Several deep sky image acquisition software offers the option of doing it by dithering. It is a process by which the guide star is moved between successive exposures, thereby moving certain noises, for example hot and cold pixels, to different places on each frame that is part of the integration.

As a general rule, the displacement is configured on one or two pixels. By proceeding in this way, it will be possible to make these imperfections disappear more easily when compositing the images using the method Sigma clip. Note that the tracking accuracy is not affected by this technique. Indeed, the displacement of the guide star on one or two pixel (s) is carried out before the second image is taken.

Here is, as an example, a GIF animation of 2 seconds per frame demonstrating the effectiveness of Dithering:

  • The image represents a portion of the galaxy NGC 7331
  • Composition of 8 images in Bin 1 × 1, each lasting 5 minutes
  • The two images are integrated using the Sigma-clip method
  • An image does not use dithering
  • The other image uses Dithering with a displacement of 2 pixels

The alt attribute of this image is empty, its file name is dithering.gif.

On the image without Dithering, we see a few hot pixels (white points) and several cold pixels (black points). These noises have all disappeared from the dithering image. Due to the high tracking precision during the acquisition of this image, the Sigma-clip integration method is not sufficient to eliminate these defects. Using Dithering and Sigma-clip, l'hot pixel elimination et cold is really effective.

Richard Beauregard
The Sky Astro-CCD

Revised 2021/01/04