Multi-adaptive optics Imaging CAmera for Deep Observations

The Multi-adaptive optics Imaging CAmera for Deep Observations (MICADO) instrument is the first light imager for the 39m diameter ESO Extremely Large Telescope currently under construction on Cerro Armazones in Chile.

Figure 1: MICADO stand-alone mode

MICADO is directly interfaced to the ELT with a temporary installed relay optic. Credit: MPE

MICADO is being build by a large consortium involving partners from Germany, Austria, France, the Netherlands, Italy and Finland, led by the Max-Planck-Institute for Extraterrestrial Physics (MPE) in Garching (see e.g. the official MICADO webpage). It will provide the ELT with the capability for diffraction limited imaging at near-infrared wavelengths. It is designed to work with the Multi-conjugate adaptive Optics Relay For ELT Observation (MORFEO), build by the MORFEO consortium, but also with a single-conjugate natural guide star adaptive optics system (SCAO). SCAO is a common effort of the MICADO and MORFEO consortia. SCAO can be used in a so called stand-alone mode, directly interfacing MICADO to the ELT using a temporary installed relay optic. Figure 1 shows the 3D design model of the MICADO instrument mounted on the Nasmyth platform of the ELT in stand-alone mode. Figure 2 gives an overview of the design also with MORFEO in place.

The main drivers for the MICADO instrument design are sensitivity, resolution, precision astrometry, and wide wave-length coverage spectroscopy. These capabilities will allow for a large number of science topics to be adressed. The five main areas if research enabled by MICADO are:

  • galaxy evolution at high redshifts,
  • black holes in galaxy centers (incl. the galactic center),
  • resolved stellar populations (photometry in galaxy nuclei, IMF in young star clusters, intermediate mass black holes in globular clusters),
  • coronographic imaging (e.g. of exo-planets),
  • solar system research.

Figure 2: MICADO with MORFEO

The final setup with MORFEO and MICADO mounted on the Nasmyth platform. Credit: MPE

To achieve these science goals, MICADO will provide 4 observing modes in the wavelength range 0.8-2.4 micron. These are

  • Standard Imaging: Low resolution 4mas/px, High resolution 1.5 mas/px,
  • Astrometric Imaging: same as standard imaging with special calibration to achieve astrometric precision of 50 microarcsec,
  • Coronographic Imaging: Classical focal plane coronagraphy and pupil plane coronagraphy with an apodizing phase plate available,
  • Slit Spectroscopy: R~10000, Slit width 16mas + 48mas, Slit length 4 arcsec + long slit with 20 arcsec (only K-Band).

Figure 3: MICADO Instrument functional diagram

MICADO instrument incl. SCAO, relay optic and calibration unit. Credit: R. Davies (MPE)

The MICADO cryostat (see Figure 3) hosts the instrument components that operate under vacuum condition and cryogenic temperature (82K). The cryogenic assembly consists of a main supporting structure where the detector array, the fixed and the moving optics are mounted. The mechanisms used to change filters, coronographic masks or slits for the spectrometer are also allocated inside the MICADO Cryostat. Finally, a cryogenic selection mechanism, called the Main Selection Mechanism (MSM), will be used to insert different optical elements into the science beam inside the cryostat, in order to switch between the instrument obervational modes.

Within the MICADO Project, USM personnel is responsible for the following workpackages.

  • Mechanics and Optics: development of the MICADO Main Selection Mechanism, incuding the cold optics allocated in the mechanism (A. Monna, J. Lange, Frank Grupp),
  • Software: development of high- and low- level Instrument Control Software (M. Wegner, J. Schlichter) and of the Cryostat Control Software (S. Annadevara, H. Kellermann),
  • Electronics: development of 4 (out of 12) Instrument Electronic cabinets (H.J. Hess, H. Kravcar).

Mechanics and Optics

Figure 4: MSM Assembly Design

The Main Selection Mechanism consists of a Support Structure Assembly (Support Structure I-IV) which holds the Rotating Platform (RP) and hosts the MSM drive/sense and the electronic board. The Cold Optics Instrument (COI) sub-systems LRI (Low Resolution Imager), SPE (Spectrograph) and PIM (Pupil Imager) are located on the MSM rotating platform. The RP is supported via small bearings installed on the MSM support structure.

The MICADO Main Selection Mechanism (MSM) allows to select the operational modes by moving the respective cold optics into the optical path of the Instrument. The module is located inside the cryostat, below the so called “NOVA Envelope” and above the camera optics level.

It consists of a main rotating platform, holding structures, drives, sensors and cable harness. Figure 4 shows the 3D model design of the MSM. The MSM design uses a stepper motor to move the MSM Rotating Platform. An indent mechanism ensures to reposition the modules always to the same location inside the science beam. Because all parts of this mechanism are located and operated inside the cryostat, it is very important to make the right choice concerning used materials and components to fullfill the performance and lifetime requirements .

The USM-Team is also responsible for the folowing optical elements located on the rotating platform.

  • Low Resolution Imager: two flat mirrors,
  • Spectrometer: two gratings,
  • Pupil viewer: two flat mirrors and one lens.

The MSM has an overall diameter of circa 1300 mm and an height of circa 350mm. The total weight is around 90kg.

Instrument Control and Observation Preparation Software

Figure 5: MICADO software architecture

MICADO software architecture with components and their mutual dependencies in the ELT control system context. Those under full or partial USM responsibility are highlighted in maroon.

For observation planning, scripting, and for the control of MICADO hardware devices, a number of dedicated tools are required which all have to fit into ESO's software ecosystem. This results in an architecture where generic subsystems and components are tailored to the specific needs of the instrument. While the MICADO Instrument Control Software is based on a customisable ESO framework, the software for observation preparation is organised as an ESO standard web application with MICADO-specific microservices. In both cases, the distinct instrument features are being implemented under full or partial USM responsibility (Fig. 5).

The purpose of the Observation Preparation Software is to create so-called Observation Blocks with a particular focus on the specific instrument configuration. For MICADO, this includes the automatic or manual selection of suitable natural AO guide stars and the definition of dither patterns that are compliant with the selected asterism.

Templates are functional units executing parts of an Observation Block's functionality and implemented as Python scripts. Users are expected to specify values for the free input parameters during observation preparation.

The Observation Coordination System is responsible for the synchronisation of all subsystems of the MICADO software which are involved in the observation process, and for the creation of the final data products (so-called FITS files) which are sent to the ESO science archive. It is implemented (mostly) in C++.

The Function Control System is the part of the software which controls the various hardware devices in MICADO (filter wheels, tracking devices, lamps, shutters, ...). It is implemented in two different layers: A lower layer, which runs on a number of PLCs, allows for the real-time capabilities usually needed in hardware control; this layer is implemented in Structured Text (one of the IEC 61131-3 PLC programming languages). A second layer (again implemented in C++) runs on the instrument workstation and is responsible for the synchronisation of the low-level software devices on the PLCs, and for the integration of the hardware control in the overall MICADO software landscape.

Science data can be visualised in real-time by means of graphical user interfaces, created on the basis of the Data Display Tool.

Finally, a dedicated MORFEO Interface enables a limited control of the AO system through MICADO while keeping both systems as separate as possible.