updated:  2021-08-17

In CCD imaging, gain refers to the magnitude of amplification a given system will produce. Gain is reported in terms of electrons/ADU (analog-to-digital unit). A gain of 8 means that the camera digitizes the CCD signal so that each ADU corresponds to 8 photo electrons.

The steps involved in reading the value of a pixel are something like this: 

  1. Electrons transferred to "amplifier"; really a capacitor. Units are coulombs
  2. The voltage induced by this charge is measured. Units are volts
  3. An Analog-To-Digital (A/D) unit or ADU converts the voltage into some other voltage, which may have only one of several discrete levels. Units are still volts
  4. The voltage is converted into a number which is passed from the hardware to the computer software as the pixel's value. Units are counts, also called "Data Numbers" (DN) or "Analog-to-Digital Units" (ADUs). 
  5. The end result is that there is some "fudge factor" which relates the initial number of electrons in a pixel to the final number of counts reported by camera software. The ratio of these two numbers is the gain of the camera: 

                      # of electrons per pixel

          gain   =  --------------------------

                      # of counts per pixel

How should one choose the gain factor? There are several criteria: 

  • Full-well depth vs. largest pixel value 
    Each CCD is designed to hold only so many electrons within a pixel before they start to leak outwards to other pixels. This maximum size of a charge packet on the chip is called the full well depth. There is also a "maximum possible number" in the Analog-to-Digital converter. Most CCDs use 14-bit, 15-bit, or 16-bit A/D units: the corresponding maximum pixel values are 2^14 = 16384, 2^15 = 32768, and 2^16 = 65536. It is logical to arrange the gain so that very roughly, the number of electrons in the full-well depth corresponds to the maximum pixel value. 

  • Readout noise vs. smallest pixel value 
    One can look at things the other way around: what are the SMALLEST values that make sense? Amplifiers on CCDs have a certain level of random noise: each time one reads the charge in a packet, one gets a slightly different value. A typical readout noise is 5 or 10 electrons RMS (meaning that the standard deviation of a bunch of measurements of the same packet would be about 5 or 10 electrons). Therefore, if two pixels have values which differ by only 3 electrons, it's not easy to tell the difference between them. 

The smallest difference one can represent in an integer image is 1 count. To some extent, it makes sense to arrange the gain so that 1 count corresponds to some moderate fraction of the readout noise. Any finer measurement of the pixel values would yield differences which would be essentially random.


See also  ­čí║ Unity Gain (for ZWO ASI cameras) on this website

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