MICADO is the Multi-AO Imaging Camera for Deep Observations, which has been designed to work with adaptive optics on the 40-m class European Extremely Large Telescope. Following a call for proposals by ESO, the MICADO consortium - led by MPE - was awarded the contract for the Phase A study. The agreement was ratified between MPE and ESO in February 2008. The Phase A Final Review was held at in December 2009 and the study is now completed. Phase B starts in autumn 2015 with the signature of the agreement with ESO and the Kick Off meeting of the entire consortium.
Early in the project, the consortium highlighted several key capabilities that exemplify the unique features of the E-ELT at which MICADO will excel in comparison to other facilities. These are at the root of the science cases and have driven the design of the camera: sensitivity and resolution, precision astrometry, and high throughput spectroscopy. By both promoting and exploiting these capabilities with a simple and robust design, MICADO has been selected as E-ELT first light instrument.
The primary observing mode is imaging, with a focus on astrometry. To achieve this, the instrument is supported on the Nasmyth platform in a gravity invariant orientation, includes an optical path comprising entirely of fixed mirrors, uses a state-of-the-art atmospheric dispersion corrector, and has a dedicated astrometric calibration plan and data pipeline. The array of detectors at the focal plane enables imaging of a small field of about 20arcsec with a fine pixel sampling that is especially useful in very crowded fields or at short wavelengths, or a large field that is nearly 1arcmin across with a coarser pixel scale that still fully samples the H- and K-band diffraction limit. In both cases, a wide selection of broad and narrow band filters are available. This mode will provide comparable sensitivity to JWST (James Webb Space Telescope) at 6 times better spatial resolution, and enable proper motions as small as 5km/s to be measured at distances of up to 100kpc. This allows proper motions of stars in globular clusters to be measured to that precision anywhere within the Galactic Halo.
High contrast imaging is enabled via a classical configuration of coronagraph and Lyot stop, and is envisaged to make use of angular differential imaging techniques. It will allow planets to be discovered and characterised at very small separation from their host star, and may permit direct imaging of planets with radial velocity measurements.
The spectroscopic mode is optimised for compact objects, and emphasises wavelength coverage at moderate resolution. It is therefore ideal for studies that rely on determining multiple emission line ratios or fitting continuum absorption features. Slit widths suitable for compact and extended objects will be provided.
MPE: Max-Planck-Institut für extraterrestrische Physik
MPIA: Max-Planck-Institut für Astronomie
USM: Universitäts-Sternwarte München
IAG: Georg-August-Universität Göttingen/Georg-August-Universität; Göttingen Stiftung Öffentlichen Rechts (Executing institute: Institute for Astrophysics)
NOVA: Netherlands Research School for Astronomy (Nederlandse Onderzoekschool voor de Astronomie). Nova is a federation of several astronomical institutes. (Specific contributions to MICADO come from the University of Groningen, the University of Leiden, and the NOVA optical/infrared instrumentation group based at ASTRON in Dwingeloo.)
CNRS/INSU: Centre National de la Recherche Scientifique/Institut National des Sciences de l’Univers (representing LESIA, GEPI and IPAG)
A*: An Austrian partnership collectively represented by University Vienna (The A* partnership comprises the University of Vienna, the University of Innsbruck, the University Graz, the University of Linz and RICAM Linz (Austrian Academy of Sciences). Contributions to MICADO come from Vienna, Innsbruck and Linz.)
INAF-OAPD: National Institute for Astrophysics at the Observatory of Padova
The consortium has highlighted the following 3 capabilities that lie at the root of the science cases and that will drive the design of the instrument:
Sensitivity and Resolution
High Throughput Spectroscopy
The superior sensitivity of MICADO@E-ELT will allow discovery and study of new or unexplored phenomena. The high angular resolution, and the astrometric and spectroscopic capabilities of MICADO will yield new insights into the structure and physics of cosmic objects. With the superb astrometric precision achieved by MICADO many astronomical objects/phenomena will no longer be static but become dynamic. Many historical examples demonstrate that attaining this additional capability often leads to dramatic new insights into the three dimensional structure and evolution.
The MICADO design to achieve these capabilities is simple, compact and robust, which minimizes risks on cost and schedule. The consortium has also identified a
as an important option. While MICADO achieves superb wide-field performance with the MCAO module MAORY, in its first phase MICADO will be combined with an internal SCAO module. As this simple on-axis, natural guide star mode sets low requirements on the telescope and AO performance (no lasers), MICADO+SCAO thus is an optimum choice for demonstrating the scientific capabilities of the E-ELT at first light.
