Article (Image Processing):
Stacking with Drizzle
For some, the Drizzle function in for example, DeepSkyStacker, is left unused when stacking images. This feature works best with images that have been captured using Dithering. Dithering is an excellent technique to employ during image capture. This involves slewing the telescope in a random direction by a few pixels between exposures. This is done easily using software such as Nebulosity or Sequence Generator Pro (for image capture) and PHD2 (for autoguiding) as they communicate perfectly together. During stacking of dithered images, pixels are overlapped but due to the transformations applied to images in the stack, slight rotations can easily lead to pixelation. This pixelation occurs because a square pixel has effectively been rotated and reproduced in an image of the same size, into square pixels. There is therefore a loss of fidelity in edges as a result. The following is an image taken from DeepSkyStacker's website to explain this loss of fidelity during stacking:
Drizzle is a simple algorithm that creates an image that is larger than the images in the stack and interpolates between pixels to ensure it reproduces fine detail in edges despite the effects of stacking transformations. Drizzle tends to have modes such as 2x Drizzle and 3x Drizzle. The former generates a final stacked image that is twice the size in each dimension. The latter is similar, but three times the size in each dimension. DeepSkyStacker's Drizzle settings are found in the first Stacking Parameters tab (Result).
As a test for its effectiveness, I stacked a set of 10 dithered Luminance images of the IC1396 Elephant Trunk Nebula, both with no Drizzle and with 2x Drizzle to check the results. The following are zoomed in segments of the comparison:
Clearly a large difference in pixelation is visible, with 2x Drizzle making all the edges much smoother across the image. This is particularly noticeable on the stars and on the star diffraction spikes (the brightest edges in the image). Overall, Drizzle is an excellent stacking algorithm to use if your images in the stack are dithered and especially if you have a good data set to stack (10 or more images, for example). The above example uses DeepSkyStacker, but the Drizzle algorithm can also be found in PixInsight.
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