This article is a companion to the article, Streamline Your Astrophotography Experience with Siril and Siril Scripts, and assumes that you have an integrated, pre-processed RGB image available on your Siril desktop. I assume that you’re completely new to Siril.
AutoStretch Your Image
At the moment, the image on your Siril desktop is very dark because it is in the linear state; Siril is showing you what your data looks without applying a stretch to it. We can apply a view-only stretch to your image to make it visible on the desktop, so you’ll have an idea of what you’re working with.
At the center of the bottom of the Siril window is a dropdown menu that says Linear. Click the arrow next to the word Linear, and select AutoStretch – your screen will change to reveal Messier 20 on a somewhat bright background.
Background Extraction
The next step is to make that background of your image more even – you’ll note that there’s significant vignetting on the corners of the image and the background in the rest of the image is somewhat uneven. We can correct this by using the Background Extraction process in Siril.
Use the following directions to extract and even-out the background in your image:
1. From the top of Siril, locate the Image Processing menu and click to open it.
2. Select Background Extraction
3. Move the window out of the way of the image so that you can see all of it
4. Click the Generate button to place an initial set of samples.
What we’re doing here is placing samples on the image; Siril will use the data in these samples to create a model of your image’s background and then subtract that background from your image, resulting in a much better image.
5. By default, Siril has avoided placing samples in the image’s corners because they vary quite bit from the rest of the image. We can correct that in this step.
Using your mouse, click in 2-3 locations in each corner of the image. You’ll notice that a red square appears wherever you click. If you make a mistake, right-click the sample to delete it, and then place another sample. You want to select samples in the bright white areas, as well as the somewhat medium areas. Your image should be almost completely covered in samples; note that the nebula itself is not included because you should avoid selecting areas of nebulosity since samples in those areas can skew the background calculation.
6. Click the Compute Background button.
Siril will work on the image for a short time, and produces an image that’s of much higher quality.
7. Click the Apply button to commit the change and remove the samples from your image.
Now that we’ve successfully extracted and evened-out the background, it’s time to further enhance our image by performing color calibration and adjusting the histogram. In this section, we’ll walk through the steps for these processes, allowing you to bring out the stunning details of Messier 20.
Color Calibration
Color Calibration is an advanced process that balances the color in your image to create a more pleasing and realistic result.
There are two types of color calibration:
- Photometric Color Calibration: this is an advanced process that compares the color of the stars in your image to the known color of stars as cataloged online by Gaia and other sources. The process corrects the color based on the deviation between your image’s stars and the actual color of the stars. This requires that you plate solve the image so that Photometric Color Calibration can perform calculations. Using this approach is preferred for more accurate results.
- Color Calibration: this is also an advanced process but uses only the information in your image to perform the color calibration. You select a background reference, and white reference and the process performs calculations based on your selections. This is the alternate process to use when you have problems with PCC.
Use the following directions to color calibrate your image:
1. From the Image Processing menu, select Color Calibration
2. From the menu that pops-up, select Photometric Color Calibration
3. Make the following selections in the Photometric Color Calibration window:
a. Under Image Parameters, type messier 20
b. For Server, select VizieR
c. Click the Find button
d. Select NED MESSIER 20 from the list that appears
e. In the Focal length field, enter 2262.
f. In the Pixel size field, enter 24.
4. Click the Ok button to color calibrate the image.
5. Click the Close button.
Your image is now accurately color calibrated. The numbers we entered are derived from the equipment used to acquire the image; you can often find this information on the telescope’s Guide page.
As I mentioned earlier, the stretch we have a applied to the image is a temporary one so that we can work with the image in its linear state. Now that we’ve finished working in the linear state, we can permanently stretch the image.
Permanently Stretching the Image
Use the following directions to permanently stretch the image:
1. Where it says AutoStretch at the bottom, center of the Siril window, click next to it and select Linear.
2. From the Image Processing menu, select Histogram Transformation
3. Move the Histogram Transformation window out of the way of your image
4. Note the following important elements on the screen:
- The Zoom level shows the histogram’s magnification
- The Black point slider shows the location of the image’s black point
- The Mid-tones slider shows the location of the image’s mid-point
We’ll adjust all three elements in the following steps, but first we’ll let Siril get us started:
5. Click the button with the gear and graph going through it, located on the right side of the Histogram Transformation window
This applies an automatic stretch to your image and it is a good place to start stretching the image.
6. Change the Zoom to 50 (you can type in the box where the number 1 appears)
7. Move the black point slider a little to the right to make the background darker; make small adjustments and release the mouse button to allow Siril to update the screen.
8. Move the mid-point slider slightly to the left to brighten the rest of the image; your goal is to make the image just a little brighter.
This is what part of my screen looks like after performing the preceding two steps:
9. Click the Apply button and close the Histogram Transformation window
Next, we’ll reduce the noise in the image.
Reducing Noise
Use the following directions to reduce the noise in your image:
1. From the Image Processing menu, select Noise Reduction
2. The defaults are appropriate for this image, so click the Apply button to have Siril reduce the noise in the image.
3. Click the Close button.
Next, we’ll boost the saturation to make the colors more vibrant.
Boosting Saturation
Use the following directions to boost saturation:
1. From the Image Processing menu, select Color Saturation.
2. Move the Amount slider to approximately 0.30 (I found this value by experimenting with various values).
3. Click the Apply button to commit the change.
Next, we’ll rotate the image so that north is up.
Rotating Your Image
Use the following directions to rotate your image:
1. From the Image Processing menu, select Geometry
2. From the menu that pops-up, select Rotate 90 degrees, clockwise
Now we’ve completed the image, we can save it in various formats for storage and sharing.
Saving Your Final Image in Various Formats
Use the following directions to save your image:
1. At the top of the Siril window, you’ll see a button marked Save – click the button immediately to the right of the Save button.
2. Name the file ‘stretched_lrgb_messier_20.fit’.
3. Click Save
4. Use the defaults and click Save
The file is now saved in FITS format – you can reopen this file using Siril in the future so that you have a full-resolution image available for further editing if you want to do that.
Now, we’ll save the image in JPG format for sharing online:
1. At the top of the Siril window, you’ll see a button marked Save – click the button immediately to the right of the Save button.
2. Where it says Supported Image Files at the bottom-right of the window, click the menu and select JPEG.
3. Click Save
4. In the window that pops-up, select a value for Quality. Higher quality results in a better image but uses more storage (larger file size); a value of 90 is a good starting point.
Closing Remarks
Congratulations, you have processed your first image using Siril’s advanced features like Background Extraction and Photometric Color Calibration to produce a high-quality image! You now have some new skills to use to apply to your images using Siril.
Conclusion
In conclusion, using Siril for processing deep-sky images can yield impressive results while providing an easy-to-follow workflow. By following the steps outlined in this tutorial, you have successfully transformed a raw Messier 20 image into a stunning representation of this celestial object. As you continue to explore and process other deep-sky targets, remember that practice makes perfect. Don’t be afraid to experiment with different settings and techniques in Siril to further enhance your astrophotography skills. The more you work with the software, the more proficient you will become at producing awe-inspiring images that truly capture the beauty of our universe. Happy stargazing!
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