The Gemini South Adaptive Optics Imager (GSAOI) to be used with the Multi-Conjugate Adaptive Optics (MCAO) system at Gemini South is currently in the final stages of assembly and testing. GSAOI uses a suite of 26 different filters, made from both BK7 and Fused Silica substrates. These filters, located in a non-collimated beam, work as active optical elements.
The optical design was undertaken to ensure that both the filter substrates both focused longitudinally at the same point. During the testing of the instrument it was found that longitudinal focus was filter dependant. The methods used to investigate this are outlined in the paper. These investigations identified several possible causes for the focal shift including substrate material properties in cryogenic conditions and small amounts of residual filter power.
The Gemini South Adaptive Optics Imager (GSAOI) is the science camera and commissioning instrument for the Multi-Conjugate Adaptive Optics (MCAO) system on the Gemini South telescope. GSAOI is required to deliver diffraction-limited performance at near-infrared wavelengths over a 85"×85" field of view. It must be delivered on a short timescale commensurate with MCAO delivery. GSAOI will use a high throughput, all-refractive optical design and a mosaic of four HAWAII-2RG detectors to form an imager focal plane of 4080x4080 pixels with a fixed scale of 0.02"/pixel. The On-Detector Guide Window (ODGW) capability of the HAWAII-2RG detectors will be used for flexure monitoring and as near-infrared substitutes for MCAO natural guide star wave front sensors. The imager will include a pupil viewer for accurate alignment to MCAO and defocus lenses to measure wave front phase errors at the science detector using the curvature technique. Non-common path wave front errors will be nulled by setting the base shapes of the three MCAO deformable mirrors. The science drivers, performance predictions, optical design issues, and detector system for the instrument are described.
The Research School of Astronomy and Astrophysics (RSAA) of the Australian National University (ANU) has designed and constructed The Gemini South Adaptive Optics Imager (GSAOI) that will be used with the Multi-Conjugate Adaptive Optics system on the Gemini South telescope in Chile. GSAOI contains three cryogenic mechanisms; two filter wheels and a utility wheel. An approach to the athermalization of cryogenic mechanisms is presented. The 280 mm diameter filter wheels are athermalized using bi-material conical bearing seats where the bearing preload is constant, irrespective of temperature. The lens mounting method is also described. Lenses up to 170 mm in diameter are mounted within precision cells such that radial clearances reduce to zero at operating temperature. The method used to derive the room-temperature lens and cell dimensions is described. Lenses are preloaded axially against conical seats using wave washers. This technique has been used successfully to mount lenses of ~100 mm diameter in the Gemini Near-infrared Integral Field Spectrograph (NIFS), also designed and constructed by RSAA.