Minus nine at 04:00, and the secondary had walked far enough to be visible on a star test by the time we closed. Third time this winter, always after the temperature crosses about minus seven.
The 2019 cell rebuild fixed the worst of it — before that we were recollimating weekly — but the residual is thermal and will not be engineered out at this budget. What we do instead is check at dusk and again at the session midpoint, which catches it before anybody notices.
Logged because guests occasionally ask why a telescope needs adjusting twice in one night. This is why.
Logged by Sofía Marchant, optics and instruments.