E-ELT Baseline Reference Design

Establishing the basic concept (Dec. 2005 - Nov. 2006)

The development of the Baseline Reference Design (BRD) of a 30 to 60m E-ELT started in December 2005. The basic goal was to define the best affordable ELT that can be built on a competitive time scale and with acceptable risks. The project is open to further international collaboration, but the intention was to define an ELT facility that could be handled at the European level alone, should no major partner be found.

Five mixed Community-ESO topical Working Groups were quickly formed, covering the main areas of Science Case, Instrumentation Design, Telescope Design, Adaptive Optics Systems and Site Evaluation. By early spring 2006 they had produced comprehensive reports, which together form a "toolbox" collating and synthesising present and projected ELT-related capabilities in their respective areas. These reports were distilled by a single "core" Working Group, producing the facility top-level requirements. They call for an instrumentation-friendly visible to mid-infrared collector with a superb image quality, a built-in adaptive optics corrector and a working field of up to 10' diameter. Special emphasis is put on getting a general-use facility with fast all-sky access and a fully flexible scheduling capability.

The BRD has been developed by the then (June 1st, 2006) newly created ESO E-ELT Project Office, still in close connection with the ESO community. It is in charge of the project and is getting advice from the ELT Scientific & Engineering (ESE) subcommittee established by the ESO STC. Furthermore, the ESO Council has established the ELT Standing Review Committee (ESRC) to oversee the whole project.

The main guiding principles towards the BRD were to:

Keep whenever possible the strong points associated with OWL, in particular cost containment through early association with industry and use of serial production;
Adopt an integrated system approach, in particular with a full-field Adaptive Optics corrector including laser guide stars for near full-sky coverage;
Develop as integral part of the facility a suite of instruments able to fulfil its most compelling science cases, and with fast instrument switching over to optimize the observing programme with respect to the current weather conditions;
Select a baseline design which minimises the technological risks, with appropriate backups and upgrades as needed;
Aggressively pursue the current extensive site evaluation programme with special attention given to start negotiations with the host country as early as possible.

E-ELT Structure

The two basic optical designs shown below have been carefully explored. Both hold a 42m F/1 aspheric primary mirror, cover a 10' diameter field and offer multiple Nasmyth foci for instrumentation. One (left), analogous to Euro50, features a 3-mirror combination with a concave (Gregorian) secondary mirror. The other (right) is a 5-mirror combination with a convex (Cassegrain) secondary mirror.

Detailed mechanical models for both approaches were developed and iteratively optimised. Their pointing performance (with realistic wind load) and telescope survival (to strong earthquakes) were established under closed-loop active optics operation. Detailed feasibility, costing and schedule for all critical components - especially the large internal deformable mirror needed for full-field guiding/imaging improvement - were obtained. Overall, the feasibility has been well established, with adequate performance, a global facility cost around 850 M€ (for a 42m primary) and a ~10-year design and construction schedule.

The full BRD was presented to the Community at large for feedback on November 29-30, 2006 at a dedicated Workshop Meeting in Marseille (France). Following hearty support by the Community, ESE/STC and ESRC, ESO Council unanimously decided in early December 2006 the start of the Detailed Design Phase for the whole facility.

A full description of the Baseline Reference Design can be found in this Messenger article.