  With significantly improved sensitivity, the Einstein Telescope (ET), along
with other upcoming gravitational wave detectors, will mark the beginning of
precision gravitational wave astronomy. However, the pursuit of surpassing
current detector capabilities requires careful consideration of technical
constraints inherent in existing designs. The significant improvement of ET
lies in the low-frequency range, where it anticipates a one million-fold
increase in sensitivity compared to current detectors. Angular control noise is
a primary limitation for LIGO detectors in this frequency range, originating
from the need to maintain optical alignment. Given the expected improvements in
ET's low-frequency range, precise assessment of angular control noise becomes
crucial for achieving target sensitivity. To address this, we developed a model
of the angular control system of Advanced Virgo, closely matching experimental
data and providing a robust foundation for modeling future-generation
detectors. Our model, for the first time, enables replication of the measured
coupling level between angle and length. Additionally, our findings confirm
that Virgo, unlike LIGO, is not constrained by alignment control noise, even if
the detector were operating at full power.
