METIS - Design

METIS - Design

May 22, 2018

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.

This figure shows the optical design of METIS including a Warm Calibration Unit (WCU) outside the cryostat and the Common Fore Optics (CFO), the Single-Conjugate Adaptive optics system (SCA), the L/M band high resolution spectrograph (LMS) and the imager (IMG) with an L/M and N/Q arm, all inside the cryostat (CRY). A Single Laser Adaptive Optics (SLAO) unit may be added but is not part of the baseline.

Design sketch of the METIS cryostat located at the ELT Nasmyth platform in 6m height. The cryostat hosts all science sub-systems. On top of it, the Warm Calibration Unit is shown. Underneath the cryostat the (warm) electronic cabinets are located.

MPIA is responsible for two sub-systems: the IMAGER and the Single-Conjugate Adaptive Optics (SCAO)

Imager unit.
The N-arm of the imager optics is mounted to the green plate on the left side. It contains three wheels, the N-camera optics and the detector. The LM arm on the right side (light yellow plate) is built similarly. In addition it contains the optical collimator unit with two fold mirrors.

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.

Opto-mechanical design of the SCAO module.
The SCAO wavefront sensor is of the pyramid type.
A first pupil image is formed on the Field Selector (2) by lens unit 1 (1) and a second pupil image on the Modulator (5) by lens unit 2 (3).
Both mirrors allow to control the position the field on the pyramid (6). Only a small field is propagated downstream of the Field Selector.
A filter wheel  (4) allows to adjust the pass bands and gray filters in case of very bright reference targets. The re-imaging lens group 3
(7) produces the four images of M4 on the Saphira detector (8) as seen through the pyramid.
Scheme for the METIS wavefront control using the ELT deformable mirror.

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