OWL's objective is to provide both resolution and sensitivity in a reliable and cost-effective manner. Individual science objectives generally imply specific requirements, from which system ones are derived. Because of the diversity of applications and the limitations of technology, conflicting requirements must be addressed by suitable trade-offs. As all scientific objectives benefit from an increased aperture size, and because the telescope size and cost are closely related, constraints which would imply overcosts must be wighted against a decrease of telescope diameter. This also applies to telescope engineering. Performance cannot be measured in technical terms only; it must encompass cost, leadtime, and risks.

OWL incorporates key fabrication technologies and system solutions that are the trademarks of the current generation of large, 8- to 10-m telescopes: active optics (NTT, VLT, Subaru, Gemini); optical segmentation (Keck, Hobby-Eberly, GTC, SALT), low-cost optics and structures (Hobby-Eberly, SALT). The only feature which truly extrapolates from state-of-the-art technology is adaptive optics.

The overall performance of a telescope depends not only on the quality of its design and construction. Technical and scientific operations must be taken into account right from the initial design phases, with a view to maximizing uptime and providing transparent operation. The user is ultimately interested in science, not technology.

For reasons of cost and ease of integration and maintenance, OWL design relies extensively on serially produced components and modules

OWL's dimensions allow to rely extensively on modular design, integration and maintenance, with large numbers of identical parts, components and modules. As a result, construction and operation costs are very low for a system of this size and capability, and mutliple supply and integration lines allow a fast construction schedule. Virtually all telescope parts can be shipped to the site in standard 40ft containers.

All major dimensions are integer multiple of the segments size, thereby simplifying interfaces and allowing a distributed transfer of loads from the optics through substructures, to the main structure and eventually to the foundations.

A byproduct of segmentation is that once the enclosure, the telescope structure, kinematics, and control systems are functional, the telescope can become operational even though its primary mirror is not yet entirely filled. Existing segmented telescopes have indeed been turned to the sky before full completion, albeit for engineering runs only. Those have, however, no more than 36 segments in total -compared to 3048 for OWL. Optical fabrication is therefore no longer on the critical path and engineering work will be essentially completed at an early stage. The telescope will progressively enter into science operation when its aperture will have reached 60m. At that point, OWL will be equipped with its first stage, first generation adaptive optics for the compensation of atmospheric turbulence, and thus be able to deliver unmatched scientific results already. According to conservative estimates, this could occur around 2017, with the telescope being completed by 2021. A faster schedule would be possible, subject to early funding.

Layout of the OWL optical design, with detailed view of the corrector (to the right). Owl primary mirror would be made of 3048 segments

The baseline design is comparable to that of the Hobbby-Eberly telescope, the main difference being a secondary, segmented flat folding mirror allowing a shorter structure. The 100-m primary mirror will be made of 3048 heaxagonal, all-identical segments, each 1.6-m in size. The 25.6-m secondary mirror will be made of 216 flat segments, also 1.6-m large. The segments will be made of low expansion glass-ceramics or silicon carbide.

Spherical and field aberrations are compensated by means of a 4-elements corrector, conveniently located mid-way in the telescope structure. The corrector includes two flexible eight-meter class active mirrors (similar to the VLT primary mirror), a 4.2-m focusing and a 2.35 flat, fast steering adaptive mirror for first stage adaptive correction. The 4.2-m mirror will eventually be replaced by an adaptive one, thereby permitting the compensation of atmospheric turbulence over larger fields.

A crucial property of the design is that, contrarily to classical designs, tight tolerances for alignment do not apply to the (flat) secondary mirror, where the structural flexures will need to be in the centimeter range -a figure deemed excellent for a steel structure of OWL size. Instead, tight tolerances apply to the alignment of surfaces inside the corrector, a subsystem of large but still much more manageable size and mass.

Design optimization is already quite advanced, with much higher stiffness and lower mass than initially thought.

The telescope will be mounted on friction drives (bogies) "rolling" on steel tracks. A large (~300) number of bogies will be used to distribute loads; each bogie will be fitted with a standard motor. The drives will be hydraulically connected to allow periodic (e.g. seasonal) re-adjustments.

.	Open air operation is ideal for low thermal turbulence but requires active compensation of the effect of dynamic wind pressure.
	Keeping OWL 3264 segments aligned to a few millionths of a millimeter is a daunting but not impossible task (artist view, courtesy Ciel et Espace)

Modern telescopes are controlled opto-mechanical systems whose optical characteristics are optimized in real time by updating dedicated active subsystems, on the basis of error signals measured by suitable metrology. Active optics, pioneered by ESO in the 1980s with its 3.5m New Technology Telescope, then with the 8-m VLT, is now a classical example of such controlled system.

The conceptually simplest control system is that ensuring that the telescope tracks i.e. that the effect of earth rotation is properly compensated by the kinematics of the telescope. With increasing size and tighter requirements, other control systems must be implemented as the telescope can no longer retain its properties passively. Permanent re-alignment and focusing, fast correction of minute tracking errors, compensation of residual fabrication errors and slowly varying flexures of optical surfaces were implemented in the active optics of the VLT, with great success. In a segmented telescope, individual segments must, in addition, be actively supported and kept aligned within extreme tolerances. The Keck telescope was the first segmented one to enter into operation (1994); the technique is proven, relies on fast evolving electro-mechanical technologies, and is highly scalable.

The most challenging system is adaptive optics, which compensates light wave distrubances generated by atmospheric turbulence. To this end, a metrology system (wavefront sensor) located at the focus of the telescope measures these disturbances, and commands the shape of a moderately sized, thin deformable mirror supported by thousands of fast actuators. The correction must be repeated every few milliseconds, as turbulence changes quickly. Bar unexpected technology development, OWL's first generation of adaptive systems will work in the infrared, where the correction requires a lower number of degrees of freedom (actuators). At shorter wavelength, it will provide partial correction only -except, perhaps, under conditions of very low turbulence. A second, smaller but more complex stage will be subsequently implemented for enhanced correction.