A digital camera takes light and focuses it via the lens onto a sensor made out of silicon. It is made up of a grid of tiny photosites that are sensitive to light. Each photosite is usually called a pixel, a contraction of "picture element". There are millions of these individual pixels in the sensor of a DSLR camera.
Digital cameras sample light from our world, or outer space, spatially, tonally and by time. Spatial sampling means the angle of view that the camera sees is broken down into the rectangular grid of pixels. Tonal sampling means the continuously varying tones of brightness in nature are broken down into individual discrete steps of tone. If there are enough samples, both spatially and tonally, we perceive it as faithful representation of the original scene. Time sampling means we make an exposure of a given duration.
Our eyes also sample the world in a way that can be thought of as a "time exposure", usually on a relatively short basis of a few tenths of a second when the light levels are high as in the daytime. Under low light conditions, the eye's exposure, or integration time can increase to several seconds. This is why we can see more details through a telescope if we stare at a faint object for a period of time.
The eye is a relatively sensitive detector. It can detect a single photon, but this information is not sent along to the brain because it does not exceed the minimum signal-to-noise ratio threshold of the noise filtering circuitry in the visual system. It requires several photons for a detection to be sent to the brain. A digital camera is almost as sensitive as the eye, and both are much more sensitive than film, which requires many photons for a detection.
It is the time sampling with long exposures that really makes the magic of digital astrophotography possible. A digital sensor's true power comes from its ability to integrate, or collect, photons over much longer time periods than the eye. This is why we can record details in long exposures that are invisible to the eye, even through a large telescope.
CCD and CMOS sensors perform similarly in absorbing photons, generating electrons and storing them, but differ in how the charge is transferred and where it is converted to a voltage. Both end up with a digital output.
The entire digital image file is then a collection of numbers that represent the location and brightness values for each square in the array. These numbers are stored in a file that our computers can work with.
The entire photosite is not light sensitive. Only the photodiode is. The percentage of the photosite that is light sensitive is called the fill factor. For some sensors, such as CMOS chips, the fill factor may only be 30 to 40 percent of the entire photosite area. The rest of the area on a CMOS sensor is comprised of electronic circuitry, such as amplifiers and noise-reduction circuits.
Because the light-sensitive area is so small in comparison to the size of the photosite, the overall sensitivity of the chip is reduced. To increase the fill factor, manufacturers use micro-lenses to direct photons that would normally hit non-sensitive areas and otherwise go undetected, to the photodiode.
Electrons are generated as long as photons strike the sensor during the duration of the exposure or integration. They are stored in a potential well until the exposure is ended. The size of the well is called the full-well capacity and it determines how many electrons can be collected before it fills up and registers as full. In some sensors once a well fills up, the electrons can spill over into adjacent wells, causing blooming, which is visible as vertical spikes on bright stars. Some cameras have anti-blooming features that reduce or prevent this. Most DSLR cameras control blooming very well and it is not a problem for astrophotography.
The number of electrons that a well can accumulate also determines the sensor's dynamic range, the range of brightness from black to white where the camera can capture detail in both the faint and bright areas in the scene. Once noise is factored in, a sensor with a larger full-well capacity usually has a larger dynamic range. A sensor with lower noise helps improve the dynamic range and improves detail in weakly illuminated areas.
Not every photon that hits a detector will register. The number that are detected is determined by the quantum efficiency of the sensor. Quantum efficiency is measured as a percentage. If a sensor has a quantum efficiency of 40 percent, that means four out of every ten photons that hit it will be detected and converted to electrons. According to Roger N. Clarke, the quantum efficiencies of the CCDs and CMOS sensors in modern DSLR cameras is about 20 to 50 percent, depending on the wavelength. Top-of-the-line dedicated astronomical CCD cameras can have quantum efficiencies of 80 percent and more, although this is for grayscale images.
The number of electrons that build up in a well is proportional to the number of photons that are detected. The electrons in the well are then converted to a voltage. This charge is analog (continuously varying) and is usually very small and must be amplified before it can be digitized. The read-out amplifier performs this function, matching the output voltage range of the sensor to the input voltage range of the A-to-D converter. The A/D converter converts this data into a binary number.
