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since filtering occurs on a background-subtracted \index{image} image. When weighting is applied
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since filtering occurs on a background-subtracted \index{image} image. When weighting is applied
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(\S\ref{chap:weight}), \index{bad pixel} \index{bad pixels} bad pixels (pixels with weight $<$ {\tt WEIGHT\_THRESH}) are interpolated
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(\S\ref{chap:weight}), \index{bad pixel} \index{bad pixels} bad pixels (pixels with weight $<$ {\tt WEIGHT\_THRESH}) are interpolated
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by default (\S\ref{chap:interp}) and should therefore not cause much trouble. It is recommended
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by default (\S\ref{chap:interp}) and should therefore not cause much trouble. It is recommended
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not to turn-off \index{interpolation} interpolation of \index{bad pixel} \index{bad pixels} bad pixels when filtering is on.
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not to turn-off \index{interpolation} interpolation of \index{bad pixel} \index{bad pixels} bad pixels when filtering is on.
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\subsection{Configuration parameters.}
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\subsection{Configuration parameters}
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Filtering is triggered when the {\tt FILTER} keyword is set to {\tt Y}. If active, a file with name
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Filtering is triggered when the {\tt FILTER} keyword is set to {\tt Y}. If active, a file with name
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specified by {\tt FILTER\_NAME} is searched for and loaded. Filtering with large retinas can be
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specified by {\tt FILTER\_NAME} is searched for and loaded. Filtering with large retinas can be
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extremely time consuming. In many cases, one is only interested in filtering pixels whose values
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extremely time consuming. In many cases, one is only interested in filtering pixels whose values
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stand out from the background noise. The {\tt FILTER\_THRESH keyword} can be given to specify the
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stand out from the background noise. The {\tt FILTER\_THRESH keyword} can be given to specify the
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range of pixel values within which retina-filtering will be applied, in units of background noise
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range of pixel values within which retina-filtering will be applied, in units of background noise
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{\tt FILTER\_THRESH} has no effect on \index{convolution} convolution.
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{\tt FILTER\_THRESH} has no effect on \index{convolution} convolution.
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The result of the filtering process can be verified through a {\tt FILTERED} \index{check-image} check-image: see
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The result of the filtering process can be verified through a {\tt FILTERED} \index{check-image} check-image: see
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\S\ref{chap:check}.
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\S\ref{chap:check}.
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\subsection{CPU cost.}
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\subsection{CPU cost}
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The {\sc SExtractor} filtering routine is particularly optimized for small kernels. It thus
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The {\sc SExtractor} filtering routine is particularly optimized for small kernels. It thus
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provides a convenient way of filtering large \index{image} image data. On a 2GHz machine, a \index{convolution} convolution by a
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provides a convenient way of filtering large \index{image} image data. On a 2GHz machine, a \index{convolution} convolution by a
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$5\times5$ kernel will contribute less than 1 second to the processing time of a $2048\times4096$
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$5\times5$ kernel will contribute less than 1 second to the processing time of a $2048\times4096$
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\index{image} image. The numbers for non-linear (retina) filtering depend on the complexity of the neural
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\index{image} image. The numbers for non-linear (retina) filtering depend on the complexity of the neural
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network, but can be a hundred times larger.
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network, but can be a hundred times larger.
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\gam{Update time?}
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\gam{Update time?}
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\subsection{Filter file formats.}
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\subsection{Filter file formats}
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As described above, two kinds of filter
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As described above, two kinds of filter
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files are recognized by {\sc SExtractor}: \index{convolution} convolution files (traditionaly suffixed with
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files are recognized by {\sc SExtractor}: \index{convolution} convolution files (traditionaly suffixed with
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``{\tt .conv}''), and ``retina'' files (``{\tt .ret}'' extensions\footnote{In {\sc SExtractor},
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``{\tt .conv}''), and ``retina'' files (``{\tt .ret}'' extensions\footnote{In {\sc SExtractor},
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file name extensions are just conventions; they are not used by the software to distinguish
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file name extensions are just conventions; they are not used by the software to distinguish
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between different file formats.}).
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between different file formats.}).
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