Working around some of the issues with image stacking, especially with Siril and Deep Sky Stacker.
Recently I’ve had a number of people comment that they have experienced issues in regards to stacking astro images. Mostly this is using Siril, but the same is true for those using Deep Sky Stacker. I’ve also heard that folks using Sequator have had similar issues.
After stacking the images in one of the above astro image stacking packages, the file is either exported or saved as a tiff or in some cases a PNG file. Typically these files show almost nothing after stacking, maybe just a few stars. Fear not your image is there just waiting for you to draw it out of its hiding place.
To do this you need an image editor that can work with levels and show a histogram. Photoshop, Gimp and others can be used for this. As I’m on a Linux system my tool of choice is Gimp, very similar to Photoshop and a software package I’m starting to use a lot more for astro image post processing. Some of the PP work can also be done in Raw Thereapee but its a lot less flexible compared to PS or Gimp.
You may have heard the term before, but you need to “Stretch” your astro images in post processing to get the most out of them. Below are three images of a stacked photo of the Scorpio/Sagittarius Star cloud area of the night sky. Parts of Scorpio including M6 & M7 is visible along with some of the Nebulae in Sagittarius. The image is comprised of 48 frames of 3 second duration taken at 18 mm – f3.5 and ISO 3200. Total capture is 2.4 minutes. And the image is a failure.
When you look at the processed images you will see what I mean. Either I had a couple of frames that were star-trailed or theres some sort of stacking error, possibly I had something set wrong. I will need to investigate further. The image does however suit our purpose for this demonstration. By shifting the black levels as well as the mid and high points using the sliders in Gimp ( this is found in the colours menu under the heading levels ) the information in the image was able to be brought out.
In the second image you can see a dark band across the bottom of the image, this is caused by the stacking overlap of the horizon at the bottom of the image. I processed this image as a black & white only. I’ve found that some of the Canon Raw files dont convert well in Siril and I have taken to converting them to Tiff files prior to processing and stacking the images. If however I want a single channel B&W image I process directly from the RAW files. I have no idea why Siril does this but the colour images have a much higher success rate after stacking if converted first. This may just be a quirk from within the Linux OS. Currently I’m running Kubuntu 20.04. As time permits I will investigate further.
In the final edited version I have stretched the image both in the shadow levels end as well as mid and highlights. The end result I was aiming for was a high contrast black & white image with an inky black background, very much along the lines of images published during the 1950’s and 1960’s and featured in many books on astronomy and our universe. Had there not been frames or a fault suggesting star trailed images the end result would have been exactly what I wanted, considering the focus and image detail was very good, especially considering it was Canon’s basic STM kit lens.
To see how to do image stretching and using very basic imaging equipment check out Nico Carvers – YouTube channel Nebula Photos. I’ve been a subscriber of Nico’s for a couple of years now and he has some of the best tutorials around for astro image processing. Be warned these are long videos, some over 2 hours.
The final link below is to a tutorial Nico has done using nothing more than camera, lens and tripod, exactly the same way as I have done the images above. For those who already have a handle on how to set up your camera for astro imaging these tutorials have well defined points to click on in the progress bar or you can scroll down to the info Nico provides when your click on the show more button in the video description. Note there are other parts to this tutorial for users with Photoshop or Gimp or Pixinsight. Its well worth subscribing to his channel if you are interested in astrophotography.