When the A/D converter digitizes the dynamic range, it breaks it into individual steps. The total number of steps is specified by the bit depth of the converter. Most DSLR cameras work with 12 bits (4096 steps) of tonal depth.
The sensor's output is technically called an analog-to-digital unit (ADU) or digital number (DN). The number of electrons per ADU is defined by the gain of the system. A gain of 4 means that the A/D converter digitized the signal so that each ADU corresponds to 4 electrons.
The ISO rating of an exposure is analogous to the speed rating of film. It is a general rating of the sensitivity to light. Digital camera sensors really only have one sensitivity but allow use of different ISO settings by changing the gain of the camera. When the gain doubles, the number of electrons per ADU goes down by a factor of 2.
As ISO is increased in a digital camera, less electrons are converted into a single ADU. Increasing ISO maps a smaller amount of dynamic range into the same bit depth and decreases the dynamic range. At ISO 1600, only about 1/16th of the full-well capacity of the sensor can be used. This can be useful for astronomical images of dim subjects which are not going to fill the well anyway. The camera only converts a small number of electrons from these scarce photons, and by mapping this limited dynamic range into the full bit depth, greater differentiation between steps is possible. This also gives more steps to work with when this faint data is stretched later in processing to increase the contrast and visibility.
For every pixel in the sensor, the brightness data, represented by a number from 0 to 4095 for a 12-bit A/D converter, along with the coordinates of the location of the pixel, are stored in a file. This data may be temporarily stored in the camera's built-in memory buffer before it is written permanently to the camera's removable memory card.
This file of numbers is reconstructed into an image when it is displayed on a computer monitor, or printed.
It is the numbers that are produced by the digitization process that we can work with in our computers. The numbers are represented as bits, a contraction of "binary digits". Bits use base 2 binary notation where the only numbers are one and zero instead of the base 10 numbers of 0 through 9 that we usually work with. Computers use binary numbers because the transistors that they are made of have only two states, on and off, which represent the numbers one and zero. All numbers can be represented in this manner. This is what makes computers so powerful in dealing with numbers - these transistors are very fast.
Digital cameras sample light from our world, or outer space, spatially, tonally and by time. Spatial sampling means the angle of view that the camera sees is broken down into the rectangular grid of pixels. Tonal sampling means the continuously varying tones of brightness in nature are broken down into individual discrete steps of tone. If there are enough samples, both spatially and tonally, we perceive it as faithful representation of the original scene. Time sampling means we make an exposure of a given duration.
Our eyes also sample the world in a way that can be thought of as a "time exposure", usually on a relatively short basis of a few tenths of a second when the light levels are high as in the daytime. Under low light conditions, the eye's exposure, or integration time can increase to several seconds. This is why we can see more details through a telescope if we stare at a faint object for a period of time.
The eye is a relatively sensitive detector. It can detect a single photon, but this information is not sent along to the brain because it does not exceed the minimum signal-to-noise ratio threshold of the noise filtering circuitry in the visual system. It requires several photons for a detection to be sent to the brain. A digital camera is almost as sensitive as the eye, and both are much more sensitive than film, which requires many photons for a detection.
It is the time sampling with long exposures that really makes the magic of digital astrophotography possible. A digital sensor's true power comes from its ability to integrate, or collect, photons over much longer time periods than the eye. This is why we can record details in long exposures that are invisible to the eye, even through a large telescope.
| Each photosite on a CCD or CMOS chip is composed of a light-sensitive area made of crystal silicon in a photodiode which absorbs photons and releases electrons through the photoelectric effect. The electrons are stored in a well as an electrical charge that is accumulated over the length of the exposure. The charge that is generated is proportional to the number of photons that hit the sensor. This electric charge is then transferred and converted to an analog voltage that is amplified and then sent to an Analog to Digital Converter where it is digitized (turned into a number). |
The entire digital image file is then a collection of numbers that represent the location and brightness values for each square in the array. These numbers are stored in a file that our computers can work with.
The entire photosite is not light sensitive. Only the photodiode is. The percentage of the photosite that is light sensitive is called the fill factor. For some sensors, such as CMOS chips, the fill factor may only be 30 to 40 percent of the entire photosite area. The rest of the area on a CMOS sensor is comprised of electronic circuitry, such as amplifiers and noise-reduction circuits.
