darkframesubtractie

Dark frame subtraction is a calibration technique used in digital imaging and astrophotography to reduce sensor-related noise. A dark frame is an exposure taken with the camera shutter closed (or a cap on the lens) using the same exposure time, ISO setting, and sensor temperature as the light frames. The dark frame captures thermal noise, dark current, and hot or “dead” pixels caused by the readout electronics.

In practice, a master dark frame is created by combining several dark frames, typically by averaging or median-combining, to reduce random noise. This master dark is then subtracted from each light frame (or from each image in a stack) to remove the dark current pattern and hot pixels. Some processing pipelines also apply a master bias or pedestal correction in addition to dark subtraction. Dark subtraction is often used alongside flat-field correction to produce cleaner calibrated images.

Requirements and limitations: dark frame subtraction relies on stable conditions, especially sensor temperature, because dark current is temperature dependent. It is important that the dark frames match the exposure time, ISO, and temperature of the light frames. It does not fix all noise types and cannot remove transient events like cosmic rays. If temperature or exposure settings vary significantly between frames, the subtraction can introduce artifacts rather than reduce them. Some cameras offer built-in dark frame subtraction, while others require manual calibration in post-processing.

Applications: widely used in astronomy for long-exposure imaging and in any workflow where sensor noise and fixed-pattern noise must be reduced to improve image quality.