= Solution
A <charge-coupled device> converts absorbed <photons> into stored charge in semiconductor potential wells. An exposure accumulates a pixelated image; clocked electrodes transfer rows toward a serial register and output amplifier. <Quantum efficiency>, pixel sampling and optical throughput connect collected <photons> to the expected signal. In ordinary optical imaging it primarily counts photoelectrons rather than independently measuring every <photon>'s <energy>.
The <CCD noise budget> combines source and sky <shot noise>, dark-current fluctuations and <read noise>. For an aperture containing $n$ pixels, known background expectation $N_b$, total dark expectation $N_d$, source expectation $N_s$ and per-pixel <read noise> $r$,
$$
\mathrm{SNR}=\frac{N_s}{\sqrt{N_s+N_b+N_d+nr^2}}.
$$
Estimating the background from a finite separate region adds its own uncertainty. Cooling reduces <dark current>; longer integration reduces the relative importance of one read but can encounter saturation, cosmic rays or changing conditions. <Full well capacity>, amplifier gain, <linearity>, blooming and charge-transfer efficiency limit useful <detector dynamic range>.
Bias frames establish the electronic offset; matched dark frames characterize time- and temperature-dependent dark signal. Flat fields calibrate pixel-to-pixel response and illumination, with appropriate attention to <wavelength>. Bad-pixel masks, repeated exposures and dithering help reject defects and cosmic rays. A calibrated <PSF> and astrometric solution are needed for reliable <photometry> and positions. High <quantum efficiency> and stable linear integration are advantages, but an uncalibrated count array is not yet an astronomical <flux> map.
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