Measuring Background

Photometry starts by determining the background level and its uncertainty for each source. At present AstroWISP uses the median average and the scatter around it (after outlier rejection) of pixels in an annulus around each source, excluding pixels too close to other sources. The user specifies two radii:

• Inner radius, defining pixels that are so close to a source that they include not just background, but source light. Pixels with centers within this radius around each source do not contribute to any source’s background value.

• Outer radius, defining pixels local enough to the source to represent the background under it. Each source’s background is then the median of all pixels within this outer radius that are not within the the inner radius of this source or any other source.

Background measurement is performed using the astrowisp.BackgroundExtractor class and requires specifying the image, list of source locations, the inner/outer background radii and returns estimates of the background for each source, its error (standard deviation), and the number of pixels that participated in the determination of the background value and error:

from astrowisp import BackgroundExtractor
import numpy.random

image = numpy.random.rand(100, 100)
x = numpy.random.rand(10) * 1000.0
y = numpy.random.rand(10) * 1000.0

measure_background = BackgroundExtractor(image,
                                         inner_radius=6.0,
                                         outer_radius=13.0)
bg_value, bg_error, bg_num_pix = measure_background(x, y)