Introduction

Valerian is free, open-source, GPU-accelerated deep-sky image processing. It takes the subframes you capture of galaxies, nebulae and clusters and turns them into a finished image: calibration, star registration, stacking with outlier rejection, plate solving, gradient removal, photometric colour calibration, stretching and star removal, on Linux, Windows and macOS, on x86_64 and ARM.

Valerian is written in Rust and is the deep-sky sibling of Helia, the solar stacker. The two share their core: file formats, GPU acceleration, the interface and the release pipeline.

Dedication

Valerian is dedicated to my son Xaviar. His middle name is Valerian, because he was a very sleepy child, which suits software made for the long, quiet hours of the night sky.

What works today

This is the foundation milestone. The book grows with the program, and each page says plainly what does not work yet. Today, on the command line and as a library:

• valerian calibrate: master bias, dark, flat and dark-flat frames with sigma, winsorised or linear-fit rejection, dark scaling, hot, cold and dead pixel repair, optional demosaicing (see How calibration works);
• valerian register: star detection, triangle matching with a RANSAC fit (similarity, affine or homography) and resampling onto a reference grid (see How registration works);
• valerian stack: reference choice, sky and transparency matching, weights by noise and star width, and per-pixel outlier rejection (see How stacking works);
• valerian background: a polynomial or thin-plate spline fitted to tile samples of the sky, with star and nebula masks, subtracted or divided out; declined when there is no clear sky to measure (see How background extraction works);
• valerian stretch: levels and a midtone, arcsinh or generalised hyperbolic curve, recorded in the header so it can be undone (see How stretching works);
• valerian starless: classical star removal, with the stars and the mask kept (see How star removal works);
• valerian solve: a TAN or TAN-SIP world coordinate system against a bundled Gaia catalogue (see How plate solving works);
• valerian photocal: white balance fitted against Gaia star colours (see How colour calibration works);
• valerian drizzle: drizzle for dithered, undersampled frames (see How drizzle works);
• valerian bench and --backend gpu: GPU kernels for calibration, the registration warp and the stack combiner (see The GPU kernels);
• valerian info: header, noise and star statistics of FITS files, the build record and the GPU adapter;
• a desktop app with the five steps of a session (calibrate, register, stack, process and export) on a rail, an image view with zoom, pan and a histogram, notices while work runs, a red night-vision palette, the tools above in its panels, and the About page (see The desktop app).

Start with Getting started, then a first session, which takes a monochrome and a one-shot-colour session from raw frames to a finished picture with the real output and the real pictures at every step: calibrate, register, stack, background, stretch, star removal, plate solving, colour calibration and drizzle. The quick start is the short version.

Copyright and licence

Copyright (C) 2026 Laurence Horrocks-Barlow. Valerian is free software under the GNU General Public License, version 3 or (at your option) any later version, with no warranty, and with the additional terms in ADDITIONAL-TERMS.md that keep the author's name and this dedication with the program. Valerian is made by Laurence Horrocks-Barlow. If you are interested in anything else I make, visit 12hobbies.studio.

LOGO-LICENCE.md is part of 12hobbies.studio.