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A charge-coupled device stores photoelectrons in potential wells and transfers the charge through clocked stages to an output amplifier. The transfer differs from independently addressed pixels in a hybrid infrared detector.

In an imaging charge-coupled device, absorbed photons produce mobile charge carriers in a semiconductor; collection stores an electric charge packet associated with each optical detector pixel. Closely spaced MOS capacitors allow applied electric potential differences to move the packet from site to site. Willard Sterling Boyle and George Elwood Smith described storage, transfer and detection as distinct operations, including imaging as one application.^[ref]

The useful architectural trick is that many optical detector pixels can send their electric charge to a shared output electronic amplifier. This makes repeatable conversion possible but makes fidelity depend on every transfer along the route. An active-pixel sensor made with CMOS technology instead uses circuitry at pixels to convert or buffer their signal and select it for readout; independent pixel addressing does not require transporting each packet through the whole image. This compares readout architectures, not whether either technology can use a photodiode.^[ref]

Digital optical phase conjugation uses a wavefront sensor and a spatial light modulator to measure an incident field and launch a beam with the opposite spatial wave phase. A phase-only spatial light modulator illuminated uniformly generally does not reproduce the incident wave amplitude as well; exact $\gamma U^{\ast }$ requires the appropriate wave amplitude and polarization of an electromagnetic wave too. The experimentally useful question is how much of the original optical mode can be recovered with the measured and controlled field components.

In 2010, Meng Cui and Changhuei Yang demonstrated digital optical phase conjugation through a prepared multiple scattering sample. They dried mixed-size polystyrene microspheres on cover glass, added immersion oil, and sealed them with another cover glass. A 532 nm laser supplied signal and reference beams. The signal crossed the sample between two objectives; the reference interfered with it on a charge-coupled device. Phase-stepped wave interference recovered the spatial wave phase; a calibrated spatial light modulator imposed its negative on the reference-derived return beam. The reference supplied playback power, without nonlinear mixing pumps. An optical camera read out the recovered signal. With the same sample and a zero-phase spatial light modulator control, the return instead produced a scattering of light pattern. This comparison supports mode recovery by spatial wave phase control, not merely increased illumination. A separate ordinary-mirror control tested source displacements. Finite sampling and collection limited recovery; the system controlled wave phase rather than the complete vector field. These restrictions distinguish demonstrated reconstruction from exact reversal of every scattered component.^[ref]

Built by André Baranne, Michel Mayor, Didier Queloz and colleagues, the ELODIE fiber-fed spectrograph carried light from Haute-Provence's 1.93-metre telescope through optical fibers. Its echelle grating dispersed light in high diffraction orders; an optical prism and grism cross-disperser separated them perpendicularly on a charge-coupled device. This cross-dispersed echelle spectrograph achieved spectral resolving power $R=42000$. A simultaneous thorium reference tracked instrumental drift. Wavelength calibration mapped detector positions to wavelengths; spectral cross-correlation combined many spectral lines into a fitted profile. A barycentric velocity correction removed the observer's motion.^[ref]

Why can a spectrograph measure a Doppler shift much smaller than one spectral resolution element? Resolution concerns separating neighbouring features, whereas locating the centre of a known profile uses information across that profile and across many spectral lines. For a chosen example, at $\lambda =500\mathrm{nm}$ a recession velocity of $60\mathrm{m}{\mathrm{s}}^{-1}$ givesAt $R=42000$, one spectral resolution element corresponds to $c/R\simeq 7.14\mathrm{km}{\mathrm{s}}^{-1}$, about $119$ times larger. A well measured centre can move by a small fraction of that width. More photons and more useful spectral lines improve the estimate, but do not remove wavelength calibration errors or stellar activity.

A simple mathematical picture of spectral cross-correlation makes the readout concrete. Define $x=c\log (\lambda /{\lambda }_0)$, with fixed reference wavelength ${\lambda }_0$, and let $d(x)$ be continuum-normalized line absorption. For small velocities, a common Doppler shift approximately translates $d(x)$. A positive absorption mask $m(x)$ then givesIf $d(x)=m(x-v_r)$, the maximum occurs at $u=v_r$; a convention correlating the received observed radiative flux instead produces a dip. A fitted correlation centre is an effective collective line position, not a claim that each individual spectral line has been resolved into a narrower feature. The lamp tracks instrumental drift, while the barycentric velocity correction removes the observer's changing line-of-sight velocity; neither removes changes in the star itself.

The charge that an integrating detector well can hold within its stated operating specification before saturation or overflow compromises the response. It differs from the digitizer limit and from the largest signal that is accurately linear. In a charge-coupled device, excess charge can cause blooming into adjacent pixels.

A hybrid infrared detector bonds a photosensitive semiconductor array to a separate silicon readout circuit. Each pixel has its own electrical connection; charge need not be transferred across the array as in a charge-coupled device.

One half went to Charles Kuen Kao for breakthroughs in transmitting light through optical fibers for communication. One quarter each went to Willard S. Boyle and George E. Smith for inventing the charge-coupled device imaging sensor.^[ref]

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 $r$,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.

