@@ -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.
.. flux_aper_def:
.. _flux_aper_def:
Fixed-aperture flux
-------------------
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@@ -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
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`.
#. 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}`
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@@ -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
have been considered.
Aperture magnitudes are sensitive to crowding.
In |SExtractor| v1, :param:`MAG_AUTO` measurements were not very robust in that respect.
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).
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.
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 use of :param:`MAG_BEST` is now deprecated as :param:`MAG_AUTO` measurements are much more robust in versions 2.x of |SExtractor|.
The first improvement is a crude subtraction of all the neighbors that have been detected around the measured source (``MASK_TYPE BLANK`` option).
The second improvement is an automatic correction of parts of the aperture that are suspected to be contaminated by a neighbor.
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 :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.
.. hint::
Aperture magnitudes are sensitive to crowding.
In |SExtractor| v1, :param:`MAG_AUTO` measurements were not very robust in that respect.
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).
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.
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 use of :param:`MAG_BEST` is now deprecated as :param:`MAG_AUTO` measurements are much more robust in versions 2.x of |SExtractor|.
The first improvement is a crude subtraction of all the neighbors that have been detected around the measured source (``MASK_TYPE BLANK`` option).
The second improvement is an automatic correction of parts of the aperture that are suspected to be contaminated by a neighbor.
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.
Photographic photometry
-----------------------
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@@ -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).
Magnitude uncertainties
-----------------------
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@@ -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
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
Unless otherwise noted, pixel values :math:`I_i` are taken from the (filtered) detection image.
.. 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.
.. _moments_iso_def:
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`.
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
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::
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`).
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@@ -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}`).
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`
-----------------------------------------------
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@@ -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.
*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`
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>`:
