Since 1997, ESO has been developing the concept of a ground-based 100-m class optical telescope. It has been named OWL for its keen night vision and for being OverWhelmingly Large (showing either the hubris of astronomers or their distorted sense of humor). The challenge and the science potential are formidable and highly stimulating: a 100m diffraction-limited optical telescope would offer 40 times the collecting power of the whole VLT with the milliarcsecond imaging resolution of the VLTI.
A few principles, mostly borrowed from recent developments in the art of telescope making, hold the key to meet the difficult OWL requirements: optical segmentation as pioneered by the Keck, massive production of standardized mirrors from the Hobby-Eberly, active optics control and system aspects from the VLT. The most critical development is diffraction-limited imaging in "large" fields (a few arcminutes in the near-IR). This is the goal of the so-called multi-conjugate adaptive optics concept, whose principles and applications have been already demonstrated (ESO is building MAD, an MCAO demonstrator, to see first light at the VLT in 2006). Tremendous pressure is building to implement this capability into existing large telescopes, and rapid progress in the underlying technologies is taking place, e.g. fabrication of low-cost deformable mirrors with tens of thousands actuators from integrated circuits techniques.
The evolution of the OWL telescope design has been marked by a few key trade-offs and subsequent decisions: (i) Segmented primary and secondary mirrors, segment dimension < 2-m; (ii) Optical design, spherical primary mirror solution; (iii) Non-adaptive main telescope optics (2 mirrors in the corrector, however, have been identified as adaptive mirrors, conjugated at 0 and 8km) (iv) Implementation of active optics and field stabilization; (v) Alt-az mount. Several optical designs have been explored, from Ritchey-Chrétien solutions to siderostats with relatively slow primary mirrors. It has been found that cost, reliability, fabrication and telescopes functionality considerations point towards a spherical primary mirror solution. In terms of fabrication, all-identical spherical segments are ideally suited for mass-production and suitable fabrication processes recommended by potential suppliers are fully demonstrated. In a modified version, these processes could also be applied to aspheric segments, however with a substantially higher cost (more complex polishing, lower predictability, tighter material requirements).
The current baseline design is based on a spherical primary mirror solution, with flat secondary mirror and aspheric, active corrector (see Fig. 6). The flat secondary mirror also has a major advantage in terms of decenters (cm rather than ^.m), which are evidently crucial with a structure the size of OWL's. Several mount options have been assessed, from a classical alt-az solution to de-coupled primary and secondary mirror structures. Cost and performance considerations point clearly towards the alt-az solution (see observatory layout in Fig. 5). The current baseline design is modular i.e. the structure is made of (nearly) all-identical, pre-assembled modules. This crucial feature allows for major cost savings.
There is no provision for a co-rotating enclosure, the advantage of which is anyway dubious in view of the enormous opening such enclosure would have. Protection against adverse environmental conditions and excessive day-time heating would be ensured by a sliding hangar, whose dimensions may be unusual in astronomy but actually comparable to or lower than those of large movable enclosures built for a variety of applications. Mirror covers sliding into the structure would provide segments cleaning and handling facilities, and local air conditioning if required.
Relevant site aspects are more complex than with previous telescope generations, mainly because of adaptive optics, telescope size, and the higher impact of seismic activity on cost and safety. AO implies, in particular, that the function of merit of the atmosphere cannot be described by a single parameter (seeing). Better understanding of site quality in relation to climatology is also essential.
Cost estimates (more than half of which supported by industrial studies) indicate that the required capital investment could be around 1 billion Euros.
Compared to "classical" telescope cost factors, substantial cost reduction occurs with the main optics (fabrication processes adapted to mass-production), the telescope structure (very low mass in proportion to dimensions, mass-produced modules), and the enclosure (reduced functionality, no air conditioning). Schedule estimates indicate that technical first light could occur within 8-9 years after project funding. Allowing for 2.5 years integration and verification of the IR adaptive module(s) the telescope could already deliver science data in the IR within 11 years after project go-ahead, with unmatched resolution and collecting power. Full completion would occur about 15 years after project start.
OWL is undergoing conceptual design review at the end of 2005. The so-called OWL Blue Book is a report detailing the studies during this phase and will be publicly available after the review.
Several projects in the optical and near infrared, both in space and on the ground, are currently underway for deployment in the second decade of the third millennium. Their scientific goals open new and exciting avenues of research that are outside the reach of current instrumentation. The detection of earth-like exo-planets and possibly of biospheres, the study of the first building blocks of stars and galaxies and their evolution, the direct measure of the deceleration of the Universe, all have driven the technical requirements to unprecedented levels, and the sensitivity increase of the new generation of telescopes with respect to the previous one will be one never seen before in astronomy.
ELTs are what "ground-based" astronomers and engineers are working on now with a view to build (at least one of) them by the next decade. Although many technical challenges remain to be solved and funds need to be found, the possibility that OWL may become a reality is far more likely today than it was when the discussions started a few years ago. The industries that built the VLT have not found any obvious showstoppers, and even adaptive optics (which we consider a "go/no-go" milestone for the entire project) is developing at a pace that allows some cautious optimism. Ten years from now, turning OWL to some nearby earth-like planet, or to the far reaches of the Universe may not be as wild a dream as it was yesterday.
To achieve the science goals, OWL or any other ELT will of course need the most advanced, large, fast readout, extremely low noise detectors, at affordable prices (i.e. at least 10 times cheaper than today), and at the tune of several gigapixels per instrument.
Jim et al, your task for the next 10 years is set!
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