Sensitivity and Resolution
MICADO is optimised for imaging at the diffraction limit, and will fully sample the 6-10mas FWHM in the J-K bands. With a throughput exceeding 60% its sensitivity at 1-2micron will, for the AO performance predicted by MAORY, be comparable to, or surpass, JWST even for isolated point sources, and be clearly superior to JWST in crowded regions. MICADO thus will realise the full power and most unique features of a 39m-AO telescope in its first light capabilities. In parallel, the consortium is pursuing a project to develop OH suppressing filters that could further improve the sensitivity by a significant factor. MICADO's superior resolution means that it will be able to probe the detailed structure of objects that are unresolved by JWST. In addition, its field of view of nearly 1 arcmin yields a significant multiplex advantage compared to other ground-based cameras such as IRIS on the TMT. Together, these characteristics make MICADO a powerful tool for many science cases, from studies of faint high redshift galaxies to performing photometry in crowded fields. In important issue in this respect is the availability of tools to extract and measure point sources, and so the consortium has initiated a study to assess the suitability and future requirements of photometry packages.
The primary imaging field of MICADO employs a catoptric design using only fixed mirrors. Together with the gravity invariant rotation and the baseline to use HAWAII-4RG detectors (developed to meet the stringent requirements of space astrometry missions), this makes MICADO an ideal instrument for astrometry. A robust pipeline, based on software already available in the AstroWISE system, will bring precision astrometry into the mainstream. An analysis of the statistical and systematic effects shows that proper motions of 40microarcsec/yr in a single epoch of observations should be achievable; and after only 3-4 years it will be possible to reach 10microarcsec/yr, equivalent to 5km/s at 100kpc. At this level, many novel science cases become feasible, including proper motions of stars in the centre of the Galaxy and other nearby galaxies, mass determinations of intermediate mass black holes, proper motions of globular clusters, and testing cold dark matter structure formation using internal kinematics of dwarf spheroidals.
High Throughput Spectroscopy
The obvious, but powerful, complement to pure imaging is spectroscopy. We have implemented a simple slit spectrometer with a high throughput that is ideal for obtaining spectra of compact objects. The resolution of R~3000 is sufficient to probe between the near infrared OH lines. This simple addition will enhance many science cases, for example: deriving stellar types and 3D orbits in the Galactic Center; using velocities of stars in nearby galaxies to probe central black hole masses and extended mass distributions; measuring absorption lines in galaxies at z = 2-3 and emission lines in galaxies at z = 4-6 to derive their ages, metallicities, and star forming histories; and obtaining spectra of the first supernovae at z=1-6.
In order to be a viable first-light instrument, the MICADO consortium has decided to implement a phased approach whereby the instrument is operated first with a more simple form of adaptive optics, SCAO, and only later upgraded to work with full MCAO. In this way the camera will be able to produce diffraction limited images leading to high quality scientific results from the very beginning. Since the E-ELT baseline is that SCAO wavefront sensing capability is not provided to the instruments by the telescope, the consortium has included the study of such a module. The optical relay and support structure have been designed so that the interface between MICADO and the SCAO module is the same as that with MAORY. Furthermore, they can in principle also be used to interface MICADO to other AO systems such as ATLAS. There are technical and fiscal benefits to this phased approach: (i) MICADO is able to make use of adaptive optics at a level of sophistication that increases as more complex AO systems are commissioned. (ii) The high capital cost of the detectors required to image the large field corrected by MCAO is spread over a longer time period.
The broadband imaging performance for the MICADO primary field has been calculated for isolated point sources using PSFs provided by the MAORY consortium and for standard broadband filters similar to those in HAWK-I. It shows that the 5sigma sensitivity will be better than a few nano-Jy (30mag AB) for the I, J, and H bands in only 1-2 hours. The K-band performance depends strongly on the thermal background and hence the ambient temperature, but is likely to be about 1mag less.
Advanced filters - including high throughput broad band filters and OH suppressing filters - will have a very significant impact on MICADO sensitivity. The prototype J-band filter pair that we have developed together with the Laser Zentrum Hannover increases the sensitivity in a given integration time by 0.3mag. More advanced design optimisation techniques could lead to a 0.5mag sensitivity gain in this band, and comparable gains may be expected for the I-band and H-band.
|Broadband imaging sensitivity of MICADO as a function of integration time|
The spectroscopic performance has been calculated for isolated point sources that are nmodded back and forth along a slit that is 8arcsec long and 12mas wide. Because of the unusually extreme core + halo shape of the adaptive optics PSF, this width maximises the signal-to-noise reached for point sources in the J and H bands. In the K-band, additional diffraction losses at the slit reduce the throughput slightly. The sensitivity calculation takes account of all effects (including the Strehl ratios predicted by MAORY, the limited coupling efficiency due to the PSF shape, diffraction losses at the slit, and the thermal background). The resulting 5sigma sensitivities are JAB=HAB=27.2mag between the OH lines in a 5 hour integration; and similarly KAB=25.7mag (which is, as before, less primarily due to the thermal background).
The table shows the predicted point sources sensitivites to 5sigma in a 5 hour integration:
|J (AB mag)||H (AB mag)||K (AB mag)|
|Imaging with advanced filters (estimated)||31.3||31.3||30.1|
|Spectroscopy (between OH)||26.7||27.2||27.2|