A cryogenic dithering stage for moving SPHERE-IRDIS' detector

Abstract

This paper describes the development of the detector motion stage for the instrument SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch). The detector movement is necessary because the instrument SPHERE has exceptional requirements on the flatfield accuracy: In order not to limit planetary detections, the photon response of every pixel with respect to the detector's mean response must be known to an accuracy of 10 -4. As only 10 -3 can be reached by calibration procedures, detector dithering is essential to apply ~100 pixels at a single spatial detection area and time-average the result to reduce the residual flatfield noise. We will explain the design of the unit including the detector package and report on extensive cold and warm tests of individual actuators. The novel, patented NEXLINE ® drive actuator design combines long travel ranges (hundreds of millimeters) with high stiffness and high resolution (better than 0.1 nm). Coordinated motion of shear and longitudinal piezo elements is what allows NEXLINE ® to break away from the limitations of conventional nanopositioning actuators. Motion is possible in two different modes: a high resolution, high dynamics analogue mode, and a step mode with theoretically unlimited travel range. The drive can always be brought to a condition with zero-voltage on the individual piezo elements and with the full holding force available to provide nanometer stability, no matter where it is along its travel range. The NEXLINE ® stage is equipped with capacitive sensors for the closed loop mode. The piezo modules are specially designed for cryogenic application.