X-shooter is a high-efficiency spectrograph capable of simultaneously observing the complete spectral range of 300-
2500 nm. The instrument will be located at the Cassegrain focus of one of the VLT UTs. To allow sky back ground
limited observations the 120 kg Optical Bench of the NIR Spectrograph and the HAWAII-2RG detector are cooled to
105 K and 82 K respectively. To ensure vibrationless operation the cooling is performed by a LN2 bath-cryostat. The
thermal stability requirements for the Optical Box are very tight (order of 100 mK) considering that the NIR-cryostat is
subject to telescope movement and LN2 level variations.
Large glass optics are limiting the cooldown. To speed up the cooldown the cooling concept of the Optical Box includes
the utilization of LN2 heat exchangers. To avoid asymptotic stabilizing times the Optical Box is cooled below the
operating temperature. When the optics reach a temperature slightly above the operating temperature the temperature of
the Optical Box is quickly brought back to stabilize the optics. Dedicated controllers, strapping and heaters are used for
temperature stabilization during steady state.
A cryostat hold time of 24 hours with the minimum amount of LN2 in view of the tight mass budget requires strict
control of the power budget and careful control of the design margins. This is ensured by precise modeling of the
temperature behavior. The thermal model is compared with the actual measured thermal behavior.