Keyboard shortcuts

Press ← or → to navigate between chapters

Press S or / to search in the book

Press ? to show this help

Press Esc to hide this help

The star catalogue file (.vcat)

valerian solve and valerian photocal read a star catalogue, gaia-dr3-g11.vcat: every Gaia DR3 source brighter than G = 11 (1,247,240 stars, 9,978,676 bytes) with its BP-RP colour. It ships with every release package and is licensed CC BY-SA 3.0 IGO (Gaia DR3 is (c) ESA/Gaia/DPAC); the licence text ships beside it.

Where Valerian looks

When --catalogue is not given, in this order, using the first that exists:

  1. the file named by the VALERIAN_CATALOGUE environment variable;
  2. gaia-dr3-g11.vcat beside the program;
  3. data/gaia-dr3-g11.vcat beside the program;
  4. ../share/valerian/gaia-dr3-g11.vcat relative to the program (where the .deb, .rpm and AppImage put it);
  5. for a build from source, crates/valerian-core/data/gaia-dr3-g11.vcat in the repository.

valerian info prints the one it found and its star count, or catalogue: not found. When none exists, solve stops with no star catalogue found; looked for gaia-dr3-g11.vcat in: followed by every place it tried.

Format

All integers are little-endian. Positions are at epoch 2016.0 (Gaia DR3); proper motion is not applied.

OffsetSizeContent
08magic VCAT0002 (VCAT0001, which has no colour byte, still reads)
84u32 star count n
124f32 magnitude limit
164u32 declination band count (180, one degree each)
204f32 epoch of the positions
248zero
324 x (bands + 1)u32 index of the first star of each band, then n
then8 x nthe stars, band by band and by right ascension within a band

Each star of version 2 is 8 bytes: u32 right ascension (0 to 2^32 for 0 to 360 degrees), u16 declination within the band (0 to 65535 for one degree), u8 (G + 2) x 10 rounded, u8 (BP-RP + 1) x 40 rounded (1 to 255, or 0 when Gaia has no BP and RP). Version 1 stars are 7 bytes without the colour. photocal needs version 2.

Building a deeper one

For fields narrower than about half a degree, G < 11 has too few stars. Build one from the ESA Gaia archive (needs network access and only the Python standard library):

python3 scripts/build_catalogue.py --mag 13 -o gaia-dr3-g13.vcat
valerian solve image.fits --catalogue gaia-dr3-g13.vcat

G < 13 is 7.4 million stars, about 59 MB. See How plate solving works.