Walking Noise stems from fixed pattern noise on the camera sensor, which is then exacerbated by imperfect polar alignment (drift) or lack of intentional shifting between exposures (dithering). It appears as faint, coloured "streaks" like "rain" across the background of the final, stacked image, often in the direction of declination drift. The camera's fixed noise moves slightly with each shot. When software stacks the images, these noisy pixels get averaged into lines. Some cameras are more susceptible to walking noise than others.
Dithering is the best preventative solution, which involves intentionally shifting the mount by a few pixels between exposures so noise does not stack on the same spot.
Using dark frames to subtract hot pixels helps mitigate walking noise as does active cooling of the sensor which reduces the thermal hot pixels that create noise.
Walking noise is often invisible on single frames but appears after stacking and aggressive stretching of the image.
However, we shall consider here how to deal with a situation where walking noise could not be or has not been avoided. This involves using a software solution to removing the walking noise.
There are a number of de-noising programs; some of them are AI, trained neural networks trained to detect noise and reduce or remove it from an image. They are all able to remove some walking noise but are really intended for the removal of more random noise, so often leave behind traces of the walking noise.
Frankin Marek’s SetiAstro contains a specific walking noise AI de-noiser in his Cosmic Clarity suite. This is trained to detect and remove walking noise specifically and so is likely to make a better job of the de-noising. This software is available for PixInsight and is fully integrated into SetiAstroSuitePro.
Paul Howat, a prominent imaging member of the Swansea Astronomical Society was king enough to provide me with a 16 bit stacked image with walking noise, unguided with a HAC125DX telescope and a QHY585C un-cooled camera. It is a good image but when aggressively stretched, walking noise is revealed.
Click on any image to get a closer view
The stretched image was loaded into SetiAstroSuitePro and Cosmic Clarity Walking noise de-noised using the maximum settings. The image has been zoomed to reveal a portion where the walking noise is clearly visible.
If this image is inspected closely (by clicking on it) the walking noise can be seen.
The de-noised image in SetiAstroSuitePro
If this image is inspected closely (by clicking on it) the walking noise can be seen to have been removed.
Animation blinking between the noisy and de-noised images
Click on the animation to get a closer view.
The Cosmic Clarity Walking noise de-noiser in SetiAstroSuitePro has done a good job of removing the walking noise from the image.
I then processed the de-noised image to show that it is a good image. Of course, image processing is always to the taste of the person doing the processing and the end result may not be what Paul would have produced. Nevertheless, it is an image, free of walking noise and showing the various regions of nebulosity in and between M42, M43 and the Running man nebula.
Processed image
In conclusion, SetiAstro's walking noise de-noiser did a good job of removing the walking noise from this un-guided, un-cooled image.
