Astron - MIRI

- Design overview
- System specifications
- Technical documentation

:: Design overview

Since 1999, various concept studies for the instrument have been carried out sponsored by ESA and NASA (see reports by Wright et al. 1999, Serabyn et al. 1999, MISC report). The earlier studies had 4–6 modules in the instrument, with two modules each for the camera and spectroscopy channels to accomodate different plate scales and cope with the different background levels.

The most recent studies focussed on simplification and lightweighting of the instrument; the minimum design that satisfies all the science goals has just one module each for the camera and spectrometer, plus a module containing the common fore-optics, with a minimum number of mechanisms. For the spectroscopy channel, both traditional long-slit designs and more innovative integral-field units (IFU) have been considered. From the science point of view, an IFU is preferred and is endorsed by the MISC report and incorporated in the current design baseline.

Scrall image to identify elements or click to enlarge.
+ Structural concept of the OM: view

An optical pre-design study of this system has been made, primarily at Service d'Astrophysique (SAP, Saclay, France) for the camera subsystem and at the UK Astronomy Technology Centre (ATC, Royal Observatory, Edingburgh, UK) for the spectrograph. All optical elements (except filters and grisms) are reflective.

Camera
This system not only provides the basic imaging mode, but also the coronographic mode and the low-resolution R ~ 100 spectroscopy. The latter mode is fed from the camera optics and uses low-dispersion grisms. All three modes make use of the same high flux level detector; in the present design this is a single 1024 x 1024 (1K x 1K) array of 27 x 27 µm2 pixel size with a single pixel field of view of 0.1'', chosen to be Nyquist sampled at ~8 µm.

Spectrometer
This module provides 'high-resolution' spectroscopy in integral-field mode, using 4 parallel wavelength channels. The spectrometer has its own low flux level detectors. Most of the Dutch effort will go in the design, building of this spectrometer sub-instrument.
See bloc diagram.

The studies and the current design have drawn heavily on the European experience in building mid-infrared instruments for ISO, Herschel and for ground-based telescopes such as the VLT (VISIR), UKIRT (image-slicer) and Gemini (MICHELLE). In fact, most of the elements have already been incorporated in a similar form in other instruments. Thus, the risk in the design and development is minimized in this approach.

The heart of the instrument is formed by the detectors. Here the NGST project profits heavily from the detector development for SIRTF, which has produced 256 x 256 pixel formats with high quantum efficiency and encouragingly low dark currents. The NASA-JWST project has funded development to extend the detector device format to 1024 x 1024 pixels, with the aim to also bring down the read-noise and dark currents to levels such that JWST is background limited at low spectral resolution as short as 6 µm.

Thanks to these rapid developments in mid-infrared detector arrays, the enormous jump forward in sensitivity and observing speed is possible; indeed, there are few other wavelength ranges where such large improvements can still be achieved. The detectors for the camera and spectrometer will be procured by the US part of the consortium.

For optimal performance and reduction of thermal background emission, the optics should be as cold as possible ( <24 K), with an enclosure temperature of ~18 K and the detectors themselves cooled to < 7.5 K. This will be accomplished by a solid-hydrogen cryostat, to be procured by ESA.

Webmaster: voegele@astron.nl
ASTRON's MIRI project pages (password required)