aperture_photometry

aperture_photometry#

astropop.photometry.aperture_photometry(data, x, y, r='auto', r_ann='auto', recenter_limit=None, recenter_method='com', gain=1.0, bkg_error=None, mask=None, bkg_method='mmm', bkg_sigma_clip=3.0, pixel_flags=None, box_size=11)#

Perform aperture photometry using sep. Output units (ADU or e-) will be the considered the same as input.

This method just wraps the ApertureStats method from photutils, assuming standard arguments and performing extra steps:

  • Calculate the total error (bkg+poisson) from input data, based on data and bkg_error.

  • It assumes the 'exact' method of subpixel sampling.

  • Compute an optimal aperture radius, considering only the gaussian background-only error. This radius is defined as r=0.6731*GFWHM.

  • Use sigma clipping for annulus background calculation.

  • Perform photometry flags generation. See PhotometryFlags.

  • Sources recentering using centroids methods. Only performed if recenter_limit is not None.

Most of the reduction is based on Astropy workshop sample notebooks [1].

Parameters:
datandarray

2D image data for photometry

x, yarray_like

Positions of the sources

rfloat, 'auto' (optional)

Aperture radius, in pixels. If ‘auto’, the value will be estimated based in the median gaussian FWHM of the sources in the image (r=0.6731*GFWHM). Default: ‘auto’

r_annarray_like([float, float]), 'auto' None (optional)

Annulus radii (r_in, r_out) for local background extraction in pixels. If ‘auto’, the annulus will be set based on aperture as (4*r, 6*r). If None, no local background subtraction will be performed. Default: ‘auto’

recenter_limitfloat (optional)

Maximum distance allowed to recenter the apertures. If None, no recentering will be performed. Default: None

recenter_method‘com’, ‘gaussian’ or ‘quadratic’ (optional)

Algorithm to recenter the apertures. If None, no recentering will be performed. 'com' method will use the center of mass of the aperture, 'gaussian' will use a gaussian fit to the aperture and 'quadratic' will use a quadratic fit to the aperture. See centroids for more information. Default: ‘com’

gainfloat (optional)

Gain to correctly calculate the error. Default: 1.0

bkg_errorfloat or ndarray (optional)

1-sigma gaussian error of the background-only error of input data. If None, zero will be used. Default: None

maskndarray (optional)

Mask badpixels and problematic ccd areas. Default: None

pixel_flagsndarray (optional)

Array with flags for each pixel. If None, no flags will be used. Flags must follow the PixelMaskFlags standard. Default: None

bkg_method‘mmm’, ‘mode’, ‘median’ or ‘mean’ (optional)

Algorith to calculate the background value. 'mode' or 'mmm' (mean, median, mode) are computed as (3*median - 2*mean) and should be better for populated fields. 'median' and 'mean' just compute these values in the annulus. All algorithms can use sigma clipping if bkg_sigma_clip is not None. Default: ‘mmm’

bkg_sigma_clipfloat, tuple(float, float) or None (optional)

Sigma clipping values for background calculation. If a tuple is provided, the first value will be used as lower sigma and the second as upper sigma. Only used if sky_algorithm='sigmaclip'. Default: 3.0

box_sizeint (optional)

Box size for median FWHM computation. Default is 11.

Returns:
res_apTable

Table containing all aperture photometry informations. - x, y: centroids of the sources. If recentering is performed,

these values will be different from input.

  • aperture: aperture radius. Same for all sources.

  • flux: flux of the sources with bkg subtracted

  • flux_error: flux error of the sources with bkg subtracted

  • aperture_area: effective aperture (considering mask) of the sources.

  • bkg: background value by pixel

  • bkg_stddev: background standard deviation inside the annulus

  • bkg_area: effective area of the annulus (considering mask and clipping)

  • flags: flag for the sources

  • original_x, original_y: original input positions

The metadata of the table will contain the following information: - photutils: photutils version used - astropy: astropy version used - astropop: astropop version used - fwhm: median FWHM of the sources - r: aperture radius used - r_in: inner annulus radius used - r_out: outer annulus radius used

Notes

  • The input data is divided by the gain prior to any calculation.

  • The centroid of the sources is computed within a circular footprint with radius 2*r+1. See centroid_sources for more information.

  • Local background subtraction is performed entirely by photutils. So, we do not touch it or compute any errors here.

References

[1]

Astropy workshop sample notebook (astropy/astropy-workshop 8a5056449b92ca4019e16/09-Photutils/03-aperture_local_bkgsub.ipynb)

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