Tutorials

These pages are numbered walk-throughs. Each one has a goal, the data it uses, the exact commands, what the terminal prints (pasted from a real run), a before and after picture, how to tell it worked and what to do when it did not. They follow one thread, so each page starts where the previous one ends.

If you want to know why a step works the way it does, read the matching "How X works" page in the user guide; the tutorials link to it. If you want one page that runs the whole chain once, read A first session. Every option is listed with its range and default in the command line reference.

One tutorial is for the desktop app only: Cancelling a running step stops Calibrate, Register, Stack or a Process tool part way and shows that nothing is left behind.

The two captures

IC 5070, the Pelican Nebula (tutorials 3 to 11). Sixty 10-second sub-exposures from a Seestar S50 (250 mm focal length, 1080 by 1920 pixels, raw Bayer data in GRBG order, light pollution filter, 2026-07-28). They are raw mosaics: Valerian demosaics them itself. A Seestar writes no darks or flats, so there is nothing to calibrate against and the thread goes straight from the lights to registration.

The California Nebula in H-alpha (tutorial 2 only). Twenty-five 300-second lights from a ZWO ASI1600MM Pro (monochrome, 4656 by 3520 pixels, Starwatcher Esprit at 840 mm, -10 C sensor), ten 300-second darks, thirty 2.04-second flats and thirty 2.04-second dark flats. This is the capture with a full set of calibration frames. The twenty-five lights are five panels of five (the header RA and DEC differ between the panels), which tutorial 2 uses to show what a mixed set looks like.

Both are real captures. You do not need them: every command takes your own folders.

How the numbers and pictures were made

Every block of terminal output is pasted from a run of the valerian binary built from this repository on a 32-core AMD EPYC 7543 Linux server with an NVIDIA A100 (the machine is shared, so times vary by a few seconds from run to run, and are not a promise for your computer). Where a page quotes a time, it says which run it came from.

Each picture is a crop or a reduction of a real output file, written by a small script that reads the FITS file, applies the stated stretch and saves the result. Linear data has to be stretched to be seen, so every linear image on these pages has a screen stretch applied and the page says so: black point at the sky level minus 2.8 robust sigma, white point at the 99.9th percentile, and a midtone transfer function that puts the sky at 20 per cent grey. Pictures of data that Valerian has already stretched are shown as they are. Photographs are saved as JPEG to keep the book small, and black-and-white pictures as PNG.

Folders used on these pages

lights/        the raw light frames (FITS)
darks/         dark frames          (tutorial 2 only)
flats/         flat frames          (tutorial 2 only)
dark-flats/    dark flats           (tutorial 2 only)

Each tutorial names the files it writes in the folder you run it from, so that the next one can read them.

The desktop app

The desktop app does the same steps in a window, apart from background extraction. The tutorials give the command line, which is where every option is spelled out, and end each page with a short note that names the matching step or tool of the app and links to its reference. The app has its own walk-through: a session in the desktop app.