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MICADO: the E-ELT Adaptive Optics Imaging Camera
R. Davies, MICADO Team
TL;DR
MICADO addresses the need for diffraction-limited, wide-field near-infrared observations on the E-ELT, with capabilities shaped by diverse science drivers. It combines MAORY-optimized imaging and early SCAO operation with astrometry and spectroscopy, reaching reported sensitivities of 30 mag AB in I–H in 1–2 hours and JAB=HAB=27.2 mag in 5 hours. The design is low-risk, while some performance-enhancing developments remain future options.
Problem
MICADO is designed to provide diffraction-limited, wide-field imaging and complementary capabilities for E-ELT science cases spanning galaxy evolution, astrometry, and spectroscopy.
Method
The paper presents a compact camera optimized for MAORY, with an early-operation SCAO module, stable imaging arms, precision-astrometry features, and a high-throughput slit spectrometer.
Results
MICADO is projected to reach 30 mag AB in I–H in 1–2 hours, with 5σ spectroscopic sensitivities of JAB=HAB=27.2 mag and KAB=25.7 mag in 5 hours.
Takeaways & Limitations
The combined resolution, sensitivity, field, astrometric precision, and spectroscopy enable studies of distant galaxies, resolved stellar populations, compact objects, and dynamical mass distributions.
Takeaways & Limitations
Future developments such as advanced filters could improve competitiveness, but none is required for successful MICADO operation.
Abstract
from arXiv · showhide
MICADO is the adaptive optics imaging camera for the E-ELT. It has been designed and optimised to be mounted to the LGS-MCAO system MAORY, and will provide diffraction limited imaging over a wide (about 1 arcmin) field of view. For initial operations, it can also be used with its own simpler AO module that provides on-axis diffraction limited performance using natural guide stars. We discuss the instrument's key capabilities and expected performance, and show how the science drivers have shaped its design. We outline the technical concept, from the opto-mechanical design to operations and data processing. We describe the AO module, summarise the instrument performance, and indicate some possible future developments.
1. MICADO OVERVIEW
MICADO is an adaptive-optics imaging camera for the E-ELT, optimized for MAORY while supporting simpler natural-guide-star AO during early operations. Its compact, gravity-invariant design combines wide-field imaging with flexible auxiliary modes.
- AO integration: MICADO is optimized for MAORY but includes a separate SCAO module using natural guide stars for early diffraction-limited operations.The SCAO and MAORY interfaces also allow use with other AO systems as they become available.
- Primary imaging: 53′′ diffraction-limited imaging is provided through selected wide- and narrow-band near-infrared filters.
- Instrument concept: The compact camera is supported beneath the AO systems and rotates in a gravity-invariant orientation.
- Primary imaging: The primary arm uses a fixed-mirror 3 mas pixel scale for high-throughput imaging and optimized astrometric stability.
- Auxiliary arm: The auxiliary arm offers 1.5 mas imaging over a smaller field and 4 mas medium-resolution long-slit spectroscopy.
2. KEY CAPABILITIES AND SCIENCE DRIVERS
MICADO’s science drivers combine diffraction-limited sensitivity and resolution with precision astrometry and complementary spectroscopy. These capabilities target crowded stellar systems, galaxy evolution, and dynamical measurements across astrophysical environments.
- Science drivers: The science cases span high-redshift galaxy assembly, stellar-population histories, Galactic Center dynamics, black holes, and dark matter.
- 2.1 Sensitivity and Resolution: Diffraction-limited 6–10 mas resolution, over 60% throughput, and a nearly 1 arcmin field support sensitive, multiplexed observations of crowded and distant systems.At 1–2 µm, sensitivity for isolated point sources is comparable to or can surpass JWST.
- 2.2 Precision Astrometry: Fixed mirrors, gravity-invariant rotation, and HAWAII-4RG detectors make MICADO suited to precision astrometry.
- 2.2 Precision Astrometry: 40 µas single-epoch accuracy is achievable, while 10 µas yr−1 proper motions may be measured after 3–4 years.The latter corresponds to 5 km s−1 at 100 kpc.
- 2.3 High Throughput Spectroscopy: A high-throughput slit spectrometer with R ∼3000 complements imaging by obtaining spectra of compact objects between near-infrared OH lines.
3. TECHNICAL DESIGN CONCEPT
MICADO’s technical design prioritizes a compact, robust, stable instrument architecture optimized for MAORY, with flexible imaging, spectroscopy, cryogenic, and data-processing subsystems.
- The design is simple, compact, and robust, minimizing cost and schedule risks.
- Optics: MICADO’s three optical subsystems are the common path, primary arm, and auxiliary arm.The common path includes a tunable atmospheric dispersion corrector.
- Detectors: HAWAII-4RG detectors support large-format imaging, astrometry, adjustable readout, and reduced saturation impacts.
