To compute the background map, |SExtractor| makes a first pass through the pixel data, estimating the local background in each mesh of a rectangular grid that covers the whole frame.
To compute the background map, |SExtractor| makes a first pass through the pixel data, estimating the local background in each mesh of a rectangular grid that covers the whole frame.
The background estimator is a combination of :math:`\kappa\,\sigma` clipping and mode estimation, similar to Stetson’s |DAOPHOT|_ program :cite:`1987PASP_99_191S,1992ASPC_23_90D`.
The background estimator is a combination of :math:`\kappa\,\sigma` clipping and mode estimation, similar to Stetson’s |DAOPHOT|_ program :cite:`1987PASP_99_191S,1992ASPC_23_90D`.
Briefly, the local background histogram is clipped iteratively until convergence at :math:`\pm 3\sigma` around its median. The mode of the histogram is estimated using:
Briefly, the local background histogram is clipped iteratively until convergence at :math:`\pm 3\sigma` around its median. The mode of the histogram is estimated using the semi-empirical relation
Using simulated images, the expression above was found more accurate with clipped distributions :cite:`1996AAS_117_393B` than the usual approximation (e.g., :cite:`stuart2009kendall`):
Using simulated images, :cite:`1996AAS_117_393B` found :math:`\alpha_{\rm{Pearson}} \approx 2.5` more appropriate for determining the mode of the background histogram in |SExtractor|.
However, extensive photometric assessments carried out in the context of the `Dark Energy Survey <https://darkenergysurvey.org>`_ later showed that this value leads to a slight overestimation of the local background :cite:`2022ApJS_258_15E`, and a higher :math:`\alpha_{\rm{Pearson}} \approx 3.5` was adopted in |DES|.
Since |SExtractor| v2.28, :math:`\alpha_{\rm{Pearson}}` can be changed using the ``BACK_PEARSON`` configuration parameter.
However, for the sake of compatibility with previous results, the default value of ``BACK_PEARSON`` remains ``2.5``.
:numref:`fig_modevsmean` shows that the mode estimation in :eq:`sexbackmode` is considerably less affected by source crowding than a simple clipped mean :cite:`1981AJ_86_476J,1987AA_183_177I` but it is :math:`\approx 30\%` noisier.
:numref:`fig_modevsmean` shows that the mode estimation in :eq:`sexbackmode` is considerably less affected by source crowding than a simple clipped mean :cite:`1981AJ_86_476J,1987AA_183_177I` but it is :math:`\approx 30\%` noisier.
Obviously :eq:`sexbackmode` is not valid for any distribution; |SExtractor| falls back to a simple median for estimating the local background value if the mode and the median disagree by more than 30%.
Obviously :eq:`sexbackmode` is not valid for any distribution; |SExtractor| falls back to a simple median for estimating the local background value if the mode and the median disagree by more than 30%.
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@@ -56,7 +63,7 @@ In parallel with the making of the background map, an *RMS background map*, that
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@@ -56,7 +63,7 @@ In parallel with the making of the background map, an *RMS background map*, that
It may be used as an internal weight map if the ``WEIGHT_TYPE`` configuration parameter is to ``BACKGROUND`` (see :ref:`weight-map_format`).
It may be used as an internal weight map if the ``WEIGHT_TYPE`` configuration parameter is to ``BACKGROUND`` (see :ref:`weight-map_format`).
Configuration and tuning
Configuration and tuning
------------------------
~~~~~~~~~~~~~~~~~~~~~~~~
.. note::
.. note::
All background configuration parameters also affect background-RMS maps.
All background configuration parameters also affect background-RMS maps.
The :param:`SPREAD_MODEL` estimator has been developed as a star/galaxy classifier for the DESDM pipeline :cite:`2012SPIE_8451E_0DM`, and has also been used in other surveys :cite:`2012ApJ_757_83D,2013AA_554A_101B`.
The :param:`SPREAD_MODEL` estimator has been developed as a star/galaxy classifier for the DESDM pipeline :cite:`2012SPIE_8451E_0DM`, and has also been used in other surveys :cite:`2012ApJ_757_83D,2013AA_554A_101B`.
:param:`SPREAD_MODEL` indicates which of the best fitting local PSF model resampled at the current position :math:`\tilde{\boldsymbol{\phi}}` (representing a point source) or a slightly ``fuzzier'' resampled model :math:`\tilde{\boldsymbol{G}}` (representing a galaxy) matches best the image data.
