In October 2022, there was an accident with the Gemini North Primary Mirror (M1) during routine mirror movements that caused it to come in contact with its washcart. The M1 came into contact with an earthquake restraint on the washcart as it was being moved in preparation for stripping and recoating activities. After the incident, analyses were conducted to determine the extent of the damage and the necessary steps to restore the telescope to normal operations. The extent of the fracture was determined by through visual inspection, and by observing stress polarization within the glass. Due to damage occurring along one of the lateral supports on the M1, it was first necessary to determine a safe method for removal. Grinding and polishing work of the damaged areas was then necessary to secure the glass and remove any stress. It was then necessary to bond a new lateral support to the M1 to prevent higher order aberrations from the asymmetrical support. Since a small change in the location of the support was necessary, a finite element analysis was performed to determine how its new location would affect optical aberrations on the M1. Test samples were produced to verify the performance of the new bond. This paper provides detailed explanations how the Gemini North telescope was returned to normal operations.
Gemini North Telescope (GN) primary mirror (M1) suffered damage due to a collision with its wash cart during uncontrolled descent onto the cart in preparation for stripping and coating in October 2022. Gemini M1 is an 8.1 m aperture ULE thin meniscus mirror. The damage consisted in two stress-induced conchoidal fractures located on the outer perimeter and the underside of the M1, one approximately 300 by 200 mm and the other approximately 100 x 50 mm. In addition to mitigating the glass damage, the repairs required relocation and modification of one of the lateral pads of the M1 support system. A multidisciplinary team was assembled to repair the GN M1, led by NOIRLab Engineering Services. The repairs were successfully accomplished without affecting the optical performance of the Telescope. This paper describes the damage assessment, the mitigation strategy, the team and resources involved, the preparatory work and tests done prior to start of the repairs, the repairs and modifications performed, the techniques employed, post-repair assessment, and the engineering analysis and experiments performed to verify long-term safety and performance of the repaired M1. After successful completion of the repairs and recoating of the M1, Gemini North telescope returned to normal science operations in June 2023. No noticeable decrease in optical or mechanical performance of the mirror has been detected.
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