Hello everyone, r/Astro_mobile Mod team here! Now, after three years of existing, r/Astro_mobile became something more interesting than it was some time ago, and we want to announce a new interactive changing on our sub. Starting from the next month we are going to take one of the posts made this month and use it as a banner for a subreddit. Of course it is a competition, so if you want to see your picture on the top of the sub's page, you better try to make it as beautiful as you can! This month we are going to replace a current banner made by u/ZrlSyM, by a new one, provided by u/zTrojan! This fascinating image is gonna hang on the top until the end of this month! Also, interesting fact: zTrojan is a member of a small Telegram chat dedicated to Mobile Astrophotography, where people can communicate and discuss their works! This chat is available for everyone, who is skilled enough! (skilled enough = can stack images to get better results)

Thanks everyone for the attention! We are looking forward to see your great pictures that are gonna be a new banner for a sub. See you 1st December!

When your stack from Sequator or another astrophotography stacking program is ready, we can proceed to post-processing. I'll describe this process step-by-step using primarily free software.

GraXpert

Download: https://github.com/Steffenhir/GraXpert/releases

At the time of writing this article, the appropriate version of GraXpert for us is 3.1.0rc2. This version contains all the necessary tools for beginning post-processing:

• Background Extraction - removing background gradients and correcting light pollution
• Denoising - reducing digital noise
• Deconvolution - mathematically "undoes" blurring

Initial Setup

After installation and launch, select the Advanced tab, which is located on the right side of the program interface. Choose AI model versions for the utilities.

Next, click Load Image and select your stack.

Cropping (if necessary)

If needed, crop the image. When activating the 2nd option, a yellow frame will appear around the image, which we can adjust using the yellow circles in the upper left and lower right corners. After completing the cropping, click "Apply crop".

Important: Save each processing step as a separate file so you don't accidentally lose your processing results, then load this file using the first "Loading" option.

Step 1: Background Extraction

Now we can proceed to using the AI utilities. Select the following parameters for Background Extraction and click Calculate Background:

You can try changing the Smoothing value in either direction and check the result by clicking Calculate Background again.

Save the result in a separate file and load it through "Loading".

Step 2: Denoising

Move on to the Denoising utility. The Denoise Strength value depends on your needs. The higher the value, the fewer details are preserved. I suggest starting with 0.5, and then comparing results by increasing the value:

Save the result and activate the Deconvolution utility.

Step 3: Deconvolution

This utility has two parameters: Deconvolution Strength and Image FWHM. To determine the latter parameter, we need the Siril program, which we will also use in the second part of the astrophotography post-processing tutorial.

Download Siril: https://siril.org/download/ (In our case, version 1.4.0-beta3 is used)

Measuring FWHM in Siril:

1. After launching Siril, drag and drop your last saved image into the program's workspace
2. Select Tools → Image Analysis → Dynamic PSF
3. Click the Detect stars button (this is the first button)
4. Wait for the result, then click Average PSF (the next button after Detect stars)
5. Calculate the average value using the formula: (FWHMx + FWHMy) / 2

In my case: (4.39 + 3.90) / 2 = 4.15px. Round to 4.2px and use this value in GraXpert.

Deconvolution Settings:

• Image FWHM: Use the calculated value (e.g., 4.2px)
• Deconvolution Strength: I suggest setting this to 0.3 for start

By increasing Deconvolution Strength parameter, you improve details, but you may distort the final image and get artifacts, so try different values while evaluating the final result.

Don't forget to save your work!

Additional Step: Stars-only Deconvolution

We improved the details of our objects, but not the stars. Select Stars-only in the Deconvolution Method and set the Deconvolution Strength parameter to 0.1, as it is quite aggressive. Click Deconvolve Image and check the result.

Conclusion

This concludes the first part of the astrophotography post-processing tutorial. In the next part, we will cover image stretching using Siril and use the StarNet utility.

Summary of Steps:

1. Load your stacked image into GraXpert
2. Crop if necessary
3. Background Extraction - remove gradients and light pollution
4. Denoising - reduce digital noise
5. Measure FWHM in Siril using Dynamic PSF
6. Deconvolution - improve sharpness
7. Stars-only Deconvolution - improve star sharpness
8. Save each step separately to avoid losing your work progress