- Detectors: The curved MCAO focal plane tilts by 4.1° with a 1500 mm radius, yet the Strehl ratio remains above 88% across the field at 0.8 µm.The ±0.32 mm focal-plane mismatch across flat detectors has only a minor impact on spot diagrams.
- Cryogenics: Continuous-flow liquid nitrogen cooling avoids cryocooler vibrations and requires an estimated 1000 L for cooldown and 72 L/day during steady state.The reversible scheme cools the heat shield, optical bench, and detectors in sequence.
- Data processing: Data processing combines calibration, archiving, analysis, and global astrometric and photometric correction, with data rates up to about 6 TB per night.
4. ADAPTIVE OPTICS
MICADO is optimized for MAORY’s wide-field MCAO but includes a simpler natural-guide-star SCAO module for early operations and on-axis diffraction-limited imaging.
- MAORY provides wide-field diffraction-limited performance using multiple lasers and natural guide stars, while MICADO’s SCAO module mitigates early-operation risk.The SCAO option enables diffraction-limited imaging before the more complex AO system is available.
- SCAO concept: SCAO uses a single natural guide star as the wavefront reference, with sufficient nearby science targets for 2–3 years of operation.
- SCAO requirements: The SCAO interfaces match MAORY, with a 0.45–0.8 µm WFS bandpass, a 45′′ patrol field, and a 27′′×27′′ transmitted science field.
- SCAO implementation: The SCAO optical relay includes an Offner relay, folding mirror, dichroic, field derotator, support structure, and wavefront-sensing optics.
- Performance estimation: SCAO performance estimates combine analytical error formulae, ESO information, and two purpose-built simulation tools.
- Performance estimation: Strehl-ratio estimates are obtained by multiplying the tabulated guide-star and anisoplanatic factors for a given magnitude and off-axis distance.
5. OPERATION AND CALIBRATION
MICADO operations use standard imaging and spectroscopic observing patterns, while calibration and reduction workflows add specialized support for astrometry and photometry.
- Operations: Imaging uses dithered exposures or sky offsets, spectroscopy uses nodding along the slit, and typical exposures range from seconds to tens of seconds.
- Calibration: Most calibrations are performed internally during the day, with twilight flats correcting illumination gradients and a special mask correcting AO-system and instrument distortions.
- Astrometry: Relative astrometric precision of about 40 µas is projected for the E-ELT from 200–300 µas achieved on an 8 m-class telescope.
- Astrometry: Astrometric calibration requires controlling ten error sources and carefully measuring instrumental effects.
- Photometry: Accurate photometry can be derived from the data itself over small fields, but larger MCAO fields require stitched sub-fields or spatially variable-PSF tools.
- Sensitivity: MICADO’s sensitivity is presented as a function of integration time across standard broad-band filters, with reference points for 5-hour integrations.
6. PERFORMANCE
MICADO’s predicted performance combines deep broadband near-infrared imaging with sensitive long-slit spectroscopy, while K-band results are limited primarily by thermal background.
- 30 mag AB: 5σ broadband sensitivity in I–H is expected in 1–2 hours for isolated point sources.K-band performance is likely about 1 mag less sensitive because of thermal background.
- 0.3 mag: a prototype J-band filter pair increases sensitivity at a given integration time.Further optimisation could yield a 0.5 mag gain in J-band.
- JAB=HAB=27.2 mag: predicted 5σ spectroscopic sensitivity between OH lines in 5 hours.The calculation includes AO Strehl ratios, PSF coupling, slit diffraction losses, and thermal background.
- KAB=25.7 mag: predicted 5σ spectroscopic sensitivity between OH lines in 5 hours.K-band sensitivity is lower primarily because of thermal background.
7. TECHNOLOGICAL DEVELOPMENTS AND RISKS
MICADO is designed as a low-risk, simple camera, while proposed technological developments could improve its competitiveness through greater sensitivity, emission-line imaging flexibility, and high-time-resolution observations.
- RISKS: No technical or programmatic risks above low level appear in the preliminary risk register.The remaining low-level risks are common to cryogenic instruments rather than specific to MICADO.
- FUTURE DEVELOPMENTS: None of the proposed future developments is required for MICADO to function successfully, but each could increase its competitiveness.
- TECHNOLOGICAL DEVELOPMENTS: Advanced filters could substantially improve near-infrared sensitivity by suppressing or avoiding sky emission lines.A prototype J-band pair uses opposite-side coatings and achieves >95% throughput.
- TECHNOLOGICAL DEVELOPMENTS: A dual imager based on Fabry-Perot etalons could provide higher-quality, higher-resolution emission-line images over a larger field.Simultaneous line and continuum imaging would avoid subtraction problems caused by variable seeing or AO performance.
- TECHNOLOGICAL DEVELOPMENTS: High-time-resolution astronomy could be added with fast detectors for applications including neutron stars, accretion disks, pulsar magnetospheres, and transients.Available detector technology covers 0.8–1.2 µm, with expected extension toward 2 µm.