{\sc SExtractor} understands that it must process \index{external \index{flags} flags} external flags when

{\tt IMAFLAGS\_ISO} or {\tt NIMAFLAGS\_ISO} are present in the catalogue

\index{parameter file} parameter file. It then looks for a FITS \index{image} image specified by the {\tt

FLAG\_IMAGE} keyword in the \index{configuration file} configuration file. The FITS \index{image} image must

contain the flag map, in the form of a 2-dimensional array of 8, 16 or

32 bits integers. It must have the same size as the \index{image} image used for

detection. Such flag maps can be created using for example the {\bf

WeightWatcher} software (Bertin 1997).

The flag map values for pixels that coincide with the isophotal \index{area} area

of a given detected object are then combined, and stored in the

catalogue as the long integer {\tt IMAFLAGS\_ISO}. 5 kinds of

combination can be selected using the

{\tt FLAG\_TYPE} configuration keyword:

\begin{itemize}

\item {\tt OR}: the result is an arithmetic (bit-to-bit) {\bf OR} of flag map pixels.

\item {\tt AND}: the result is an arithmetic (bit-to-bit) {\bf AND} of non-zero flag map pixels.

\item {\tt MIN}: the result is the minimum of the (signed) flag map

  pixels. \gam{How can they be negative?}

\item {\tt MAX}: the result is the maximum of the (signed) flag map pixels.

\item {\tt MOST}: the result is the most frequent non-zero flag map pixel-value.

\end{itemize}

The {\tt NIMAFLAGS\_ISO} catalogue parameter contains a number of

relevant flag map pixels: the number of non-zero flag map pixels in

the case of an {\tt OR} or {\tt AND} {\tt FLAG\_TYPE}, or the number

of pixels with value {\tt IMAFLAGS\_ISO} if the {\tt FLAG\_TYPE} is

{\tt MIN},{\tt MAX} or {\tt MOST}.