@@ -67,7 +68,8 @@ A “total” magnitude :param:`MAG_ISOCOR` estimate is then
...
@@ -67,7 +68,8 @@ A “total” magnitude :param:`MAG_ISOCOR` estimate is then
Clearly this cheap correction works best with stars; and although it is shown to give tolerably accurate results with most disk galaxies, it fails with ellipticals because of the broader wings of their profiles.
Clearly this cheap correction works best with stars; and although it is shown to give tolerably accurate results with most disk galaxies, it fails with ellipticals because of the broader wings of their profiles.
.. flux_aper_def:
.. _flux_aper_def:
Fixed-aperture flux
Fixed-aperture flux
-------------------
-------------------
...
@@ -80,10 +82,13 @@ before measuring the flux within the aperture. If :param:`FLUX_APER` is provided
...
@@ -80,10 +82,13 @@ before measuring the flux within the aperture. If :param:`FLUX_APER` is provided
vector :param:`FLUX_APER[n]`, at least :math:`n` apertures must be
vector :param:`FLUX_APER[n]`, at least :math:`n` apertures must be
specified with ``PHOTOM_APERTURES``.
specified with ``PHOTOM_APERTURES``.
Automatic aperture magnitudes
-----------------------------
:param:`MAG_AUTO` provides an estimate of the “total magnitude” by integrating the source flux within an adaptively scaled aperture.
.. _flux_auto_def:
Automatic aperture flux
-----------------------
:param:`FLUX_AUTO` provides an estimate of the “total flux” by integrating pixel values within an adaptively scaled aperture.
|SExtractor|’s automatic aperture photometry routine is inspired by Kron’s “first moment” algorithm :cite:`1980ApJS_43_305K`.
|SExtractor|’s automatic aperture photometry routine is inspired by Kron’s “first moment” algorithm :cite:`1980ApJS_43_305K`.
#. An elliptical aperture is defined whose elongation :math:`\epsilon` and position angle :math:`\theta` are defined by second order moments of the object’s light distribution. The ellipse is scaled to :math:`R_{\rm max}.\sigma_{\rm iso}`
#. An elliptical aperture is defined whose elongation :math:`\epsilon` and position angle :math:`\theta` are defined by second order moments of the object’s light distribution. The ellipse is scaled to :math:`R_{\rm max}.\sigma_{\rm iso}`
...
@@ -110,18 +115,21 @@ The user has full control over the parameters :math:`k` and :math:`R_{\rm min}`
...
@@ -110,18 +115,21 @@ The user has full control over the parameters :math:`k` and :math:`R_{\rm min}`
(see text). Only isolated galaxies (no blends) of the simulations
(see text). Only isolated galaxies (no blends) of the simulations
have been considered.
have been considered.
Aperture magnitudes are sensitive to crowding.
.. hint::
In |SExtractor| v1, :param:`MAG_AUTO` measurements were not very robust in that respect.
Aperture magnitudes are sensitive to crowding.
It was therefore suggested to replace the aperture magnitude by the corrected-isophotal one when an object is too close to its neighbors (2 isophotal radii for instance).
In |SExtractor| v1, :param:`MAG_AUTO` measurements were not very robust in that respect.
This was done automatically when using the :param:`MAG_BEST` magnitude: :math:`{\tt MAG\_BEST} = {\tt MAG\_AUTO}` when it is sure that no neighbor can bias :param:`MAG_AUTO` by more than 10%, and :math:`{\tt MAG\_BEST} = {\tt MAG\_ISOCOR}` otherwise.
It was therefore suggested to replace the aperture magnitude by the corrected-isophotal one when an object is too close to its neighbors (2 isophotal radii for instance).
Experience showed that the :param:`MAG_ISOCOR` and :param:`MAG_AUTO` magnitude would loose about the same fraction of flux on stars or compact galaxy profiles: around 0.06 % for default extraction parameters.
This was done automatically when using the :param:`MAG_BEST` magnitude: :math:`{\tt MAG\_BEST} = {\tt MAG\_AUTO}` when it is sure that no neighbor can bias :param:`MAG_AUTO` by more than 10%, and :math:`{\tt MAG\_BEST} = {\tt MAG\_ISOCOR}` otherwise.
The use of :param:`MAG_BEST` is now deprecated as :param:`MAG_AUTO` measurements are much more robust in versions 2.x of |SExtractor|.
Experience showed that the :param:`MAG_ISOCOR` and :param:`MAG_AUTO` magnitude would loose about the same fraction of flux on stars or compact galaxy profiles: around 0.06 % for default extraction parameters.
The first improvement is a crude subtraction of all the neighbors that have been detected around the measured source (``MASK_TYPE BLANK`` option).
The use of :param:`MAG_BEST` is now deprecated as :param:`MAG_AUTO` measurements are much more robust in versions 2.x of |SExtractor|.
The second improvement is an automatic correction of parts of the aperture that are suspected to be contaminated by a neighbor.
