Effect of the lack of the windscreen at Subaru Telescope

The windscreen of the Subaru Telescope, which had been installed to prevent strong wind from directly hitting the telescope, was damaged and dropped out during the night operation at around 12:40 am, April 10, 2017. After that all observations at Subaru Telescope have been forced to be done without the windscreen. Due to the lack of the windscreen, the telescope would vibrate with strong wind, and the shape of the primary mirror would be deformed, thus the star image would become worse. Here we have investigated the effect of the lack of the windscreen statistically. The seeing at zenith at 500 nm was calculated from the FWHM of the star image obtained from the HSC on-site analysis software, and compared with the DIMM seeing at the east ridge of Maunakea simultaneously. Although the median seeing value varied by ∼0.100 each year, the median seeing of DIMM before and after the incident is almost the same value (0.6100 and 0.6400), while the median seeing of HSC after the incident is ∼0.0700 larger than before (0.6800 to 0.7500). The wind speeds at the telescope top ring, telescope center section, and outside the dome roof were investigated from the telescope telemetry data, and the correlations with the seeing were also investigated. Although the wind speed outside the dome did not change significantly before and after the incident, it was found that the wind speed inside the dome increased after the incident. Although it is not clear whether the degradation of the HSC seeing can be attributed to the absence of the windscreen, the seeing values over the last three years have statistically become ∼10% worse than before.