- used extensively for high-speed work in the 1970s and 1980s
- photon-counting: zero readout time and noise
- sky background and comparison stars require multiple channels
- poor QE: 25% at peak and only a few per cent in the red
- one band only
- requires fixed and typically large apertures
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- started in 1970s
- simultaneous measurement of sky and comparison stars
- high QE: up to 95% over wide wavelength range
- readout noise and slow readout time
- one band only
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