Download and install

arcsec is a single program: there is no installer and nothing else to set up. Getting to a first solve takes three steps: put the program somewhere, fetch a star catalogue, solve.

Using N.I.N.A.?

Do steps 1 and 2 here, then follow Use with N.I.N.A. to point N.I.N.A. at arcsec. You do not need step 3: N.I.N.A. runs arcsec for you.

1. Install arcsec

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

Platform	Archive	Size
Windows (x86-64)	arcsec-v0.4.0-x86_64-pc-windows-msvc.zipDownload .zip	3.3 MB
macOS (Apple silicon)	arcsec-v0.4.0-aarch64-apple-darwin.tar.gzDownload .tar.gz	3.3 MB
Linux (x86-64)	arcsec-v0.4.0-x86_64-unknown-linux-gnu.tar.gzDownload .tar.gz	3.8 MB
Linux (arm64)	arcsec-v0.4.0-aarch64-unknown-linux-gnu.tar.gzDownload .tar.gz	3.6 MB

Each archive holds the arcsec program (arcsec.exe on Windows) with its README, licence and changelog.

Windows

1. Download the Windows .zip from the table above and extract it.

2. Move arcsec.exe to a permanent folder, for example C:\Program Files\arcsec\.

3. Optionally, add that folder to your PATH (Settings, search for “environment variables”, edit Path) so you can type arcsec in any terminal. N.I.N.A. does not need this: it is given the full path.

4. Open a new PowerShell or Command Prompt window and check it runs:

   arcsec --version

The Windows build is for x86-64. Windows on Arm runs it under emulation.

macOS

1. Download the macOS .tar.gz from the table above. It is built for Apple silicon (M1 and later). On an Intel Mac, build it with cargo instead.

2. Unpack it and move the program onto your PATH:

   tar -xzf arcsec-v*-aarch64-apple-darwin.tar.gz
   sudo mv arcsec-v*-aarch64-apple-darwin/arcsec /usr/local/bin/

3. Check it runs:

   arcsec --version

   If macOS refuses to open it because the developer cannot be verified, clear the download quarantine flag and try again: xattr -d com.apple.quarantine /usr/local/bin/arcsec.

Linux

1. Download the .tar.gz for your processor from the table above: x86-64 for most PCs, arm64 (aarch64) for 64-bit Arm boards. Both need glibc 2.35 or newer, as on Ubuntu 22.04 and later.

2. Unpack it and move the program onto your PATH:

   tar -xzf arcsec-v*-x86_64-unknown-linux-gnu.tar.gz
   mkdir -p ~/.local/bin
   mv arcsec-v*-x86_64-unknown-linux-gnu/arcsec ~/.local/bin/

3. Check it runs:

   arcsec --version

Other platforms

There are ready-made downloads for the four platforms above. On anything else, including Intel Macs, arcsec can be built from source: see For developers, which also explains how to check a download’s checksum.

2. Install a star catalogue

arcsec recognises your stars by matching them against a star catalogue, and cannot solve without one. Which catalogue depends on how much sky your camera sees; the Which catalogue do I need? page works it out from your focal length and camera, or ask arcsec directly, giving the field size in degrees:

arcsec catalog recommend --fov 1.5

For a 1.50° field:

  solving      d05         102.2 MB  Gaia DR3 to 500 stars/deg². Smallest useful solving database.
  blind        anet-4100   355.5 MB  optional: solve with no position hint

  arcsec catalog install d05

Then install what it suggests. It shows the download size and asks before starting:

arcsec catalog install d05

Catalogues go into a per-user folder that the solver searches by default, so from now on you need neither -d nor -D. arcsec catalog path prints where that is.

Already have ASTAP?

arcsec reads ASTAP’s star databases as they are. Point it at ASTAP’s folder instead of downloading again: see sharing with ASTAP.

3. Solve an image

arcsec -f image.fits

arcsec needs a rough idea of where the image is and how big the field is. It reads both from the image header when your capture software wrote them: the pointing from RA/DEC (in degrees), and the pixel scale from FOCALLEN, XPIXSZ and XBINNING. Otherwise give them on the command line:

arcsec -f image.fits --ra 5.58 --spd 82.0 --fov 1.5 -r 10

--ra is in hours, --spd is south pole distance (90 + Dec) in degrees, --fov is the image height in degrees and -r the radius to search around that position. These are ASTAP’s conventions.

A successful solve prints ASTAP’s report, ending with the position of the image centre:

217 of 2203 quads selected matching within 0.007 tolerance.
Solution["] x:=1.241346*x+ -0.000061*y+ -1799.243147,  y:=-0.000200*x+ 1.241438*y+ -1799.174846
Solution found: 03: 25  36.0 +49d 51  00
Solved in 0.4 sec. Δ was 0.0".  Mount Δα=-0.2",  Δδ=0.3".  Used stars down to magnitude: 14.5

and writes two files next to the image: image.wcs, the solution as a FITS header, and image.ini, ASTAP’s summary. Output files has the details, and --update writes the solution into the FITS file itself.

Next

• Use arcsec from N.I.N.A.
• Use arcsec with Siril
• Every command-line option
• Which catalogue for which field