7.1 The Detector
77
The ADU bit number refers to the processing limitations of your camera. The
ADU, or analogue to digital unit, effectively registers the impact of the electrons
trapped in each pixel and converts the result into a number. The number of bits per
pixel limits the maximum number the ADU can report. With an 8-bit camera, this
limit is 256; for 16 bits, it is 65,536; and for 32 bits, it 4,294,967,296. Note that counts
are dimensionless nonnegative integers. The full well capacity of a camera is the
number of electrons each pixel can hold before it becomes unresponsive. A typical
full well depth for a modern astronomical camera is 100,000–200,000 electrons.
Having a higher full well capacity means that you can expose for longer times, which
can improve the quality of the data within the image. However, most astronomical
cameras have an ADU that cannot count as high as the full well capacity, and as
a consequence, the number of electrons in each pixel has to be scaled. The scaling
Fig. 7.1 An SBIG astronomical CCD camera. The CCD is clearly visible inside its window. The
black tube on the side is full of desiccate to keep the air or inert gas dry. The I-beam-shaped metal
object in the centre of the image is the shutter
77
The ADU bit number refers to the processing limitations of your camera. The
ADU, or analogue to digital unit, effectively registers the impact of the electrons
trapped in each pixel and converts the result into a number. The number of bits per
pixel limits the maximum number the ADU can report. With an 8-bit camera, this
limit is 256; for 16 bits, it is 65,536; and for 32 bits, it 4,294,967,296. Note that counts
are dimensionless nonnegative integers. The full well capacity of a camera is the
number of electrons each pixel can hold before it becomes unresponsive. A typical
full well depth for a modern astronomical camera is 100,000–200,000 electrons.
Having a higher full well capacity means that you can expose for longer times, which
can improve the quality of the data within the image. However, most astronomical
cameras have an ADU that cannot count as high as the full well capacity, and as
a consequence, the number of electrons in each pixel has to be scaled. The scaling
Fig. 7.1 An SBIG astronomical CCD camera. The CCD is clearly visible inside its window. The
black tube on the side is full of desiccate to keep the air or inert gas dry. The I-beam-shaped metal
object in the centre of the image is the shutter