Equipping the European Extremely Large Telescope

The UK expects to design and build one of the European Extremely Large Telescope's two first-light instruments.
15 May 2012
Isobel Hook

The European Extremely Large Telescope (E-ELT) will be the largest optical-IR telescope in the world, equipped with state-of-the-art instrumentation (see Figure 1). It will provide exquisitely sharp images of the universe in the optical and IR, with resolution close to its theoretical diffraction limit.

The combination of massive collecting area and image quality means this telescope will be capable of making significant observations ranging from the discovery and characterization of extrasolar planets to measurements of the internal motions of distant galaxies, or even of the expansion of the universe itself. The largest existing optical-IR telescopes, including the European Southern Observatory (ESO) Very Large Telescope, and the Gemini and Keck telescopes, have diameters of about 8--10m. The light collecting area of E-ELT will be 16--25 times larger than the current largest telescopes (area is proportional to diameter squared). In addition, the larger diameter of E-ELT means it can reach higher spatial resolution, about four to five times sharper than what can be obtained with existing instruments. The combination of collecting area and improved resolution is very powerful. The E-ELT will perform some types of observations more than 200 times faster than any existing telescope. We expect the E-ELT to see first light early in the next decade.

Figure 1. Artist's impression of the European Extremely Large Telescope (E-ELT). (Image courtesy European Southern Observatory, ESO.)

The telescope will sit on Cerro Armazones in Chile, an excellent astronomical site close to the existing ESO Very Large Telescope. The E-ELT project is also run by the ESO, and the two facilities will share some infrastructure.

The telescope is a novel five-mirror design with adaptive optics (AO) built into the main telescope optics. The basic optical design is that of a folded three-mirror anastigmat (see Figure 2). The fourth and fifth mirrors are both flat, and direct the beam to either of the Nasmyth foci along the elevation axis of the telescope. The 39m diameter primary mirror will be built up of almost 1000 hexagonal segments, each 1.4m across. The AO correction will be performed by rapid deformation of the fourth mirror by around 6000 actuators. In this way the telescope will compensate for blurring by the Earth's atmosphere.

Figure 2. The E-ELT's basic optical design is that of a folded three-mirror anastigmat, with two flat mirrors that direct the beam to the foci. (Image courtesy ESO.)

The recently completed construction proposal includes a roadmap for delivering the telescope's instrumentation suite. A pair of first-light instruments will consist of a near-IR imaging camera capable of fully exploiting the telescope's diffraction-limited images (provided by the telescope's AO system), and an optical-IR integral field spectrograph, capable of providing spectra of multiple positions in an object simultaneously. A third instrument following soon after will enable imaging and spectroscopy at mid-IR wavelengths, providing a broad balance of capabilities during the early life of the telescope. These first three instruments will allow the E-ELT to detect wavelengths from 0.5 to 14 microns.

The instrumentation suite will gradually build, with new instruments arriving approximately every two years. It will include a multi-object spectrograph, a high-resolution spectrograph, and a extrasolar planet-finding instrument with its own extreme-AO system. This will provide the very high order correction that is required to directly detect and characterize extra-solar planets, possibly even those similar to our own, in the vicinity of the much brighter parent stars.

The UK expects to lead the design and construction of one of the two first-light instruments, which will be based on the integral field spectrograph concept High Angular Resolution Monolithic Optical Near-infrared Integral field spectrograph (HARMONI) shown in Figure 3.1 This scientifically flexible instrument will work at both optical and IR wavelengths, and in the latter range will be able to exploit the diffraction-limited images provided by the telescope's AO system. The UK is also heavily involved in the design of the potential multi-object spectrographs for the E-ELT. The Extremely Large Telescope Adaptive optics for GaLaxy Evolution instrument (EAGLE) would provide the exciting possibility of simultaneous near-IR observations of 20 objects over a wide patrol field, each observed with integral field units that are individually deployable and corrected with AO.2 The open-loop, multi-object AO required for such an instrument is being tested now by the Canary project on the William Herschel Telescope in La Palma.3 A second, fiber-fed multi-object spectrograph concept is being developed, the OPTIcal Multi Object Spectrograph--Extreme Visual Explorer (OPTIMOS-EVE), also with strong UK involvement. This instrument would cover both optical and IR wavelengths and would make use of the telescope's own AO system.4 Initial investigations into whether EAGLE and OPTIMOS-EVE, which are both multi-object spectrographs, could be merged are under way.

Figure 3. Concept design for the High Angular Resolution Monolithic Optical Near-infrared Integral field (HARMONI) spectrograph, showing the inside of the cryostat.1 This design will form the basis for one of the first-light instruments on the E-ELT. (Image courtesy HARMONI consortium.)

In September 2011 the UK's Science and Technology Facilities Council announced funding of $5.6 million (3.5 million pounds) over two years to support the UK's leading role in E-ELT instrumentation. A dedicated UK E-ELT Project Office manages the work, liases with industry, and works to inform and prepare the scientific community for use of the E-ELT.

The E-ELT Project Office has completed the detailed design and submitted a construction proposal to ESO's governing body. The full construction proposal and an executive summary can be found on the ESO's website.5 Funding was approved in December 2011 for preparatory construction, including design of the road to the site, and studies for the adaptive mirror. Formal approval of the roughly $1.3 billion (e1 billion) project by the ESO Council is expected later this year.

The E-ELT project status and its instrumentation will be presented and discussed at the SPIE conference Astronomical Telescopes and Instrumentation 2012 from 1 to 6 July and at a special session of the European Week of Astronomy and Space Science the same week. A dedicated ESO conference, Shaping E-ELT Science and Instrumentation, will be held in February 2013.

Isobel Hook
University of Oxford
Oxford, United Kingdom
National Institute for Astrophysics (INAF) Observatory of Rome
Rome, Italy

Isobel Hook chaired the E-ELT science working group and is the UK E-ELT project scientist. She holds a joint position between the University of Oxford and INAF Observatory of Rome.

1. N. Thatte, S. Arribas, M. Tecza, T. Goodsall, F. Clarke, R. L. Davies, R. Bacon, Expected performance and simulated observations of the instrument HARMONI at the European Extremely Large Telescope (E-ELT), Proc. SPIE 7735, p. 77355H, 2010. doi:10.1117/12.857548
2. J.-G. Cuby, S. Morris, T. Fusco, EAGLE: a MOAO fed multi-IFU NIR workhorse for E-ELT, Proc. SPIE, p. 77352D, 2010. doi:10.1117/12.856820
3. R. Myers, Progress in adaptive optics, SPIE Newsroom, 2010. doi:10.1117/2.1201005.002953
4. R. Navarro, F. Chemla, P. Bonifacio, H. Flores, I. Guinouard, J.-M. Huet, M. Puech, Project overview of OPTIMOS-EVE: the fibre-fed multi-object spectrograph for the E-ELT, Proc. SPIE 7735, p. 77352L, 2010. doi:10.1117/12.857638
5. http://www.eso.org/sci/facilities/eelt/ ESO's construction plans and timeline for the E-ELT. Accessed 12 April 2012.