Readout noise

In many ways, today s optical and infrared detectors are nearly perfect, with high quantum efficiency, low readout noise, high dynamic range and large arrays of pixels. However, as good as the detectors are, there are limitations that must be understood and respected in order to produce the best astronomical instm-ments and thereby, the best science. 

Detect 100% of photons Photon detected as a delta function Large number of pixels Time tag for each photon Measure photon wavelength Measure photon polarization No detector noise fr Up to 99% detected fr One electron for each photon fr Over 377 million pixels 0 No - framing detectors 0 No - provided by optics 0 No - provided by optics 0 Readout noise and dark current... 

Detector noise - The two most signihcant noise sources of a detector are readout noise and dark current. 

Readout noise is the noise that comes from the amplification of the small amount of electrical charge that is produced by the light. 

Amplifiers are not perfect. There is always random fluctuation in the current through an amplifier, even if there is no change in the charge on the gate of the amplifier. This random fluctuation of the current produces a false signal that looks like a real signal. The readout noise is the rms variation of the signal, and... 

The CCD MOSFET is a destructive readout device - there is only one measurement per charge packet. However, an infrared amplifier can be read ouf several times, with averaging and corresponding reduction in the effective readout noise (16 reads can reduce the noise by a factor of a/Tg or 4). In theory, multiple readout of an infrared amplifier could achieve extremely low noise, but in practice, due to other complications, the noise reduction usually reaches a limit of 4-5 improvement (achieved after 16-32 reads). 

Figure 21. Noise spectrum of detector amplifiers. Note that both axes have logarithmic scale. There are two main components of noise - the white noise which is present at all frequencies, and the 1// noise that is dominant at low frequencies. 1// noise has a fractal structure and is seen in many physical systems. The bandpass of a measurement decreases for slower readout, and the readout noise will correspondingly decrease. A limit to reduction in readout noise is reached at the knee of the noise spectrum (where white noise equals l/f noise) - reading slower than the frequency knee will not decrease readout noise.

In real curvature sensors, a vibrating membrane mirror is placed at the telescope focus, followed by a collimating lens, and a lens array. At the extremes of the membrane throw, the lens array is conjugate to the required planes. The defocus distance can be chosen by adjusting the vibration amplitude. The advantage of the collimated beam is that the beam size does not depend on the defocus distance. Optical fibers are attached to the individual lenses of the lens array, and each fiber leads to an avalanche photodiode (APD). These detectors are employed because they have zero readout noise. This wavefront sensor is practically insensitive to errors in the wavefront amplitude (by virtue of normahzing the intensity difference). 

The accuracy with which a wavefront sensor measures phase errors will be limited by noise in the measurement. The main sources of noise are photon noise, readout noise (see Ch. 11) and background noise. The general form of the phase measurement error (in square radians) on an aperture of size d due to photon noise is... 

An electronic noise component is also generated by the transfer of charges and by the preamplifier. For each readout process, one readout noise is generated. This readout noise is not very sensitive to temperature but increases with reading-out speed. Readout noise for a HCCD is about 10 electrons RMS or less. 

As noted briefly above, the readout noise level is sharply decreased when reading-out rate is low. Cameras allowing a slow readout are called slow-scan CCD cameras. 

Some cameras allow multiple-mode readout. The user may switch modes between ultralow-readout noise for a high dynamic range and high-speed frame capture at reduced resolution. This is the case, for instance, for the Hamamatsu Orca II (high sensitivity at 14 bits vs. 5.3 fps at 12 bits). 

The slow-scan CCD, also called the scientific CCD, or in the spectroscopy literature simply CCD, is the detector of choice for most applications of Raman spectroscopy. A well-designed CCD has essentially zero dark current, very low readout noise, and high quantum efficiency (peak 45—70% near 700 nm) in the visible region of the spectrum. However, the response drops quickly above 800 nm and there is no photon response above 1.05 J m. For routine spectroscopy or process control, thermoelectrically cooled (to about —40° C) CCDs are adequate. Although these detectors are somewhat noisier than detectors operated at —100° C or lower, the former do not require liquid nitrogen cooling. The general properties and spectroscopic applications of the CCD have been reviewed (22). 

Although the dark signal contributes to the observed signal, it is distinguished from the background defined in Section 4.2.2 by the fact that it does not depend on laser intensity or sample variables. In fact, one test for detector noise is to run a spectrum with the laser completely off. The observed noise in this situation is due to detector and readout noise. Figure 4.6 shows a spectrum dominated by detector noise. The detector noise remains after the contributions of the cell and water are subtracted. 

Calculated from Eq. (4.11), assuming negligible dark and readout noise. 

The readout noise limit is not encountered frequently with modern spectrometers, but can be important for samples with very weak scattering. If cTr in Eq. (4.11) is dominant, the SNR is linear in S ... 

Figure 4.7. Spectra of solid dextrose, obtained with 785 nm excitation and a dispersive/CCD spectrometer. Spectrum C is an average of fifty O.I sec CCD integrations and shows no improvement in SNR over a single 0.1 sec integration spectrum A), due to the dominance of readout noise.

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