Development and performance validation of a cryogenic linear stage for SPICA-SAFARI verification

In the context of the SAFARI instrument (SpicA FAR-infrared Instrument) SRON is developing a test environment to verify the SAFARI performance. The characterization of the detector focal plane will be performed with a backilluminated pinhole over a reimaged SAFARI focal plane by an XYZ scanning mechanism that consists of three linear stages stacked together. In order to reduce background radiation that can couple into the high sensitivity cryogenic detectors (goal NEP of 2•10 -19 W/√Hz and saturation power of few femtoWatts) the scanner is mounted inside the cryostat in the 4K environment. The required readout accuracy is 3 μm and reproducibility of 1 μm along the total travel of 32 mm. The stage will be operated in “on the fly” mode to prevent vibrations of the scanner mechanism and will move with a constant speed varying from 60 μm/s to 400 μm/s. In order to meet the requirements of large stroke, low dissipation (low friction) and high accuracy a DC motor plus spindle stage solution has been chosen. In this paper we will present the stage design and stage characterization, describing also the measurements setup. The room temperature performance has been measured with a 3D measuring machine cross calibrated with a laser interferometer and a 2-axis tilt sensor. The low temperature verification has been performed in a wet 4K cryostat using a laser interferometer for measuring the linear displacements and a theodolite for measuring the angular displacements. The angular displacements can be calibrated with a precision of 4 arcsec and the position could be determined with high accuracy. The presence of friction caused higher values of torque than predicted and consequently higher dissipation. The thermal model of the stage has also been verified at 4K.