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arcsec

arcsec

Finds where your telescope is pointing. arcsec reads the stars in your image and works out exactly which part of the sky it shows, so your imaging software can centre your target.

Download v0.4.0Use with N.I.N.A.GitHub

Free and open source, for Windows, macOS and Linux. All downloads

A greyscale telescope image of faint stars and galaxies in Coma Berenices, overlaid with the right ascension and declination grid that arcsec solved for it. The field is 13.5′ × 9.8′, centred on RA 12h59m32s, Dec +27°27′58″.
Solved by arcsec: centre RA 12h59m32s, Dec +27°27′58″. The grid shows the sky coordinates it worked out.Image: Sloan Digital Sky Survey
Download and installWindows, macOS and Linux. Step by step.
Use with N.I.N.A.Set arcsec up as N.I.N.A.'s plate solver.
Use with SirilSolve your images with arcsec, then process them in Siril.
Which catalogue do I need?Enter your telescope and camera; get the catalogue and the command to fetch it.
Catalogue guideEvery catalogue, where they are stored, and sharing them with ASTAP.
FAQCommon questions, from blind solving to file formats.
Command lineEvery option, output file and exit code.

What is plate solving?

Section titled “What is plate solving?”

Plate solving is how imaging software finds out exactly where a photo was taken on the sky: it matches the pattern of stars in the image against a star catalogue. Software such as N.I.N.A. uses the answer to centre a target after a slew, to return to the same framing on another night, and to correct the mount’s idea of where it is pointing.

arcsec is the program that does the matching. It runs on your own computer, needs no internet connection to solve, and is free.

Get started in three steps

Section titled “Get started in three steps”

  1. Download arcsec

    Unzip it and keep arcsec.exe in a folder of your choice. There is no installer and nothing else to set up.

    Downloads

  2. Fetch a star catalogue

    arcsec recognises your stars by comparing them with a catalogue. One command fetches it, for example arcsec catalog install d50.

    Which catalogue do I need?

  3. Point N.I.N.A. at it

    In N.I.N.A.'s plate solving options, choose ASTAP and enter the path to arcsec.exe. Solve as usual.

    Set-up guide

Why arcsec

Section titled “Why arcsec”

Works with N.I.N.A.

arcsec is a drop-in replacement for ASTAP’s plate solver: it takes the same options and writes the same result files. In N.I.N.A., choose ASTAP as the solver and point it at arcsec. Nothing else changes.

Uses the catalogues you have

arcsec reads ASTAP’s star catalogues as they are, so an existing ASTAP folder works straight away. Starting fresh? One command picks the right catalogue for your field and fetches it.

Works offline

Solving happens entirely on your computer. arcsec needs the internet only to download a catalogue, and sends nothing anywhere.

Free and open source

MIT licensed. No account, no subscription, no telemetry. The source is on GitHub.

Windows, macOS and Linux

One small program for each, with nothing else to install.

Telescopes to all-sky lenses

Fields from about 0.15° to 80° across. Reads FITS from any capture program, PixInsight’s XISF and ASDF, compressed or not.

Download

Section titled “Download”

Latest release: arcsec 0.4.0, published 2026-10-02.

PlatformArchiveSize
Windows (x86-64)arcsec-v0.4.0-x86_64-pc-windows-msvc.zipDownload .zip3.3 MB
macOS (Apple silicon)arcsec-v0.4.0-aarch64-apple-darwin.tar.gzDownload .tar.gz3.3 MB
Linux (x86-64)arcsec-v0.4.0-x86_64-unknown-linux-gnu.tar.gzDownload .tar.gz3.8 MB
Linux (arm64)arcsec-v0.4.0-aarch64-unknown-linux-gnu.tar.gzDownload .tar.gz3.6 MB

Unzip the archive and keep arcsec (arcsec.exe on Windows) in a folder of your choice. Download and install walks through it for each system. For other platforms, checksums and building from source, see For developers.