Adventures in Astrophotography Post-Processing

 These days I do most of my blog-type posting on aus.social, a Mastodon instance. Sometimes though the limited formatting makes that a poor choice. This is one such case.

Today I want to take you through the post-processing steps I go through to create a final astrophotographic artwork. And these are artworks, not scientific records. I could use the same data to perform astrometric observations, but I don't. I like to create beautiful images. Knowing the things I am creating pictures of is part of creating that beauty, and that does require some scientific knowledge, but it is only a means to an end. 

So, let's start with what comes out of my telescope. I use either a DwarfII or Dwarf3 smart telescope for my imaging, and what comes out of that is already pre-processed in a variety of ways. The telescope does its own stacking, and has a built-in link to a cloud post processor. So what I get is two images - one denoised and with basic image optimisations performed, and another with stars removed. Here they are:

Denoised and Optimised

Stars removed

This is IC4592 - a reflection nebula known as the Blue Horsehead. Strictly speaking, this is only part of the nebula, as it is 1.5 times the size of my field of view, but this covers the key section.

Straight out of the telescope, there is not a huge amount going on. In the starless version, you can see some of the blue, and a bit of the general shape of things, but it is all rather washed out. So let's pull it into my photo editor of choice, Snapseed, and do three things - deepen the shadows, boost the colours, and perform a general light/dark stretch (called 'ambience' in Snapseed). I generally have to go through the saturation boost a couple of times to really bring out the colours. So this is step 2:

Colours boosted, shadows darkened, and general boosting of light areas.

Now we are getting somewhere, but the starless image by itself lacks reference points. So lets go across to the other image, and pull that into the editor. This time we are looking to slightly reduce the number of stars, so we make the shadows as dark as possible and drop the overall brightness. We lose almost all of the nebula in this process, but that is fine, as you will see.

Darkend base image

If I were doing this in Siril or other dedicated astrophotography tool, this would be called a Star Mask. I would then use a recombination tool to join this to the star-removed version. In Snapseed we don't have that as such, so we just use the double-exposure tool to layer one on top of the other. In general I use the Additive process, and tune the levels of the two halves of the image to get the right mix of nebula and stars. 

Recomposed image

This is 99% of the way there now. The final step is to take this version and do some final tweaking of levels, saturation, and perhaps a bit of careful dodging and burning to bring out specific details.

The Final Result

And there you have it. This started with nine hours of image capture over three nights (for which I was asleep for the most part), 2 hours of re-stacking in the telescope itself (while I was at work), 10 minutes running the restacked image through the internal Stellar Studio tools, and about 15 minutes doing the post-processing. 

This compares with doing it all through Siril, where, even with scripts, I would be spending five or six hours tweaking processes before finally feeding the result into Snapseed for the final step.

Would the Siril version be better? Maybe. Certainly if I were using a professional camera, filter stack, and a decent telescope, it would be better. But then it would take me many hours of fiddling settings in Siril or whatever. 

This is about as close as you can get to instant gratification in Astrophotography. And I think this is a good thing, because it makes it so much more accessible.