While it seems odd that you want to shrink or remove stars from a photo about stars, this process aims to make the multitude of visible stars in the image less imposing and distracting.īy reducing enlarged stars due to the histogram stretching and by removing the smallest stars, you make the deep sky objects in the image more visible, as shown in the image below.

Star reduction is another process that is standard when editing deep-sky astrophotography. The process allows us to take full advantage of the image stacking process, and it results in a cleaner, brighter image with a lot of details that were not visible (or barely visible) in the single exposures. The effect of the Histogram Stretching process is illustrated in this image, moving from the top left to the bottom left image in a clockwise direction. Thus pushing details that were crammed in the blacks towards the middle tones.Īnd you do that slowly, in small steps, to ensure retaining the best possible image quality. To keep it simple, let’s say that when you perform the stretching of the histogram, you are broadening the histogram. Histogram stretching can be done manually using Adobe Photoshop or in automatic/semi-automatic way using astrophotography software such as Astro Pixel Processor, Star Tools, or His Majesty PixInsight.Ī rigorous explanation on how digital data are recorded and how the histogram works can become fairly technical and is beyond the scope of this article. The same image right after the background extraction, gradient and light pollution removal, and star color calibration performed in Astro Pixel Processor. And this is why this process also goes under the name of background extraction. There is nothing wrong with it, as all the details and information are there, but hidden in the dark background. A typical result from stacking 28 exposures, each 3 minutes long, with Deep Sky Stacker (DSS). With deep sky astrophotography, this stacked image can be surprisingly dark, with only a few bright stars visible. This scheme illustrates the essence of image stacking. With image stacking, you have combined all your light frames (the actual images of the sky) into a single image with an enhanced signal-to-noise ratio. What Histogram Stretching Is And Why You Need It Of the steps mentioned above, it is worth to spend a few words on the Histogram Stretching, as it is of utmost importance in deep-sky astrophotography. In astrophotography, the post-processing includes steps that are crucial to the quality of the final image. 4) Conclusions What Does Post-Processing Mean In Astrophotography?

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