Because the light-sensitive area is so small in comparison to the size of the photosite, the overall sensitivity of the chip is reduced. To increase the fill factor, manufacturers use micro-lenses to direct photons that would normally hit non-sensitive areas and otherwise go undetected, to the photodiode.
Electrons are generated as long as photons strike the sensor during the duration of the exposure or integration. They are stored in a potential well until the exposure is ended. The size of the well is called the full-well capacity and it determines how many electrons can be collected before it fills up and registers as full. In some sensors once a well fills up, the electrons can spill over into adjacent wells, causing blooming, which is visible as vertical spikes on bright stars. Some cameras have anti-blooming features that reduce or prevent this. Most DSLR cameras control blooming very well and it is not a problem for astrophotography.
The number of electrons that a well can accumulate also determines the sensor's dynamic range, the range of brightness from black to white where the camera can capture detail in both the faint and bright areas in the scene. Once noise is factored in, a sensor with a larger full-well capacity usually has a larger dynamic range. A sensor with lower noise helps improve the dynamic range and improves detail in weakly illuminated areas.
Not every photon that hits a detector will register. The number that are detected is determined by the quantum efficiency of the sensor. Quantum efficiency is measured as a percentage. If a sensor has a quantum efficiency of 40 percent, that means four out of every ten photons that hit it will be detected and converted to electrons. According to Roger N. Clarke, the quantum efficiencies of the CCDs and CMOS sensors in modern DSLR cameras is about 20 to 50 percent, depending on the wavelength. Top-of-the-line dedicated astronomical CCD cameras can have quantum efficiencies of 80 percent and more, although this is for grayscale images.
The number of electrons that build up in a well is proportional to the number of photons that are detected. The electrons in the well are then converted to a voltage. This charge is analog (continuously varying) and is usually very small and must be amplified before it can be digitized. The read-out amplifier performs this function, matching the output voltage range of the sensor to the input voltage range of the A-to-D converter. The A/D converter converts this data into a binary number.
When the A/D converter digitizes the dynamic range, it breaks it into individual steps. The total number of steps is specified by the bit depth of the converter. Most DSLR cameras work with 12 bits (4096 steps) of tonal depth.
The sensor's output is technically called an analog-to-digital unit (ADU) or digital number (DN). The number of electrons per ADU is defined by the gain of the system. A gain of 4 means that the A/D converter digitized the signal so that each ADU corresponds to 4 electrons.
The ISO rating of an exposure is analogous to the speed rating of film. It is a general rating of the sensitivity to light. Digital camera sensors really only have one sensitivity but allow use of different ISO settings by changing the gain of the camera. When the gain doubles, the number of electrons per ADU goes down by a factor of 2.
As ISO is increased in a digital camera, less electrons are converted into a single ADU. Increasing ISO maps a smaller amount of dynamic range into the same bit depth and decreases the dynamic range. At ISO 1600, only about 1/16th of the full-well capacity of the sensor can be used. This can be useful for astronomical images of dim subjects which are not going to fill the well anyway. The camera only converts a small number of electrons from these scarce photons, and by mapping this limited dynamic range into the full bit depth, greater differentiation between steps is possible. This also gives more steps to work with when this faint data is stretched later in processing to increase the contrast and visibility.
For every pixel in the sensor, the brightness data, represented by a number from 0 to 4095 for a 12-bit A/D converter, along with the coordinates of the location of the pixel, are stored in a file. This data may be temporarily stored in the camera's built-in memory buffer before it is written permanently to the camera's removable memory card.
This file of numbers is reconstructed into an image when it is displayed on a computer monitor, or printed.
It is the numbers that are produced by the digitization process that we can work with in our computers. The numbers are represented as bits, a contraction of "binary digits". Bits use base 2 binary notation where the only numbers are one and zero instead of the base 10 numbers of 0 through 9 that we usually work with. Computers use binary numbers because the transistors that they are made of have only two states, on and off, which represent the numbers one and zero. All numbers can be represented in this manner. This is what makes computers so powerful in dealing with numbers - these transistors are very fast.
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