A dry air purge and condensation monitor for the CFHT primary mirror

Over the typical 3-year primary mirror (M1) recoating schedule at the Canada France Hawaii Telescope (CFHT), throughput losses due to contamination and degradation of the M1 bare-aluminum coating exceeding 0.4 mag in U band are observed with MegaCam. It is hypothesized that extended periods of wet weather, leading to clusters of heavy condensation events on the primary mirror, leads to accelerated degradation of the bare aluminum coatings. In a pre-emptive effort to address this problem, a dry air purge system was implemented for the CFHT M1, prior to CFHT M1 recoating in 2020. Additionally, a camera system was implemented to detect condensation on the primary mirror using a simple image processing algorithm. As of May 2022, no significant condensation events have been observed on the CFHT M1.