= Aperture photometry
{title2=$\widehat r=(S-nb)/\Delta t$}
<Aperture photometry> estimates brightness by summing calibrated detector pixels in an extraction region and subtracting the estimated background in that region. For $n$ equally weighted pixels, let $S$ be their total accumulated signal in <Electrons>, $b$ the estimated background <Electrons> per pixel over the same exposure, and $\Delta t$ the exposure duration. Then $\widehat r=(S-nb)/\Delta t$ is a background-subtracted electron rate. Kepler applied <aperture photometry> to calibrated pixels with a fitted sky background. https://arxiv.org/pdf/1001.0258[Jenkins et al. (2010), photometric analysis, section 3]{ref}.
This measurement trades source collection against background: enlarging the extraction region can retain more starlight but includes more <sky brightness> and neighbouring sources. <Photon shot noise>, <read noise>, and uncertainty in <background subtraction> contribute to its uncertainty. Dividing $\widehat r$ by a consistently measured reference rate gives a dimensionless <light curve>; it does not supply an absolute <observed radiative flux>. If contaminating light remains, the reference includes it and the inferred <exoplanet transit depth> suffers <transit dilution>.
In the first toy figure, the source's reference rate is $100{,}000$ electrons per second, the summed sky contribution is $4{,}000$ electrons per second, and a known multiplicative response is $g(t)=1+0.0003t$ with $t$ in hours from mid-transit. The artificial <exoplanet transit depth> is $0.010$, its total duration four hours, and its flat-bottom duration $2.8$ hours; the straight ingress and egress are a toy approximation, not an exact <exoplanet transit photometry> model. Each synthetic sample averages a $0.5$-hour exposure and adds seeded <normal distribution> noise with relative <standard deviation> $0.0006$. Subtracting the known sky rate and dividing by the known response and reference rate recovers the dip. Real observations require estimating those corrections and carrying their uncertainty.
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