FAQ
Why use arcsec?
Section titled “Why use arcsec?”It is an alternative for the plate-solving step: a small, self-contained program that fetches its own catalogues, runs on Windows, macOS and Linux, and is open source under the MIT licence. It takes the same options and writes the same files as ASTAP’s command-line solver, so trying it in N.I.N.A. means changing one path, and changing it back is just as easy. If ASTAP already works well for you, there is no need to switch.
Does it replace ASTAP?
Section titled “Does it replace ASTAP?”Only ASTAP’s command-line solver, astap_cli. ASTAP is also an image viewer, stacker
and photometry tool, and arcsec does none of that. If you use those parts of ASTAP,
keep it; the two can share one set of star databases.
arcsec is an independent program. It is built on the star-pattern method Han Kleijn published for ASTAP and reads ASTAP’s database files, but contains no ASTAP code and is not affiliated with the ASTAP project.
How fast is it?
Section titled “How fast is it?”Typically well under a second per image; the median across the test images is 0.15 seconds, about the same as ASTAP.
Does it need an internet connection?
Section titled “Does it need an internet connection?”Only to download catalogues with arcsec catalog install. Solving is entirely
offline, and arcsec sends nothing anywhere.
Which catalogue do I need?
Section titled “Which catalogue do I need?”It depends on your field of view. The Which catalogue do I need? page works it
out from your focal length and camera. For most telescopes it is one of the D-series;
camera lenses need g05, and all-sky lenses w08.
Where are the catalogues stored?
Section titled “Where are the catalogues stored?”In a per-user folder that the solver searches automatically:
| Platform | Default folder |
|---|---|
| Linux | ~/.local/share/arcsec/catalogs (or under $XDG_DATA_HOME) |
| macOS | ~/Library/Application Support/arcsec/catalogs |
| Windows | %LOCALAPPDATA%\arcsec\catalogs |
arcsec catalog path prints it, and the ARCSEC_CATALOG_DIR environment variable
moves it. See where they are stored.
Can I use the star databases I already have for ASTAP?
Section titled “Can I use the star databases I already have for ASTAP?”Yes. Set ARCSEC_CATALOG_DIR to ASTAP’s database folder, or pass it with -d. arcsec
reads ASTAP’s .1476, .290 and .001 files directly and never modifies them. See
sharing with ASTAP.
Can it solve with no position at all?
Section titled “Can it solve with no position at all?”Yes, with Astrometry.net index files and the -i option: arcsec estimates the
position from the index, then finishes the solve against a star database as usual.
arcsec catalog install anet-4100arcsec -f image.fits -i "$(arcsec catalog path)" --fov 3It works best on wider fields. The anet-4100 set (355 MB) covers fields of about
0.7° and wider. For narrower fields the anet-5200 set is needed, and it is an 8.8 GB
download. You still need to know the field size (--fov, or FOCALLEN and XPIXSZ
in the header). If your mount reports where it is pointing, you do not need blind
solving at all.
Which image formats does it read?
Section titled “Which image formats does it read?”FITS, XISF (PixInsight) and ASDF (used by the Roman telescope and astropy). The format
is recognised from the file’s contents, not its name. Compressed FITS is read directly
in gzip (.fits.gz), bzip2 and Unix compress (.Z) form. XISF colour images are
averaged to greyscale for solving.
Which imaging programs can use it?
Section titled “Which imaging programs can use it?”N.I.N.A. has been tested: set arcsec as its ASTAP solver. Other programs that run ASTAP’s command-line solver may work too, since arcsec takes the same options and writes the same files, but that is untested. If you try one, please report how it went.
Siril has also been tested, in a different way: it has no setting for an
external ASTAP solver, so you solve the image with arcsec --update and Siril reads the
solution from the FITS header when it opens the file.
Does it correct for lens distortion?
Section titled “Does it correct for lens distortion?”Only if you ask for it. By default arcsec fits a linear (TAN) solution, which suits
most telescopes. With --sip it also fits third-order distortion terms and writes them
to the .wcs file (and the header with --update), but only when the stars show
significant distortion across the frame: on an undistorted field extra terms just add
noise at the corners. Strongly distorted fields, such as very wide camera lenses, are
the hardest case for any linear solve.
What is --method tetra?
Section titled “What is --method tetra?”An experimental matcher that uses three-star triangles instead of ASTAP-style four-star quads. It solves far fewer images than the default (29 of the 98 solvable benchmark images, against 90) and is not recommended yet.
Which platforms does it run on?
Section titled “Which platforms does it run on?”Prebuilt downloads are available for Windows (x86-64), macOS (Apple silicon) and
Linux (x86-64 and arm64, glibc 2.35 or newer). Anywhere else that Rust supports,
including Intel Macs, cargo install arcsec builds it from source with Rust 1.96 or
newer.
How do I report a problem?
Section titled “How do I report a problem?”Open an issue on GitHub. For a failed solve, include:
- the full command line (it is on the
CMDLINE=line of the.inifile), - the output of the same command run with
--progress, or the.logfile from--log, - the arcsec version (
arcsec --version), - and the image, if you can share it.
Security problems should be reported privately, as described in the security policy.