METIS - Design
METIS is a cryogenic instrument including the IMAGER subsystem, an LM band spectrograph and a wavefront sensor for adaptive optics control. Also included is a sophisticated coronagraph system for high contrast imaging.
The sub-systems as well as the common fore-optics are entirely encased in a cryostat of roughly 4m height to maintain the stable low temperatures required for good performance at mid-infrared wavelengths. The cryostat is located at the ELT Nasmyth platform.
MPIA is responsible for two sub-systems: the IMAGER and the Single-Conjugate Adaptive Optics (SCAO)
The IMAGER sub-system is providing diffraction-limited imaging at 3-13
microns with a field of view of approximately 11 x 11″. The imager also includes low/medium resolution slit spectroscopy as well as coronagraphy for high contrast imaging. It mainly consists of a collimator, an LM-arm (3-5 microns) including a Hawaii-2RG detector and an N-arm (5-13 microns) including a GeoSnap detector.
The Single-Conjugate Adaptive Optics (SCAO) corrects atmospheric distortions and is thus essential for diffraction limited observations with METIS. It consists of a wavefront sensor, the control software, and the Real Time Computer (RTC). The wavefront sensor measures the disturbed incoming wavefront. Its signal is processed by the Real Time Computer, which, in turn, sends control signals to the deformable ELT mirror M4.
We are using a novel approach to wavefront control, in which the reconstruction of the wavefront from the sensor signals is logically separated from the projection onto the deformable mirror and the temporal filtering. In this approach, a virtual deformable mirror is introduced, which is kept fixed in orientation with respect to the wavefront sensor and serves as an intermediate layer in which the wavefront is first reconstructed. In a second step, the alignment of the wavefront sensor (and thus the virtual mirror) with the actual M4 is taken into account and the wavefront is projected onto the corresponding command signals. This projection is time-variant and requires continuous monitoring of the system parameters to keep the command matrix up to date and adapted to the respective conditions. This allows a multitude of parameters to be considered (e.g. scaling, rotation, displacement), completely independent of fixed compensation elements in the beam path.