SVOM/VT: Preliminary Calibration Analysis

TL;DR

This paper details the in-orbit calibration process of the SVOM/VT instrument, highlighting astrometric precision, contamination effects, and calibration stability crucial for gamma-ray burst observations.

Contribution

It provides the first comprehensive calibration analysis of the VT instrument, including astrometry, photometry, and contamination management strategies.

Findings

Astrometric precision better than 0.03'' for bright stars

Contamination reduced system transmission by ~40% initially

Photometric stability of 0.02 mag maintained over time

Abstract

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.