Using an electron-multiplied CCD in a digital camera has many advantages, but it also poses some unique challenges. The high sensitivity of an electron-multiplied CCD is a major advantage, but it can also lead to image degradation when there are light leaks. In order to avoid these issues, it is important to know how to properly adjust the CCD for your camera and Neuromorphic Camera
High- and low-intensity regions of the histogram
These include quantum efficiency, readout noise, dark current, and gain ageing. In order to deliver photon counting performance in the visible, these factors need to be taken into account in the design of the device.
The best way to achieve this is to perform gain calibration. This involves the calibration of the effective gain multiplier (ADU factor), which is a function of the offset parameter (S_0) and the row-position. Calibration of the effective gain multiplier is crucial for the proper scaling of raw signals to true amplification values. This calibration is also necessary in order to compare intensity levels among pixels.
The best calibration is to simulate a high-end electron multiplying charge coupled device using a commercially available software package. This software package enables users to simulate four different gain levels. This software package also includes an interactive tutorial that demonstrates the physics of gain multiplication.
A quick glance at the histogram of the simulated images will reveal a couple of key points. The histogram is divided into two regions: the upper region is for the illuminated part of the image, while the lower region is for the darkest part. This is due to the fact that a portion of the signal is lost to adjacent pixels. This is not expected to have a significant effect at these signal levels.
Noise contribution of clock-induced charge (CIC)
Depending on the type of CCD, the noise contribution of clock-induced charge (CIC) is either relatively low or more significant than dark noise. The amplitude and edges of the clock waveform can be used to reduce or eliminate the noise contribution of CIC.
The noise contribution of clock-induced charge is inversely proportional to temperature. For example, at a temperature of 60 degrees Celsius, the probability of secondary electron generation is increased. However, the noise contribution of CIC is much lower than in the case of a temperature below 50 degrees Celsius.
The noise contribution of clock-induced charge is also impacted by the relative state of the clock signals. For example, a clock signal with a very sharp inflection will produce impact ionization and increase the noise contribution of CIC. In contrast, the probability of a secondary electron generation will be increased when the clock waveform has a less sharp inflection.
The other noise contribution of clock-induced charge is the statistical variation of the multiplication process. This is attributed to various loss mechanisms that occur when a low-signal detector is used. With increased gain settings, the statistical variation can add noise to the output signal.
The noise contribution of clock-induced charges can be reduced by a spurious noise filter. The filter is used to detect problematic pixels and replace the signal value with the mean value of pixels immediately surrounding the problematic pixel.
Photon pulse height distribution is quasiexponential
Using a CCD in a photon counting fashion requires a fairly long exposure time to get a signal of significance. In a typical scientific grade CCD, this is typically around 25 photons per pixel. However, when the exposure time exceeds this threshold, photon noise becomes the dominant noise source.
There are three primary broad components of noise in a CCD, including photon noise, read noise, and dark current. Photon noise, also known as shot noise, is a statistical phenomenon that reflects similar statistical variation in photon arrival rates.
One way to reduce the magnitude of the photon noise is to reduce the number of photons that reach the sensor. Two approaches are employed to accomplish this: spatial oversampling, and temporal oversampling. Both sacrifice signal/noise for simplicity, but spatial oversampling is more useful.
In a simulation, the Gaussian intensity distribution was superimposed on particle positions. The program then updated the positions with a Gaussian random number generator. This was a surprisingly robust model, given the complexities of a real world system.
The multi-exponential random number generator was then used to calculate a single-photon response. The results were then summed together to get an analog distribution. The resulting equation yielded a feasibly correct estimate of the square-root of the magnitude of the photon signal.
The simplest example is the inverted mode operation of the multi-pinned phase (MPP) technology. This method has several advantages, including a shortened exposure time and reduced bulk dark current.
Photon pulse height distribution is proportional to frame rate
Several types of scientific imaging applications require multi-megapixel focal plane sensors with high sensitivity, high dynamic range and rapid frame rates. These sensors have a variety of advantages, but their drawbacks are also common. One of the most common is read noise, which can reduce the dynamic range and QE of the sensor. This can be a major concern in many applications, particularly in low light.
Fortunately, there are several options for multi-megapixel sensors. Some of these include interline CCDs, sCMOS, and backilluminated EMCCDs. Interline CCDs are a popular choice for low light applications because of their high performance and low cost. This type of sensor uses microlenses, which direct most incident photons onto the active silicon area of the CCD.
Another advantage of Interline CCDs is their low read noise. This type of sensor has a read noise of five or six electrons at 20 MHz. In contrast, sCMOS sensors have a read noise of 1.3 electrons at 560 MHz.
The amplification mechanism of an EMCCD is designed to reduce read noise, but this also introduces multiplicative noise. The combination of shot and read noise increases the photon to photon variability in the EMCCD, which lowers the sensor’s S/N.
sCMOS sensors are known for exhibiting better contrast between the signal and the read noise. They also have a larger field of view and higher SNR. However, their read noise is comparable to that of Interline CCDs.
Photon pulse height distribution is dependent on voltage levels and the CCD temperature
X-ray radiation has a number of effects on silicon-based electronic devices. These effects vary depending on the type of radiation. X-ray energy can be detrimental to CCDs. It can be deposited in adjacent columns, reduce CTI, and distort the PSF.
ACIS CCDs have readout noise as low as two electrons RMS. This is due to the fact that the photon is accelerated before hitting the CCD chip. During the readout, charge is transferred from the photon to the photoelectron. These electrons are generated in the epitaxial layer.
The standard thicknesses of CCDs can absorb x-rays from 30 eV to 20 keV. These thicknesses are measured in mm. However, this does not tell you how much energy the CCD absorbs. The amount of energy deposited in the CCD depends on the other components of the CCD.
ACIS CCDs have a “chip average energy resolution”. In other words, the energy resolution of the CCD is improved with distance from the readout. This is illustrated in Figure 6.27.
ACIS CCDs have an ‘active imaging section’. This section is shielded to protect the photons from energetic particles. This section contains an epitaxial layer, a buried channel, and a shielded frame store region. The photons are accelerated through the active section and reach the shielded region before reaching the photocathode.
The standard thicknesses of CCDs are also important for determining the optimal thickness of the epitaxial layer. The ideal layer thickness depends on the wavelength. For example, the thickest epitaxial layer may be 20 mm, but this may not be desirable in a soft x-ray application.
Photon pulse height distribution depends on voltage levels and the CCD temperature
X-ray cameras can appear to be little more than isolated X-rays. In reality, they are a composite of X-rays and optical light. The photons and optical light are scattered into the focal plane detector. The X-rays and optical light are gathered and amplified using a channel plate and a bundle of tiny pipes.
In terms of CCDs, the largest contributor to radiation damage will be protons trapped in the earth’s magnetic field. In the experiment CCDs, the triumvirate of electrons, photons and neutrons make up a total system noise of a few electrons RMS. For ACIS CCDs, readout noise is a mere two electrons RMS.
It is important to note that the best way to mitigate the effects of pile-up is to observe with fewer CCDs. This will reduce the number of hot pixels occupying valuable telemetry bandwidth.
The best way to determine whether the octave of a CCD is a good match for your observations is to run a MARX simulator. This is especially important for observers with concerns about energy. The MARX simulator will also give you a sense of the quality of the data you are about to receive. Lastly, it will show you that the octave of X-rays at your target location is highly dependent on the temperature and voltage levels of the source. This is the best time to adjust your observation plan accordingly.
