BSL Deep-Sky Imaging – Conversion Gain

Conversion Gain

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Conversion Gain is a fundamental sensor property in digital imaging and astrophotography that defines how many photoelectrons generated in a pixel are converted into a digital signal unit (Analog-to-Digital Unit, or ADU), or the voltage change produced per electron.
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(Often measured in… e⁻ /ADU …or… uV/e⁻..)

Analogue-to-Digital Conversion

Analogue-to-Digital Conversion:
When photons strike a camera sensor (CCD or CMOS), they release electrons ($e^-$). The camera’s readout electronics measure this charge and convert it into a digital number (ADU).

Formula:
Conversion Gain (g)

(g) = Signal in Electrons (e⁻) ÷ Signal in Digital Units (ADU)
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System Gain vs Conversion Gain (g)

Higher gain setting:
(For example: high ISO or increased camera gain) – Higher gain setting results in a lower numerical value of e⁻ /ADU, meaning fewer electrons are needed to produce 1 ADU (higher sensitivity per digital unit).

Lower gain setting:
A lower gain setting results in a higher numerical value of e⁻ /ADU, requiring more electrons to register 1 ADU.

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Impact on Image Parameters

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Full Well Capacity (FWC):
Higher conversion gain (lower e⁻ /ADU) causes pixels to reach maximum digital output (ADU_max)faster, reducing the overall full well capacity before digital saturation occurs.

Read Noise (RN):
Increasing conversion gain reduces the relative contribution of post-amplification read noise when measured in electrons (e⁻), lowering the camera’s absolute noise floor for faint signals.

Dynamic Range (DR):
Base dynamic range is highest at low gain (maximum headroom for bright highlights) – however, modes like High Conversion Gain (HCG) optimise sensor circuitry to drastically drop read noise while preserving a high dynamic range for faint targets.

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HCG (High Conversion Gain) Mode
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Modern CMOS sensors (such as those from Sony Semiconductor used in astrophotography cameras) feature dual conversion gain readout circuits:
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LCG (Low Conversion Gain):
Used for bright targets, wide dynamic range, and maximum star colour retention.

HCG (High Conversion Gain):
Automatically engages at a specific gain threshold to significantly drop read noise without sacrificing significant dynamic range, making it ideal for deep-sky narrowband imaging and weak signal detection.

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