OSIRIS: A diffraction limited integral field spectrograph for Keck

Abstract We present an overview of the OSIRIS integral field spectrograph which was recently commissioned on the Keck II Telescope. OSIRIS works with the Keck Adaptive Optics system and utilizes an infrared transmissive lenslet array to sample a rectangular field of view at close to the Keck diffraction limit. By packing the spectra very closely together (2 pixel rows per spectrum) and using the Rockwell Hawaii-2 detector (wavelengths between 1 and 2.5 μm), we achieve a relatively large field of view (up to 6.4″) while maintaining full broad-band spectral coverage at a resolution of 3800. Among the challenges of the instrument are: a fully cryogenic design (approximately 250 kg are brought down to 55 K); four spatial scales from 0.02 to 0.10″; extremely low wavefront error (approximately 25 nm of non-common path error); large all aluminum optics for the spectrograph; extremely repeatable spectral formats; and a sophisticated data reduction pipeline.