Satellite based quantum key distribution (QKD) enables the delivery of keys for quantum secure communications over long distances. Maturity of the technology as well as industrial interest are ever increasing. Same is true for satellite free-space optical communications (FSOC). In order to enable a robust channel for transmission it is indispensable to account for static and dynamically changing misalignments between the transmitter and receiver pair. This work will focus on the transmitter terminal (Alice) and the design and verification process of the active beam steering system. The novelty is a recently developed variable reluctance fine steering mirror (FSM) including eddy current sensors (ECS) to measure its tip and tilt. A cascaded architecture was chosen in order to combine the optical stabilization objective with the dynamics of the mirror platform. The inner control loop makes use of an observer model whose estimated output is fed into a state controller allowing for an increased responsiveness. While high gains increase the closed loop bandwidth the eigenfrequency of the system introduces a pole to the plant which has to be avoided by the controller output. A digital notch filter was introduced to reject the excitation of the critical frequency band which gets obsolete in a system with high frequency sampling capabilities. The outer loop is engaged when a valid optical signal is received and a transition from a closed loop pointing to a closed loop tracking mode is performed. A proportional-integral (PI) controller keeps the received beam at the 4-quadrant-diode (4QD) whose center is used as the main reference through prior calibration with the transmit beam launching on the same path. The presented cascaded control scheme allows improvements in system performance and reliability.
MICADO, the Multi-AO-Imaging-Camera and Spectrometer for Deep Observations, is one of the first light instruments for the future 40 m class Extremely Large Telescope (ELT). MICADO utilizes the advanced laser guide star multiconjugate adaptive optics system MCAO developed by the MAORY consortium and the jointly developed singleconjugate adaptive optics system (SCAO). We present an overview on the conceptual design of the MICADO Cold Optical Instrument (COI) which comprises the infrared focal plane imager with its 3 x 3 4k2 HgCdTe detector array and a compact cross-dispersing slit spectrometer operating in the spectral range of 0.8 to 2.4 μm. High contrast imaging is enabled via a classical configuration of coronagraph and Lyot stops. The paper summarizes the MICADO COI interchangeable optics, its cryogenic implementation together with the modular opto-mechanical configuration of the cryo-mechanisms and the cryo-vacuum cooling system, which consists of a continuous LN2 flow cryostat.
MICADO will equip the E-ELT with a first light capability for diffraction limited imaging at near-infrared wavelengths. The instrument’s observing modes focus on various flavours of imaging, including astrometric, high contrast, and time resolved. There is also a single object spectroscopic mode optimised for wavelength coverage at moderately high resolution. This contribution provides an overview of the key functionality of the instrument, outlining the scientific rationale for its observing modes. The interface between MICADO and the adaptive optics system MAORY that feeds it is summarised. The design of the instrument is discussed, focusing on the optics and mechanisms inside the cryostat, together with a brief overview of the other key sub-systems.
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