Aperture photometry

Besides isophotal, PSF and model-fitting flux estimates, SourceXtractor++ can currently perform two types of aperture flux measurements: fixed-aperture and adaptive-aperture.

Both types are applied to the measurement image(s). In SourceXtractor++ runs with only a detection image it is necessary to define the detection image as measurement in a python configuration files such as:

from glob import glob
import numpy as np
from sourcextractor.config import *

top = load_fits_image(
   '../EUC_MER_BGSUB-MOSAIC-VIS_TILE79171-SM.fits',
   weight='../EUC_MER_MOSAIC-VIS-RMS_TILE79171-SM.fits',
   weight_type='rms',
   constant_background = 0.0,
   weight_absolute=1
)

mesgroup = MeasurementGroup(top)

all_apertures = []
for img in mesgroup:
   all_apertures.extend(add_aperture_photometry(img, 10 ) )
add_output_column('aperture', all_apertures)

Fixed-aperture flux

The fixed aperture flux measurements are requested with output-properties=...,AperturePhotometry,.... The diameter of the aperture is specified in the python configuration file (see above) with all_apertures.extend(add_aperture_photometry(img,<diameter [pix]>)). It is possible to give a vector with several diameters [diam_1, diam_2, diam_3, ...].

It is also necessary to append the aperture measurements to the output with a given column name such as add_output_column('aperture', all_apertures). The measurements have the dimension n x m for each object with n the number of measurement images and m the number of diameters.

The fixed aperture checkimage is specified with check-image-aperture=<name.fits> and provides a visual impression of the apertures for each measurement image.

Automatic aperture flux

SourceXtractor++’s automatic aperture photometry routine derives from Kron’s “first moment” algorithm [10]:

[10] and [9] have shown that for stars and galaxy profiles convolved with Gaussian seeing, \(\ge 90\%\) of the flux is expected to lie inside a circular aperture of radius \(k r_{\rm Kron}\) with \(k = 2\), almost independently of the magnitude. Experiments have shown [1] that this conclusion remains unchanged if one replaces the circular aperture with the “Kron elliptical aperture” \({\cal K}\) with reduced pseudo-radius \(k r_{\rm Kron}\).

Automatic aperture flux measurements are requested with output-properties=...,AutoPhotometry,.... The scale factor \(k\) for the Kron radius \(r_{\rm Kron}\) and the minimal Kron radius \(r_{\rm Kron, min}\) can be adjusted with the parameters auto-kron-factor= and auto-kron-min-radius=, respectively. The measurements have the dimension n for each object with n the number of measurement images.

The automatic aperture checkimage is specified with check-image-auto-aperture=<name.fits> and provides a visual impression of the automatic apertures for each measurement image.