ao:camera_noise
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ao:camera_noise [2018/07/06 13:37] 127.0.0.1 external edit |
ao:camera_noise [2018/07/07 08:39] (current) jones |
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| + | ====== Camera Noise ====== | ||
| The noise properties of an EMCCD can be tricky to understand. First, for a given photon rate, shot-noise gives a Poisson distribution of electrons entering the readout amplifier. Each electron gives an exponential distribution of charge with a mean equal to the //EM Gain//, and then readout noise is added to the end result. Typically, high gains correspond to the //EM Gain// being 10 or 20 times higher than the readout noise.\\ \\ In addition, there is a dark current that is proportional to the readout rate, i.e. a number of "fake photons" | The noise properties of an EMCCD can be tricky to understand. First, for a given photon rate, shot-noise gives a Poisson distribution of electrons entering the readout amplifier. Each electron gives an exponential distribution of charge with a mean equal to the //EM Gain//, and then readout noise is added to the end result. Typically, high gains correspond to the //EM Gain// being 10 or 20 times higher than the readout noise.\\ \\ In addition, there is a dark current that is proportional to the readout rate, i.e. a number of "fake photons" | ||
| ==== Noise Factor ==== | ==== Noise Factor ==== | ||
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| \\ The noise factor for an EMCCD can be reduced by thresholding. In the simplest form, all pixel values less than some threshold at 2 to 3 times the readout noise are called 0, and all other pixel values are called 1. This simple thresholding only works well at photon rates less than about 0.25 photons per pixel per frame - the rate at which nonlinearity becomes significant at the 10% level (i.e. there are 0.1 times as many 2-photon events as 1-photon events). For slightly higher photon rates, there are lots of options to threshold but reduce nonlinearity, | \\ The noise factor for an EMCCD can be reduced by thresholding. In the simplest form, all pixel values less than some threshold at 2 to 3 times the readout noise are called 0, and all other pixel values are called 1. This simple thresholding only works well at photon rates less than about 0.25 photons per pixel per frame - the rate at which nonlinearity becomes significant at the 10% level (i.e. there are 0.1 times as many 2-photon events as 1-photon events). For slightly higher photon rates, there are lots of options to threshold but reduce nonlinearity, | ||
| - | | {{/ | + | | {{ao:EMCCD_0_05_photons.gif|EMCCD_0_05_photons.gif}} | |
| | as | | | as | | ||
| - | \\ //Figure 1: Expected number of photons versus EMCCD output for an input where pixels have uniformly distributed photon arrival rates between 0 and 0.1 photons/ | + | \\ //Figure 1: Expected number of photons versus EMCCD output for an input where pixels have uniformly distributed photon arrival rates between 0 and 0.1 photons/ |
| ==== Optimal Photon Rates ==== | ==== Optimal Photon Rates ==== | ||
| \\ The above arguments mean that there are 2 obvious ways to run an EMCCD as an AO sensor: | \\ The above arguments mean that there are 2 obvious ways to run an EMCCD as an AO sensor: | ||
ao/camera_noise.1530898652.txt.gz ยท Last modified: 2018/07/06 13:37 by 127.0.0.1
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