Use with Siril

Siril has its own plate solver, and can also run a local Astrometry.net installation. Unlike N.I.N.A., it has no setting to run ASTAP’s command-line solver, so arcsec cannot take a solver’s place inside Siril. What does work is solving the file with arcsec first: arcsec writes the solution into the FITS header, and Siril reads it from there when it opens the image, exactly as it reads one of its own.

Note

This was tested with Siril 1.4.4 on Linux: single images, Siril’s own 32-bit and RGB FITS files, compressed .fit.fz files, and sequences. The header route does not depend on the operating system, but Siril on Windows and macOS has not been tried.

Solving an image for Siril

1. Install arcsec and a star catalogue for your field of view, as described in Getting started. Which catalogue do I need? picks the catalogue.

2. Solve the FITS file with --update, which writes the solution into the file itself:

   arcsec -f M101_stacked.fit --update

   arcsec takes the pointing from the RA/DEC keywords and the field size from FOCALLEN and XPIXSZ, which most capture programs record. If the file lacks them, give them on the command line, for example --ra 14.05 --spd 144.35 --fov 1.0 (RA in hours, south pole distance = 90 + Dec, field height in degrees).

3. Open the file in Siril. If it was already open, open it again: Siril reads the header when it loads a file, not afterwards. The image is now plate solved: the annotation buttons become available, the coordinates under the cursor are shown, and tools that need a solution, such as photometric and spectrophotometric colour calibration (PCC, SPCC), can run.

Siril accepts arcsec’s solution as its own. Its platesolve command reports Image is already plate solved. Nothing will be done. unless you pass -force, and saving the image from Siril keeps the solution. With --sip, arcsec adds distortion terms when the field needs them, and Siril applies them too.

Solving a sequence

To use Siril’s astrometric registration, or to build a mosaic, solve every frame before Siril does. In a terminal, in the folder holding the frames:

for f in *.fit; do arcsec -f "$f" --update; done

or in PowerShell on Windows:

Get-ChildItem *.fit | ForEach-Object { arcsec -f $_.FullName --update }

Then build the sequence in Siril and solve it with Skip already solved images ticked (the seqplatesolve command skips them by default). Siril keeps arcsec’s solution for each frame, reporting for example (3 were already solved and skipped), and still records the star measurements it needs for registration.

Siril still prepares a star catalogue at the start of a sequence solve, even when it then skips every frame, so it needs its usual catalogue source for that step.

Build the sequence from the FITS files themselves, as convert does by default. For a sequence converted into a single FITS cube (-fitseq), Siril solved the frames again instead of keeping arcsec’s solutions.

What Siril reads

Siril reads the solution with the same library most astronomy software uses, wcslib, and accepts it only when the header has a CD or PC matrix: CDELT and CROTA2 alone are not enough. arcsec writes the full set: CRVAL, CRPIX, CD, CDELT and CROTA, plus SIP terms with --sip, and PLTSOLVD = T.

Siril writes its own solutions as a PC matrix with CDELT. When arcsec updates a file that Siril (or another solver) solved before, it first removes the old PC matrix and SIP terms, so the header holds only the new solution.

The .wcs and .ini files arcsec writes next to the image are for programs that drive ASTAP, such as N.I.N.A.; Siril does not use them.

Why not set arcsec as Siril’s solver?

Siril’s Image Plate Solver (Tools > Astrometry > Image Plate Solver) offers two solvers: Siril’s own, which matches the image against an online or local catalogue, and Astrometry.net’s solve-field, set up under Preferences > Astrometry. There is no ASTAP option to point at arcsec.

arcsec cannot stand in for solve-field either. Siril does not give solve-field the image, only a table of the stars it has detected, and expects Astrometry.net’s own options, output messages and files back. arcsec solves images, not star lists.

Troubleshooting

Siril says the image is not plate solved

• Check that arcsec solved it. arcsec prints Solution found: and exits with code 0; anything else means no solution was written. The exit code table says why.
• Check --update was given. Without it, arcsec writes the solution only to the .wcs and .ini files, which Siril does not read.
• Open the file again in Siril after solving it.
• The file is not FITS. --update writes into FITS files only (.fit, .fits, .fts, and tile-compressed .fit.fz). For an XISF or TIFF file, save it as FITS from Siril first.

The solve fails or is slow

• With no position in the header and none on the command line, arcsec searches the whole sky, which can take several minutes. Give the position with --ra and --spd, or install Astrometry.net index files for blind solving.
• Check the field size: a wrong focal length or pixel size in the header is the most common cause of a failed solve. --fov overrides it.
• Check the catalogue covers your field: see Which catalogue do I need?.

If a solve still fails, please open an issue with the command line, its output and, if you can share it, the image.