Stacking and stretching
Note:
Much of the important information in the sections on image processing was significantly influenced by Frank Sackenheim's Astrophotocast (https://www.youtube.com/hashtag/astrophotocast). Every beginner in the field of astrophotography is recommended to watch some of these very informative and interesting videos.
In this way, many thanks to Frank Sackenheim for his hard work in producing the videos.
After a successful observation night, more or less many individual images (sub-frames or light-frames) have been created. In a further step, these have to be stacked before they can be used for further image processing.
Before stacking, the images should be examined with any astro software (e.g. ASI FITs Viewer) and failed images (elongated stars, airplane or satellite tracks...) should be sorted out.
A popular and free stacking program for this is DeepSkyStacker. A step by step tutorial can be found at this link.
Another stacking tool includes the PixInsight program. This software is very comprehensive, but not free. It has many tools integrated that are still necessary for further image processing. A step by step guide to stacking with PixInsight can be found at this link.
However, various other astro programs (e.g. Fitswork, SiriL, SharpCap...) can also be used for stacking.
Why the stacked image is black
After the stacking is finished, a beginner will look full of excitement at the final result. But then comes disillusionment. Where is the object? Why, after several hours of exposure, is the image black, with a few white pixels? In the capturing software or on the display of the DSLR, an object could be seen. The pictures of other amateur astrophotographers look so great, even though they sometimes have less exposure time.
58 images with 180 s exposure each of M76 (Small Dumbbell Nebula) after stacking
The reason for the dark image is that a dark sky was captured despite the long exposure time. As described in the menu point 'Basics' - 'Physical quantities' - 'Exposure time and noise', the electrons fill the memory of the pixels, which are subsequently output as signal information (ADU-value).
The capturing software for astrophotos or even the DSLR use an auto-stretching to display the images at all.
If the histogram is viewed with these values (tone value curve), it becomes clear that almost all pixels have a very low signal value, because very many pixels are very dark. They are all on the left edge of the histogram.
Histogram of M76 (small Dumbbell Nebula) - output with PixInsight
In addition, monitors always output color values in 8 bit format only. This means that the many gray shades of the 14 or 16 bit camera are now also compressed to the 8 bit.
The way to get a brighter image is to spread the dark image information over the entire histogram area. This process is called stretching and is described below using the free program GIMP. A step by step tutorial can be found at this link. But the process can also be done with many other image editing programs (e.g. PixInsight, Adobe Photoshop...).