BSL Deep-Sky Imaging – Dark Current

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Dark Current

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Dark Current:
It is a persistent background electric current generated within an image sensor (CMOS or CCD) even when no light is entering the camera (i.e., in complete darkness).
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What Causes It?

Indistinguishable from Light:
To the camera sensor, thermally generated electrons look identical to electrons generated by incoming light photons.

Thermal Energy:
Random vibrations in the sensor’s silicon structure thermally excite electrons, knocking them loose into the pixel wells.

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Signal Build-Up Over Time:
Dark current accumulates linearly with exposure time. During short daytime snapshots (for example: 1/500 seconds), it is imperceptible.
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During long exposures (for example: 300 seconds) required for faint deep-sky targets (galaxies, nebulae), dark current creates a noticeable thermal background layer.
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Thermal Shot Noise:
The random quantum variation in dark current creates dark current shot noise (grainy background noise).
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Hot Pixels & Non-Uniformity:
Pixels do not accumulate dark current at the exact same rate.
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Defective or sensitive pixels accumulate charge very rapidly, showing up as bright white or coloured specks (“hot pixels”).

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Sensor Cooling (-10° or -20°):
Dedicated astronomy cameras use Thermoelectric Coolers (TEC / Peltier cooling) to chill the sensor down to -10° or -20°, dropping dark current to negligible levels.
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Dark Frames Subtraction:
In astrophotography image processing, long-exposure images (“dark frames”) taken with the telescope/lens covered at the same temperature and exposure time are subtracted from light frames to remove the fixed thermal background and hot pixels.
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Dark frame:
It is a calibration image taken with the lens cap on (or the camera shutter closed) using the exact same exposure time, ISO/gain, and sensor temperature as your actual astrophotography shots.
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See more information about Dark Frames (Page ##).
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