We present the in-orbit calibration of the Visible Telescope (VT), one of the key instruments aboard the Space Variable Objects Monitor (SVOM) mission for gamma-ray burst (GRB) studies. Using Gaia Data Release 3 (DR3) as a reference, the VT achieves an astrometric precision better than 0.03'' for bright stars, degrading to ~0.25'' for faint targets. Shortly after launch, contamination was detected, reducing system transmission by ~40%. An initial bake-out successfully restored performance, but gradual recontamination caused transmission to decline by ~20% over the following 100 days before stabilizing. Despite this effect, routine standard star observations maintain precise zero-point calibration, ensuring a photometric stability of 0.02 mag. Using synthetic stellar spectra, we derived photometric transformations to the Gaia, SDSS, and Johnson-Cousins systems with typical residuals of 0.03 mag. These results demonstrate the VT system's capability and reliability in calibrating GRBs and other transients.
@article{arxiv.2604.24256,
title = {SVOM/VT: Preliminary Calibration Analysis},
author = {Zhu-Heng Yao and Yu-Lei Qiu and Jin-Song Deng and Li-Ping Xin and Chao Wu and Hua-Li Li and Jing Wang and Yi-Nuo Ma and Hong-Bo Cai and Xu-Hui Han and Jian-Yan Wei and Betrand Cordier},
journal= {arXiv preprint arXiv:2604.24256},
year = {2026}
}
Comments
Accepted for publication in the SVOM special issue in Research in Astronomy and Astrophysics; 11 pages, 10 figures