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Temperature‐dependent simultaneous measurement of birefringence and optical rotatory power (ORP) for orthorhombic crystals of potassium titanyl phosphate (KTP) and potassium titanyl arsenate (KTA) has been achieved by using the high‐accuracy universal polarimeter method at 632.8 nm wavelength. The birefringence and ORP changes along the (110) planes for KTA and KTP crystals were found to have a nearly parabolic form for the temperature range 297–493 K. The thermal variation coefficients were found to be 0.9 (5) × 10−5 K−1 and 5.6 (3) × 10−8 K−2 for KTA and 0.9 (5) × 10−5 K−1 and 5.7 (3) × 10−8 K−2 for KTP. The ORPs at 297 K were found to be 20.0 (20) and 20.5 (15)° mm−1 for KTA and KTP, respectively. The thermal variation coefficients of the ORP were found to be 1.8 (2) × 10−3° mm−1 K−1 and 1.1 (1) × 10–5° mm−1 K−2 for KTA and 1.9 (2) × 10−3° mm−1 K−1 and 1.2 (1) × 10−5° mm−1 K−2 for KTP.

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EMIR is a intermediate resolution near infrared (1.0 -2.5 microns) multiobject spectrograph with image capabilities, to be mounted on the Gran Telescopio Canarias (GTC). EMIR is being built by a consortium of Spanish, French and British institutions, led by the Instituto de Astrofísica de Canarias. EMIR is being funded by GRANTECAN and the Plan Nacional de Astronomía y Astrofísica (National Plan for Astronomy and Astrophysics, Spain) as one of the first common user instruments for the GTC.The instrument shall deliver images and spectra from a large FOV (6x6 arcmin in imaging mode, and 6x4 arcmin in multislit spectroscopic mode). Due to the telescope image scale (1 arcmin = 52 mm) and the spectral resolution required (around 4000), one of the major challenges of the instrument is the optical design and the manufacture.The detailed optical design and its expected performance will be presented. In particular the main risk areas will be identified and our risk control strategy will be outlined.

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