GN and GS reflectivity tracking is not at present standardized, either in sampling frequency, equipment used, bandpasses. This may converge in future, though we note that the more dusty summit environment at Cerro Pachon means that more measurements are typically needed to ensure that trends are detected.
Gemini North uses an Surface Optics Corporation reflectometer
Gemini South uses a CT7
At both sites, if the optic is small or delicate, only one measurement location may be used.
Measurements are taken to track coating health and degradation over time. This helps to identify maintenance issues, environmental effects and to schedule recoating. Measurements need to be accurate enough to identify trends at the percent level. Error can be introduced by:
Incorrect calibration
Unstable power supply
Instrument temperature changes after calibration
Measurements being taken at a location that does not represent the average surface of the optic.
Occasional jumps are seen in the measurements, for example between 17-Apr-2025 and 22-Apr-2025 at Gemini North. These may be calibration-related and not real changes in reflectivity.