The software that controls these different pieces of hardware enables them to communicate with each other to get the job done properly. We use a computer to control the telescope mount, imaging camera, and autoguider. Thankfully, in today’s modern digital world we can automate many processes in astrophotography. I used to dither by turning off the autoguider, manually moving the scope a little bit with the hand controller, turning on autoguiding again, and finally starting another exposure.
How to Ditherīack in the days of film, I used an SBIG ST-4 autoguider, and I manually triggered a cable release for every exposure. This allows them to be removed with Sigma stacking. When the image is aligned on the stars, hot pixels are in a different place from frame to frame.
Hot pixels appearing the same place on your camera’s detector, but stars shifted from frame to frame with dithering. For the Sigma algorithm to work, you need a minimum of 10 frames. This means that if you have 10 frames and a hot pixel is only showing up in one particular location on a given frame, the algorithm replaces it with an average from the other frames. The Sigma rejection stacking method, available in most astronomical image-processing programs, applies a mathematical algorithm to examine every pixel and discard outliers that are very different from the average in your group of images. This shift places the hot pixels in a different place in every frame. Later, as you process your images, you’ll align and stack individual frames based on the stars in each image. By moving the pointing of the telescope, dithering shifts the stars to a slightly different place in each frame. Hot pixels, which are individual pixels that look brighter than they ought to, lie in the exact same place in every exposure. This allows hot and cold pixels, cosmic ray artifacts, and fixed pattern noise, and even satellite or airplane trails to be removed during the stacking process. In astrophotography, to dither means to shift the pointing of the telescope slightly in random directions between exposures.