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The infrared imaging spectrograph (IRIS) for TMT: imager design

We describe an optical design of an imager mode of the IRIS instrument for the Thirty Meter Telescope. IRIS is a fully-cryogenic diffraction-limited infrared camera and integral field spectrograph working in the wavelength coverage from 0.84 to 2.4 microns. The imager mode covers 16.4" × 16.4" FOV with a 4096 × 4096 detector array with sampling 4 milli-arcsec/pix. There are two challenges in performance which the science cases require in the imager mode. 1) rms wavefront error should be less than 30 nm, and 2) optical distortion should be corrected sufficiently to achieve astrometric accuracy of 10 micro-arcsec. Among possible optical configurations consisting of reflective and refractive solutions, a refractive solution with apochromatic triplets best meets the requirements. The optical system consists of a collimator and camera both of which have a BaF2-Fused Silica- ZnSe apochromatic triplet and a single BaF2 lens near the focus. The rms wavefront error of the system including the telescope, adaptive optics, and imager mode is less than 22 nm with ideal optical parameters. A sensitivity analysis shows that reasonable amount of errors in fabrication and alignment will give the rms wavefront error of less than 30 nm in 90 % of all cases. We also investigate accuracy of the distortion correction and how movable parts affect the correction accuracy. We find that uncorrectable distortion correction errors are well below 10 micro-arcsec with reasonable stability and repeatability of the movable parts.