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Aperture Photometry

What you are seeing: a synthetic star (Moffat PSF) sits on a sky background with Poisson + read noise. Move the aperture and annulus radii to see how the recovered flux compares to the true flux. Too-small aperture undercounts the star; too-large aperture is contaminated by sky noise.

Figure 1. Synthetic CCD image; circles mark aperture (cyan) and sky annulus (orange).
aperture r6.0
sky r_in / r_out14.0
FWHM (px)2.5
F_true8000
sky / pixel100

WHAT TO TRY

  • Grow the aperture radius: the flux climbs and plateaus at F_true once it captures the star's Moffat wings, but the noise keeps growing, so the SNR curve rises then falls. Park your radius near the SNR peak.
  • Too small an aperture throws away light (the error goes negative); too large drowns the star in sky-noise pixels. The growth curve shows both failure modes at once.
  • Resample the exposure: the frame reshuffles its photon and read noise, the measured flux scatters around F_true, but the SNR-optimal radius barely moves. That stability is why aperture photometry works.