The first improvement is a crude subtraction of all the neighbors that have been detected around the measured source (``MASK_TYPE BLANK`` option).
This is done by mirroring the opposite, cleaner side of the measurement ellipse if available (``MASK_TYPE CORRECT`` option, which is also the default).
The second improvement is an automatic correction of parts of the aperture that are suspected to be contaminated by a neighbor.
Figure [figphot] shows the mean loss of flux measured with isophotal (threshold :math:`= 24.4\ \mbox{\rm magnitude\,arsec}^{-2}`), corrected isophotal and automatic aperture photometries for simulated galaxy :math:`B_J` on a typical Schmidt-survey plate image.
This is done by mirroring the opposite, cleaner side of the measurement ellipse if available (``MASK_TYPE CORRECT`` option, which is also the default).
Figure [figphot] shows the mean loss of flux measured with isophotal (threshold 24.4 magnitude.arsec\ :sup:`-2`), corrected isophotal and automatic aperture photometry for simulated galaxies on a typical Schmidt-survey B\ :sub:`J` plate image.
The automatic adaptive aperture photometry leads to the lowest loss of flux.
The automatic adaptive aperture photometry leads to the lowest loss of flux.
Photographic photometry
Photographic photometry
-----------------------
-----------------------
...
@@ -140,6 +148,7 @@ where :math:`\gamma` (``MAG_GAMMA``) is the contrast index of the emulsion, :mat
...
@@ -140,6 +148,7 @@ where :math:`\gamma` (``MAG_GAMMA``) is the contrast index of the emulsion, :mat
One advantage of using a density-to-intensity transformation relative to the local sky background is that it corrects (to some extent) large-scale inhomogeneities in sensitivity (see :cite:`1996PhDT_68B` for details).
One advantage of using a density-to-intensity transformation relative to the local sky background is that it corrects (to some extent) large-scale inhomogeneities in sensitivity (see :cite:`1996PhDT_68B` for details).
Magnitude uncertainties
Magnitude uncertainties
-----------------------
-----------------------
...
@@ -167,6 +176,7 @@ Making the assumption that plate-noise is the major contributor to photometric e
...
@@ -167,6 +176,7 @@ Making the assumption that plate-noise is the major contributor to photometric e
where :math:`I(x,y)` is the contribution of pixel :math:`(x,y)` to the
where :math:`I(x,y)` is the contribution of pixel :math:`(x,y)` to the
total flux (Eq. [eq:dtoi]). ``GAIN`` is ignored in ``PHOTO`` mode.
total flux (Eq. [eq:dtoi]). ``GAIN`` is ignored in ``PHOTO`` mode.
@@ -13,7 +11,11 @@ The following parameters are derived from the spatial distribution :math:`\cal S
...
@@ -13,7 +11,11 @@ The following parameters are derived from the spatial distribution :math:`\cal S
Unless otherwise noted, pixel values :math:`I_i` are taken from the (filtered) detection image.
Unless otherwise noted, pixel values :math:`I_i` are taken from the (filtered) detection image.
.. note::
.. note::
Unless otherwise noted, all parameter names given below are only prefixes. They must be followed by _IMAGE if the results shall be expressed in pixel coordinates or :param:`_WORLD`, :param:`_SKY, :param:`_J2000` or :param:`_B1950` for |WCS|_ coordinates (see :ref:`coord_suffix`).
Unless otherwise noted, all parameter names given below are only prefixes.
They must be followed by _IMAGE if the results shall be expressed in pixel coordinates or :param:`_WORLD`, :param:`_SKY, :param:`_J2000` or :param:`_B1950` for |WCS|_ coordinates (see :ref:`coord_suffix`).
It is sometimes useful to have the position :param:`XPEAK`, :param:`YPEAK` of the pixel with maximum intensity in a detected object, for instance when working with likelihood maps, or when searching for artifacts. For better robustness, PEAK coordinates are computed on *filtered* profiles if available. On symmetrical profiles, PEAK positions and barycenters coincide within a fraction of pixel (:param:`XPEAK` and :param:`YPEAK` coordinates are quantized by steps of 1 pixel, hence :param:`XPEAK_IMAGE` and :param:`YPEAK_IMAGE` are integers). This is no longer true for skewed profiles, therefore a simple comparison between PEAK and barycenter coordinates can be used to identify asymmetrical objects on well-sampled images.
It is sometimes useful to have the position :param:`XPEAK`, :param:`YPEAK` of the pixel with maximum intensity in a detected object, for instance when working with likelihood maps, or when searching for artifacts. For better robustness, PEAK coordinates are computed on *filtered* profiles if available. On symmetrical profiles, PEAK positions and barycenters coincide within a fraction of pixel (:param:`XPEAK` and :param:`YPEAK` coordinates are quantized by steps of 1 pixel, hence :param:`XPEAK_IMAGE` and :param:`YPEAK_IMAGE` are integers). This is no longer true for skewed profiles, therefore a simple comparison between PEAK and barycenter coordinates can be used to identify asymmetrical objects on well-sampled images.
