Hot pixels in images from a colour astronomical camera appear as bright, fixed spots of red, green, or blue in deep sky astronomical images. These spots are caused by individual pixels on the camera's sensor that retain a higher-than-average charge, even in the absence of light. This excess charge manifests as a bright spot in the image.
Causes of Hot Pixels:
Sensor Defects: Hot pixels are inherent defects in the camera sensor. They are more noticeable in long-exposure astrophotography because the sensor is exposed to light for extended periods.
Cosmic rays: When a cosmic ray strikes a pixel on the sensor, it can ionize atoms within the silicon, creating electron-hole pairs. These excess charge carriers can become trapped in the pixel's structure, causing it to retain a higher-than-normal charge even in the absence of light. This trapped charge manifests as a persistent bright spot, or a permanent hot pixel.
Age: As the camera sensor ages, the number of hot pixels can increase.
Dark frame calibration should remove most or all of the hot pixels but this may not occur satisfactorily.
For one reason or another an imager might end up with hot pixels in her/his deep sky image
In the current case, the powerful open source software G’MIC Qt will be used to remove the hot pixels from the image.
G'MIC (GREYC's Magic for Image Computing) is a comprehensive open-source framework for image processing. It offers a variety of tools and interfaces for manipulating and processing images. G'MIC provides a number of user interfaces, including a command-line tool, a GIMP plug-in, a web service, and a Qt-based interface. G'MIC-Qt is compatible with several image editing programs, including GIMP, Krita, Photoshop, Affinity Photo, PaintShop Pro, and Paint.NET. It can also be used as a standalone application. G'MIC is developed in Caen, France, by the IMAGE team at the GREYC research laboratory, which is a joint research unit of CNRS, ENSICAEN, and the University of Caen. Since its inception in 2008, the development of G'MIC has resulted in numerous academic research articles focused on the design of new image processing algorithms.
Here the stand alone version is used in a Linux environment, but looks and behaves just like the Gimp plugin.
100% zoomed of part of a deep sky image showing bright red, green and blue hot pixels
Click on an image to see a closer view with more details
To remove hot pixels the Repair group of filters in G’MIC is selected and from within that group, Remove Hot Pixels is selected.
Remove Hot Pixels parameters set to mask size 5 and Threshold 4
The hot pixels have been removed.
Of course, it is advisable to experiment with the values of the filters’ parameters used to evaluate the results in each case.
A second filter called Recursive Median is capable of removing hot pixels and is selected from within the Repair group of filters.
Parameters set to Median Radius 3 and Repeats 7
The hot pixels have been removed.
Although either of these methods would suffice, doing them both and 50% blending the results in the Gimp may be preferable.
Blended of images with hot pixels removed by both filters
Animation showing the hot pixel removal by G'MIC
G'MIC Online web service interface
GREYC's Magic for Image Computing is extremely powerful and in a separate article I shall discuss other aspects of deep sky image enhancement using this software.