A charge-coupled device converts incident optical photons into collected electrons in pixel potential wells. Exposure accumulates charge; clocked transfers move it to readout amplifiers. Quantum efficiency, gain, read noise, dark current, saturation and charge-transfer behavior determine the usable signal. Bias or overscan estimates remove electronic offsets, dark frames characterize dark signal where needed, and flat fields correct relative response and illumination. Multiple exposures help identify cosmic rays and defective pixels, while astrometric alignment permits stacking.

For aperture photometry, sum the source aperture and subtract an estimated local sky contribution. A point-spread-function fit can instead use the expected spatial shape, particularly in crowded fields. Detector gain converts recorded units to electrons before applying a Poisson noise calculation. Let the aperture contain $n$ pixels, total expected source charge $S$, sky charge $B$ per pixel, dark charge $D$ per pixel and rms read noise $r$ per pixel. With a perfectly known background mean,If the background is estimated from $M$ independent comparable blank pixels, its subtraction adds approximately $n^2(B+D+r^2)/M$, givingThe extra term is uncertainty in the estimated mean; it does not replace the sky shot noise already present in the source aperture. A more exact budget propagates the actual bias, dark and flat calibrations and any pixel correlations. The existing aperture photometry with an estimated background derives this distinction.

For one exposure in the photon-limited regime, signal grows as $t$ and its counting noise as $\sqrt{t}$, so SNR grows as $\sqrt{t}$. Read-noise domination can favor longer individual exposures, but full-well saturation, cosmic rays, tracking and changing sky constrain them. Extraction uses an aperture correction or a calibrated PSF model to recover light outside the measured region. Flat-field errors, nonlinearity, imperfect sky gradients, blending and atmospheric transparency can dominate bright-source accuracy even when the formal photon error is tiny.

An H2RG detector is a hybrid infrared detector: a photosensitive array is bonded, pixel by pixel through indium contacts, to a separate silicon readout circuit. In infrared use the absorber is normally mercury cadmium telluride, whose composition sets the absorption cutoff. The H2RG readout architecture can also be used with other absorbing materials, so the readout name does not fix the detector’s wavelength response. The manufacturer’s H2RG specification gives a $2048\times 2048$ grid and $18\mu \mathrm{m}$ pixel pitch, together with reference-pixel and guide-window capabilities.

Each illuminated pixel contains a photodiode connected to its own readout node. Absorbed photons generate carriers; their accumulated charge changes the node voltage, with $|\Delta V|=|\Delta Q|/C$ for node capacitance $C$. A reset establishes the baseline; row and column addressing and multiplexed output amplifiers sample the pixel voltage. This is not the serial transfer of charge packets through adjacent pixels used by a charge-coupled device.

Nondestructive detector readout allows repeated measurements during an integration. Correlated double sampling removes the reset baseline, Fowler sampling differences groups of reads at the beginning and end, and up-the-ramp sampling estimates the signal from its slope and can flag cosmic-ray steps or saturation. The “R” denotes detector reference pixels, which monitor electronic offsets; “G” denotes a programmable guide window that can be read rapidly between full-array operations. Reference pixels do not measure sky background. Cooling suppresses dark current and thermal background. Read noise, nonlinearity, persistence and interpixel coupling still require calibration.

The 1998 High-Z Supernova Search Team paper by Adam Riess, Brian Schmidt and collaborators presented supernova evidence for acceleration.^[ref]

The 1999 Supernova Cosmology Project paper by Saul Perlmutter and collaborators independently analysed 42 distant events. Alexei V. Filippenko coauthored both papers.^[ref]

The High-Z Supernova Search Team search used the CTIO Blanco 4-m telescope and a $2048\times 2048$ charge-coupled device, comparing fields across dark runs. Difference images located rising candidates; Keck, MMT and ESO spectroscopy classified them and measured host redshifts. Follow-up photometry used filters approximating emitted B and V bands. Calibrated field stars transferred the magnitude zero point; late host images removed galaxy light, point spread function fits measured brightness, and inserted artificial stars tested recovery.^[ref]

The Supernova Cosmology Project likewise discovered rising events with CTIO 4-m charge-coupled device imaging, then followed their light curves with several telescopes. Keck LRIS and ESO spectroscopy supplied classification and redshift, preferably from narrow host lines. Cross-filter K corrections connected received R/I observations to emitted B/V light.^[ref]

Why does fainter mean accelerating? First it means a larger inferred luminosity distance only at fixed standardized luminosity and corrected attenuation. Next it favours acceleration only after a comparison family is specified: an FLRW metric, representative propagation of light, common nearby and distant standardization, and controlled measurement calibration and selection. For the usual general relativity models containing nonnegative pressureless matter, spatial curvature of an FLRW universe and a cosmological constant, the shape of the Hubble diagram constrains ${\Omega }_{m,0},{\Omega }_{\Lambda ,0}$ and thus $q_0={\Omega }_{m,0}/2-{\Omega }_{\Lambda ,0}$. A positive residual relative to one decelerating model rejects that prediction; it does not alone prove $q_0<0$ across arbitrary expansion histories or arbitrary opacity laws.

The essential advance is that a nearby sample fixes an intercept while a range of distant redshifts measures the shape. The two teams supplied independent observations and analysis, although common nearby calibrators and Type Ia supernova physics can produce shared uncertainties. The dark-energy acceleration criterion explains the interpretation within general relativity; the worked supernova distance comparison shows the size and sign of the effect without fitting any survey data.