.. _moments_iso_def:
2nd order moments: :param:`X2`, :param:`Y2`, :param:`XY`
2nd order moments: :param:`X2`, :param:`Y2`, :param:`XY`
Note that :param:`A` and :param:`B` are exactly halves the :math:`a` and :math:`b` parameters computed by the COSMOS image analyser :cite:`1980SPIE_264_208S`.
Note that :param:`A` and :param:`B` are exactly halves the :math:`a` and :math:`b` parameters computed by the COSMOS image analyser :cite:`1980SPIE_264_208S`.
Actually, :math:`a` and :math:`b` are defined in :cite:`1980SPIE_264_208S` as the semi-major and semi-minor axes of an elliptical shape with constant surface brightness, which would have the same 2nd-order moments as the analyzed object.
Actually, :math:`a` and :math:`b` are defined in :cite:`1980SPIE_264_208S` as the semi-major and semi-minor axes of an elliptical shape with constant surface brightness, which would have the same 2nd-order moments as the analyzed object.
.. _elong_iso_def:
By-products of shape parameters: :param:`ELONGATION` and :param:`ELLIPTICITY`
Measurements performed through a *window* function (an *envelope*) do not have many of the drawbacks of isophotal measurements. |SExtractor| implements “windowed” versions for most
Measurements performed through a *window* function (an *envelope*) do not have many of the drawbacks of isophotal measurements. |SExtractor| implements “windowed” versions for most
of the measurements described in the :ref:`previous section<position_iso_def>`:
of the measurements described in the :ref:`previous section<pos_iso_def>`:
.. note::
.. note::
Unless otherwise noted, all parameter names given below are only prefixes. They must be followed by _IMAGE if the results shall be expressed in pixel coordinates or :param:`_WORLD`, :param:`_SKY`, :param:`_J2000` or :param:`_B1950` for |WCS|_ coordinates (see :ref:`coord_suffix`).
Unless otherwise noted, all parameter names given below are only prefixes. They must be followed by _IMAGE if the results shall be expressed in pixel coordinates or :param:`_WORLD`, :param:`_SKY`, :param:`_J2000` or :param:`_B1950` for |WCS|_ coordinates (see :ref:`coord_suffix`).
...
@@ -30,7 +30,7 @@ the object’s isophotal footprint.
...
@@ -30,7 +30,7 @@ the object’s isophotal footprint.
The Gaussian window is scaled to each object; the Gaussian FWHM is the diameter of the disk that contains half of the object flux (:math:`d_{50}`).
The Gaussian window is scaled to each object; the Gaussian FWHM is the diameter of the disk that contains half of the object flux (:math:`d_{50}`).
Note that in double-image mode (§[chap:using]) the window is scaled based on the *measurement* image.
Note that in double-image mode (§[chap:using]) the window is scaled based on the *measurement* image.
.. _xywin:
.. _pos_win_def:
Windowed centroid: :param:`XWIN`, :param:`YWIN`
Windowed centroid: :param:`XWIN`, :param:`YWIN`
-----------------------------------------------
-----------------------------------------------
...
@@ -86,10 +86,12 @@ It has been verified that for isolated objects with Gaussian-like profiles, thei
...
@@ -86,10 +86,12 @@ It has been verified that for isolated objects with Gaussian-like profiles, thei
*Left*: histogram of the difference between :param:`X_IMAGE` and the true centroid in x.
*Left*: histogram of the difference between :param:`X_IMAGE` and the true centroid in x.
*Right*: histogram of the difference between :param:`XWIN_IMAGE` and the true centroid in x.
*Right*: histogram of the difference between :param:`XWIN_IMAGE` and the true centroid in x.
.. _moments_win_def:
Windowed 2nd order moments: :param:`X2`, :param:`Y2`, :param:`XY`
Windowed 2nd order moments: :param:`X2`, :param:`Y2`, :param:`XY`
Semi-major axis :param:`ERRAWIN`, semi-minor axis :param:`ERRBWIN`, and position angle :param:`ERRTHETAWIN` of the :math:`1\sigma` position error ellipse are computed from the covariance matrix elements :math:`{\rm var}(\overline{x_{\tt WIN}})`, :math:`{\rm var}(\overline{y_{\tt WIN}})`, :math:`{\rm cov}(\overline{x_{\tt WIN}},\overline{y_{\tt WIN}})`, similarly to the :ref:`isophotal error ellipse <poserr>`:
Semi-major axis :param:`ERRAWIN`, semi-minor axis :param:`ERRBWIN`, and position angle :param:`ERRTHETAWIN` of the :math:`1\sigma` position error ellipse are computed from the covariance matrix elements :math:`{\rm var}(\overline{x_{\tt WIN}})`, :math:`{\rm var}(\overline{y_{\tt WIN}})`, :math:`{\rm cov}(\overline{x_{\tt WIN}},\overline{y_{\tt WIN}})`, similarly to the :ref:`isophotal error ellipse <poserr_iso_def>`:
