What (TF) is ADU?

Long story short: ADU stands for Analogue-to-Digital Unit

As you probably remember, the camera has a sensor, usually CMOS. Such a sensor acquires photons, charging each pixel proportionally to the number of photons landed on it. There is an article that describes how the process works. Anyway, the ADU represents the numerical value assigned to a pixel’s brightness after the camera’s electronics convert an analogue voltage signal into a digital number. When light hits a sensor, photons are converted into electrons. These electrons are measured as a voltage, which is then digitised into ADU by the camera’s Analog-to-Digital Converter (ADC).

The maximum possible ADU value depends on your camera’s bit depth:

• 12-bit: 0 to 4,095 ADU
• 14-bit: 0 to 16,383 ADU
• 16-bit: 0 to 65,535 ADU

There is another parameter involved: the well-known Gain. The relationship between electrons (e) and ADU is defined by Gain (measured in e/ADU). For example, a gain of 2 means every 2 electrons collected in a pixel results in 1 ADU.
In daily imaging, you will encounter ADU primarily when managing your exposure levels:

• When getting a Flat Frames: to correct for vignettes and dust, “flats” are typically exposed to reach a specific “Target ADU,” often around 30%–50% of the sensor’s maximum (e.g., ~20,000–30,000 for a 16-bit camera) to ensure the sensor stays in its linear range.
• The sky background: to ensure you are “swamping” the camera’s read noise without overexposing, the sky background in your sub-exposures should ideally have a minimum ADU value slightly above the camera’s base Offset level.
• To avoid saturation: if a pixel reaches its maximum ADU (e.g., 65,535), it is “saturated” or “clipped,” and all detail in that area (like the core of a bright star) is lost. This is a critical element to consider during planning the log exposure sessions, even with the narrow-band filters.

A few other questions may arise here. For instance, what is offset? It is a small positive ADU value added by the manufacturer to prevent noise from dropping signal values below zero.

What is Linearity? Linearity is a range in which doubling the light accurately doubles the ADU count. It is essential for scientific calibration.

And the last element is a Full Well Capacity. This is the maximum number of electrons a pixel can hold before it saturates the ADU count.

About The Author

Artur

An IT network engineer by profession. A passionate diver. Active diving instructor and EFR (Emergency First Response & Secondary Care) instructor. Technical diver TDI Advanced Trimix Open Circuit and X-CCR Normoxic Trimix. Closed-circuit diving enthusiast. Yet also an astrophotographer. After all these years, I’m still a beginner, learning the stuff to do things better.

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