Siril and star catalogues
With the astrometry (plate solving), and photometric colour correction (PCC) features, Siril uses star catalogues data to identify stars in an image, determine exactly the framing of the images in equatorial sky coordinates and adjust the image's white balance to get as close as possible to real colour.
These features appeared in the 0.99 series of versions and up to the 1.0 series, the data were obtained from a Web service, in general VizieR.
With version 1.1, starting in June of 2022, it is now possible to rely on a locally installed star catalogue, for disconnected or more resilient operation. The star catalogue we found to be the most adapted to our needs is the one from KStars. It is in fact composed of four catalogues (documented here in KStars), two of them not being directly distributed in the base KStars installation files:
- namedstars.dat, the brightest stars, all of them have names
- unnamedstars.dat, also bright stars, but down to magnitude 8
- deepstars.dat, fainter stars extracted from The Tycho-2 Catalogue of the 2.5 Million Brightest Stars, down to magnitude 12.5
- USNO-NOMAD-1e8.dat, an extract of the huge NOMAD catalogue limited to B-V photometric information and star proper motion in a compact binary form, down to magnitude 18.
The first two files are available in KStars source, the Tycho-2 catalogue from a debian package and the NOMAD catalogue from KStars files too, as documented in this small article for KStars installation. It is has multiple worldwide mirrors as indicated in the articles.
To make things easier to Siril users, and possibly to KStars users too, we redistribute the four files in a single place, and in a more compressed format. With LZMA (used by xz or 7zip) algorithm, the file size is 1.0GB instead of the 1.4GB with the original gzip file.
To make it available from anywhere faster, it is distributed with bittorrent, using this torrent file or the following magnet link: magnet:?xt=urn:btih:0d0d891df2779bcfc691638d216551bf9e44cb59&dn=kstars-siril-catalogues&tr=http%3A%2F%2Ffree-astro.org%3A6969%2Fannounce&tr=udp%3A%2F%2Ftracker.openbittorrent.com%3A80%2Fannounce&tr=udp%3A%2F%2Fbt1.archive.org%3A6969%2Fannounce&tr=udp%3A%2F%2Ftracker.opentrackr.org%3A1337%2Fannounce
Installation in Siril
The files can be put anywhere and their paths given to Siril in the settings, but there is a default location for the four files: ~/.local/share/kstars/. They can be linked there to avoid unnecessary copies. Settings can be changed from the command line now, using the set command.
When available and readable, Siril will not use the Web service to retrieve astrometric or photometric data. See the messages in the log tab or on the console to verify that the catalogue files are used as expected.
Only SIMBAD will be used to convert object names into coordinates if required, but that should only be needed if the acquisition software did not record the target coordinates in the FITS header, or when using SER file format which cannot hold this information.
With the addition of the new link between Siril's plate solver and the local catalogue and the new link between Siril's PCC and the local catalogue, a new command nomad was created to display which stars in a plate solved image contain photometric information (the B-V index) and can be used for calibration.
his is a good way to verify that the plate solving and the image are aligned, in addition to the object annotation feature.
For photometry, Siril only uses the B-V index, which gives information about star colour. The three image channels are then scaled to give the best colour representation to all stars in the image.
Sha1 sums for the 4 catalogue files:
4642698f4b7b5ea3bd3e9edc7c4df2e6ce9c9f7d namedstars.dat 53a336a41f0f3949120e9662a465b60160c9d0f7 unnamedstars.dat d32b78fd1a3f977fa853d829fc44ee0014c2ab53 deepstars.dat 12e663e04cae9e43fc4de62d6eb2c69905ea513f USNO-NOMAD-1e8.dat
Licenses for the 4 catalogue files.