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1, My personal parameters when using SirilIntroduction:I have done the pre processing of astrophotos in many different software through the years. Pre processing is when you calibrate, align and stacking your astrophotos. In earlier software there was a big problem, they couldn't handle 32-bits floating points figures which is needed if doing the processing of the images with no rounding errors. The first software I used was Iris, it could only handle 15 bit images. Interesting is that Siril is based on this old software but now added 32-bit support and a lot of new powerful functions. Other software I have used with 32-bit support were: AstroImageJ, Fitswork, DSS and I also used Matlab with 64-bit precision with my own functions in the middle of 1990s. There is already a good tutorial about how to use Siril, here I more explain things where I divert from the normal workflow. Siril homepage: It's free to download too ! A couple of years ago I did all Post Processing in Gimp which also is a free software to download. Here is a tutorial I have written: Gimp tutorial. Post processing, when you make your astroimage good looking. But Siril has developed a lot the last years. I can do some of the Post processing in Siril with good results, take a look at page 9. |
Workflow:I never use Siril alone, always in combination with other software. It depends a bit of the complexity of the object. Simple objects star cluster etc I do something like this:
Difficult objects like nebulas, deformed stars I do like this:
I frequently update the procedure, these ones are from March 2026.
Equipment: Siril pre processing:First of all, install Siril: Siril Installation. Read this tutorial from Siril with its workflow when calibrate, align and stacking astro images: Siril Manual pre-processing Siril is very easy to understand and use. When I wrote this I had the Siril version 1.2.0 Beta 2 installed and later 1.2.1 and even later 1.4.2. Convert raw files to fits:
First of all the camera's raw files must be converted to a file format that Siril can work with, FITS file format. In my case a Canon/Nikon DSLR camera with cr2/nef raw files that has to be converted. Conversion is done under the "Conversion" tab. Click on the "+" button and a file system window opens. 
Look for your astro image files you want to process. Mark all of them that shall be processed and click "Add" button. 
In the Sequence window, give your processed files a name, this is how I normally name them. But feel free to use something better for your case. When finished, click the "Convert" button and Siril starts to convert your files. Replace MasterDark with a bias, a constant:
In next step, when calibrating the images I do a little bit different. With a DSLR camera which doesn't have a stabilized cooler your sensor will always have different temperatures which make it complicated to calibrate. The image files and the dark files should have the same temperature. You must built a library with lot of dark images to match the temperature, at least no more difference than 3 degrees Celsius. Even worse, you need 30 to 50 of each temperature and ISO setting. And more, every extra calibration you do increase the random noise, dark calibration can only reduce the static pattern. I skip this dark calibration ! I can do this because my cameras, a Canon 6D/Nikon D800 has relative low static pattern and no amp glow. In the case of Canon I subtract a constant, 2048 as bias. This work best if the photos is taken with dithering technique, about 10 to 20 pixels random move between the photos. 
Here is how I do it. Instead of a MasterDark calibration file I use the bias and in this case it's only a constant. My Canon camera has a bias of 2048 ADU, whatever ISO and exposure time I use. The Nikon D800 I use doesn't have a bias and I don't subtract any bias. There is a more deeper tutorial on Siril's homepage here: Siril Synthetic biases. This is a bit old, from Siril ver 1.4.0 I don't Debayer, I use Drizzling x1 in the Registration later. Background extraction:
Background extraction is when removing an uneven light polluted background. It's done in two steps, first a simple tilted plan that is subtracted from all subs. The second one is done on the stacked image later. A tilted plan is setup as Polynomial of first degree. It's function is to subtract the light pollution gradient. When the telescope follow the object the altitude change between the images and so does the light pollution, that's why this subtraction is done on every single sub image. In the image above can be seen how it has placed the test points, the red small squares. Not good to have them along the edge, I have to remove them manually and place new ones a bit inside the edge. Earlier Siril didn't place them like this, some error was introduced in the new Beta version and will be fixed later I hope. If there are some nebulosity in the image, remove all markers that cover it, you don't want to subtract your nebula! Note: I always add dither, I forgot it here. If not it can made some moiré pattern. Sometimes I can try other settings. 
This is an alternative when the flats are not perfect. Flat calibrated images can some times give a pit or a peak in center. Then I use a second degree Polynomial. Added 9 groups of points like the above. Give sometimes much better result, but even better if the flat is correct from the beginning. It depends a lot of the optical design if it gets better or worse. With complex optics it can be ripples in the corrected image which will be visible at high contrast. Normally I correct more complex problem on the stacked image later, I then often use GraXpert. Registration:
I have found that Siril register the images very good, even when they are upside down which happens when a meridian flip has been done. Register is when aligning the images against each other. It shift, rotate, stretch and distort them to fit each other. I use the Global Star Alignment setting when there are no moving objects like comets. With later versions of Siril, 1.4.0 the Drizzle function is more advanced. I always use it with Drizzle x1 on color cameras and then no need of debayer. 
After the images are registered a graph can be plotted to show different data of your choose. In this image I have plotted the stars FWHM value, the stars sharpness. This object was only 18 degrees above the horizon when I started. When time goes on it comes higher above the horizon and the stars sharpness will be better. The temperature change the focus too and bad guiding give even more problems. Bad images can be excluded, right click and exclude. Stacking:
Nothing complicated with the stacking process, I use the "Average staking with rejection" which is recommended. But when building Master calibration files other settings are used. With stacking of many images you reduce the noise, the same effect as taking one photo with longer exposure time. Smooth the background:
Note: This is the stacked image. Normally the background is uneven because of light pollution. I can correct this in Siril or at the later post processing in GraXpert / Gimp. If I do it in Siril I do it like this with the RBF function. I place a lot of test points, the red small squares. Siril place them automatically, but some times it come above a star or nebula and it must be moved. In linear mode it's difficult to see the background, apply histogram stretch or similar, hidden behind the menu here. I have replaced this step with GraXpert witch do it better, I run that software separately even if it can be run from inside of Siril. See page 9. 
Zoom in and check that there are no stars behind the rectangles. Right click to remove them and left click to add a new one. If there are areas with dark or light that shouldn't be there I add more test points. Note this is done on the stacked files.
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