Siril

Siril is a photo processing program oriented toward astrophotography. With it, you can transform the raw captures obtained with your telescope or camera into detailed, noise-free final images, applying corrections to your nightly sessions that eliminate imperfections and extract the maximum light signal from nebulae, galaxies, or star clusters.

Automation with custom scripts

To streamline your work, you can use predefined scripts that execute all routine tasks without you having to be on top of everything. Just load the files into the corresponding folders and select a calibration script for the program to generate the stacked result while you spend your time on something else. If you have sufficient technical knowledge, you can edit those text files yourself to adapt the commands to the specific details of your equipment, creating routines for monochrome cameras or configurations with narrowband filters.

Image calibration and stacking

After spending some time watching the night sky, the first step is to clean the RAW files using calibration frames: four groups of photos (Bias, Darks, Lights, and Flats) must be taken to then stack them. In general, the process is similar to creating an HDR, but this involves combining shots with the lens cap on the camera (black frames) and others completely white with a flashlight. This allows the program to filter out the dust and noise captured by the sensor (temperature-dependent) under the same conditions in which the rest of the photos were taken. Once your files have been organized into those four folders, the preprocessing script aligns each frame through an advanced registration algorithm that uses the star pattern in the field of view as a reference. By combining tens or hundreds of individual frames using various pixel rejection methods, the signal-to-noise ratio is drastically increased, allowing faint background structures to stand out against the digital grain typical of long exposures.

Histogram stretching and deconvolution

The resulting image after stacking usually appears completely dark or linear until a histogram stretch is applied using AutoStretch, where asinh transformation curves or adaptive equalization methods reveal the subtle brightness of the galaxies without burning out the center of the brightest stars. On top of this, there are deconvolution and green noise reduction processes that recover sharpness in structures that have been slightly blurred as a result of tracking micro-errors of the equatorial mount, as well as precise adjustments to calibrate colors using data from real online stellar catalogs, achieving natural and scientifically rigorous tones in the final palette.

Lucky imaging and videos in SER format

For bright objects like the planets of the solar system or the Moon, you can use high-speed videos in SER format, which the program supports natively, processing thousands of frames in a matter of minutes thanks to the use of multiple processor threads. Each individual frame is analyzed to discard those that are distorted, and by aligning only the sharpest parts of the video, you see very precise details in lunar craters or Jupiter's cloud bands, exceeding what traditional photographic captures offer.

My experience with Siril

I set a working directory to a folder containing dozens of RAW files of two nebulae and, after importing the four types of frames (bias, dark, lights, and flats), I ran a basic preprocessing script from the menu at the top. After a few minutes, I had the final file ready with all the image information. I added the resulting FITS and ran the AutoStretch script, which revealed the nebulosity I had captured with my star tracker, and I switched the display mode to auto-adjust to see it better before opening the photometric color calibration tool. I selected the object in the internal viewfinder, applied the calculation, and the white balance was instantly adjusted, returning very defined reddish and bluish tones. I finished by exporting the result in TIFF format with maximum bit depth to retouch it later in Camera Raw.

What I liked most about Siril and what could be improved

· What I liked most: I appreciated the processing speed when making use of all the processor threads during stacking and the convenience of using scripts to automate all the repetitive calibration steps.

· What could be improved: The initial learning curve to master the working folders required by the scripts can be a little confusing and requires a review of the documentation before the first use.

Download Siril and put the program's calibration, stacking, and detail extraction scripts to work for your nighttime captures, from the first raw frame to the final deep-sky image.