Detect & Remove Residual Structured Noise after Calibration and Stacking

Calibration and stacking reduces pixel non-uniformity and random noise from dark current, shot noise, read noise, and camera off-chip noise since random noise does not build up as fast as signal from the objects in the image. But structured noise from electronic interference do build up as fast as signal from the objects and are not removed by calibration and stacking. Below is a method to detect if your camera has residual structured noise and a method to reduce or remove structured noise from an image.


Detecting Structured Noise

The QSI 583 structured noise spectrum above has no structured noise which would show up as bright vertical or horizontal lines or spikes. Only the bright center DC pixel is shown which represents the average color of the image. This is what the structured noise spectrum looks like from a camera with little or no noise from electronic interference.




The structured noise spectrum of a Canon 60Da is shown above. The horizontal and vertical line through the center bright DC pixel is from periodic noise that is not removed by calibration and stacking. The amplitude of frequencies along the horizontal and vertical lines can be reduce using the method shown below and will slightly improve the image.




The structured noise spectrum above is from a SBIG ST-10 and shows several bright vertical lines on each side of the center DC pixel. These lines are from structured or periodic noise that is not removed by calibration and stacking. The Frequency Filter can be used to remove the frequencies marked by the vertical lines to reduce structured noise in the calibrated and stacked image as shown below.



Reducing Structured Noise

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