Jacqueline Macinnes Wood Nipple

The "gain" and "full well capacity"

Before setting out the important subject of gain of a CCD camera, we introduce an equally important characteristic of a CCD: the Full Well Capacity . The photoelements that make up the matrix of a CCD can be viewed as microscopic containers of electrons: the number of electrons that can fit in each photoelement is usually indicated by the manufacturers of CCD with the term Full Well Capacity (FWC). The analogy of the container is more suitable if we think that the larger the size of photoelement, the greater its ability to contain electrons. So for example a sensor KAF-0401E of Kodak composed photoelements 9 micron square of side a FWC of about 100000 and the sensor-SITE Room Apogee Ap7p has photoelements square size of 24 microns with a FWC of about 300,000 e-. E ' clear that when a light sensor is no longer able to contain electrons, the CCD camera will no longer be able to count them: the system has reached saturation. But that is another topic that we will see later.

The gain of a CCD camera is a number that expresses how many electrons per ADU in the image are generated from the same room. Recall that with ADU (Analog to Digital Unit ) denote the unit of luminous intensity of a pixel CCD. In practice the number corresponding to the pixels of a digital image.

The gain is a parameter that is set by the manufacturer of the CCD camera according to the choice the analog-digital converter: electrons captured during the exposure are converted to ADU integrity ADC (Analog to Digital Converter or analog to digital converter). The "accuracy" of this converter is measured in bits: the higher the number of bits of the converter, the greater the device's ability to distinguish the signal in electrons formed by the exposure of CCD 12 bit = 2 ^ 12 = 4096 values, 15 bit = 2 ^ 15 = 32768, 16 bit = 2 ^ 16 = 65536 values, etc. ..

A method for determining the gain to be used in a particular CCD camera is to compare the FWC photoelements of the sensor with the largest number that can count the analog-digital converter: thus, for example, assuming the sensor chamber SITE Apogee Ap7p (FWC = 300000 and-drive with a 16-bit), we have:

gain = 300000/65536 = 4.6 e-/ADU

And in fact in the data-sheet of the CCD camera are shown a gain of 4.4 e-/ADU: This value is then set correctly to take advantage of the characteristics of the analog-digital converter based on ability that each photoelement to collect electrons.

should make sure, however: the gain is also the minimum unit of discretization, namely that the system is unable to distinguish lower values \u200b\u200bto it (eg a number of electrons below 4.4 as above). This fact introduces a new concept of noise: the sound of discretization. The higher the gain, the greater the noise dicretizzazione. This type of noise can be important because it affects the precision of photometric measurements, especially in the faint and extended objects like comets and galaxies.