Siril

Siril processes astronomical images. It takes the individual frames a camera produced at the telescope, removes the sensor and optical defects using calibration frames, aligns the exposures on the stars, combines them into a single low-noise stack, and then stretches and colour-balances that stack until the object is visible. It is free, open source, and built specifically for the linear data that astronomy cameras and modified DSLRs produce.

What the program does

The workflow is organised around sequences: a folder of frames becomes a named sequence, and every later operation applies to the whole sequence. Calibration uses master bias, dark and flat frames to remove hot pixels, vignetting and dust shadows. Registration aligns frames, optionally with a full astrometric solution that also writes coordinates into the header. Stacking combines them with a choice of rejection algorithms that discard satellite trails and cosmic ray hits. After stacking, tools for background extraction, photometric colour calibration, deconvolution, denoising and histogram transformation turn a flat grey frame into a presentable image, and scripts chain the whole process into one command.

The working rhythm

Most users follow the same path each time: copy the night's frames into a working folder, build the masters from the calibration frames, convert the light frames to a sequence, register, stack, then process the result. The command line at the bottom of the window shows every operation that the interface performed, so the sequence of steps can be copied into a script and reused on the next target. Because the operations are deterministic, re-processing a night with different settings is cheap once the frames are in place.

Settings and limits

Siril assumes linear, uncalibrated frames and expects them to be reasonably consistent; mixing different exposure times, filters or cameras in one sequence causes trouble. It is not a general photo editor: there are no layers, no cloning brush and no selective masking in the sense a terrestrial editor provides. Large stacks need disk space and memory, and some functions rely on local catalogues that are downloaded the first time they are used. Documentation is thorough but assumes you know why you are taking darks and flats.

Who should choose something else

Anyone editing ordinary photographs will be much better served by a conventional photo editor, and a fully automated capture suite is the right tool if you want the computer to run the whole night's imaging. Users who want an all-in-one commercial package get a more polished experience, but for a free, scriptable processing pipeline that handles calibration, stacking and stretching, Siril is the specialist choice.