:param:`SPREAD_MODEL` indicates which of the best fitting local PSF model resampled at the current position :math:`\tilde{\boldsymbol{\phi}}` (representing a point source) or a slightly "fuzzier" resampled model :math:`\tilde{\boldsymbol{G}}` (representing a galaxy) matches best the image data.
:math:`\tilde{\boldsymbol{G}}` is obtained by convolving the local PSF model with a circular exponential model with scalelength = 1/16 |FWHM|, and resampling the result at the current position on the pixel grid. :param:`SPREAD_MODEL` is normalized to allow comparing sources with different PSFs throughout the field:
:math:`\tilde{\boldsymbol{G}}` is obtained by convolving the local PSF model with a circular exponential model with scalelength = 1/16 |FWHM|, and resampling the result at the current position on the pixel grid. :param:`SPREAD_MODEL` is normalized to allow comparing sources with different PSFs throughout the field:
.. math::
.. math::
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@@ -306,6 +306,7 @@ In order to maintain a certain level of purity or completeness across the whole
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@@ -306,6 +306,7 @@ In order to maintain a certain level of purity or completeness across the whole
author={{Everett}, S. and {Yanny}, B. and {Kuropatkin}, N. and {Huff}, E.~M. and {Zhang}, Y. and {Myles}, J. and {Masegian}, A. and {Elvin-Poole}, J. and {Allam}, S. and {Bernstein}, G.~M. and {Sevilla-Noarbe}, I. and {Splettstoesser}, M. and {Sheldon}, E. and {Jarvis}, M. and {Amon}, A. and {Harrison}, I. and {Choi}, A. and {Hartley}, W.~G. and {Alarcon}, A. and {S{\'a}nchez}, C. and {Gruen}, D. and {Eckert}, K. and {Prat}, J. and {Tabbutt}, M. and {Busti}, V. and {Becker}, M.~R. and {MacCrann}, N. and {Diehl}, H.~T. and {Tucker}, D.~L. and {Bertin}, E. and {Jeltema}, T. and {Drlica-Wagner}, A. and {Gruendl}, R.~A. and {Bechtol}, K. and {Carnero Rosell}, A. and {Abbott}, T.~M.~C. and {Aguena}, M. and {Annis}, J. and {Bacon}, D. and {Bhargava}, S. and {Brooks}, D. and {Burke}, D.~L. and {Carrasco Kind}, M. and {Carretero}, J. and {Castander}, F.~J. and {Conselice}, C. and {Costanzi}, M. and {da Costa}, L.~N. and {Pereira}, M.~E.~S. and {De Vicente}, J. and {DeRose}, J. and {Desai}, S. and {Eifler}, T.~F. and {Evrard}, A.~E. and {Ferrero}, I. and {Fosalba}, P. and {Frieman}, J. and {Garc{\'\i}a-Bellido}, J. and {Gaztanaga}, E. and {Gerdes}, D.~W. and {Gutierrez}, G. and {Hinton}, S.~R. and {Hollowood}, D.~L. and {Honscheid}, K. and {Huterer}, D. and {James}, D.~J. and {Kent}, S. and {Krause}, E. and {Kuehn}, K. and {Lahav}, O. and {Lima}, M. and {Lin}, H. and {Maia}, M.~A.~G. and {Marshall}, J.~L. and {Melchior}, P. and {Menanteau}, F. and {Miquel}, R. and {Mohr}, J.~J. and {Morgan}, R. and {Muir}, J. and {Ogando}, R.~L.~C. and {Palmese}, A. and {Paz-Chinch{\'o}n}, F. and {Plazas}, A.~A. and {Rodriguez-Monroy}, M. and {Romer}, A.~K. and {Roodman}, A. and {Sanchez}, E. and {Scarpine}, V. and {Serrano}, S. and {Smith}, M. and {Soares-Santos}, M. and {Suchyta}, E. and {Swanson}, M.~E.~C. and {Tarle}, G. and {To}, C. and {Troxel}, M.~A. and {Varga}, T.~N. and {Weller}, J. and {Wilkinson}, R.~D. and {Wilkinson}, R.~D.},
title="{Dark Energy Survey Year 3 Results: Measuring the Survey Transfer Function with Balrog}",
journal={\apjs},
keywords={343, 1464, 351, 1857, Astrophysics - Cosmology and Nongalactic Astrophysics, Astrophysics - Instrumentation and Methods for Astrophysics},
