M. Colleen Gino

Introduction

While the concept of a huge mirror as wide as an American football field supported by a beamed structure half as tall as the Eiffel Tower and housed in a roll-top enclosure larger than a dirigible hanger may sound like somebody's pipe dream, plans are in the making for just such a telescope. Dubbed OWL ("for its keen night vision and for OverWhelmingly Large," according to the concept study ⁽¹⁾), the project participants have been conducting a series of feasibility studies since 1998, and expect to have decided on the primary design elements by the year 2002. The question on everyone's mind is this: can an affordable 100-meter telescope be built using existing technology? While some in the astronomical community are skeptical, the members of the OWL project team give the answer as a resounding yes!

Background

During the 20^th century, every several decades saw the next generation of optical telescopes double in size. From the 100" (2.5m) Hooker telescope at Mt. Wilson constructed in 1917, to the 200" (5m) Hale telescope at Palomar completed in 1949, to the 10m (390") Keck telescopes on Mauna Kea, the first of which was operational in 1993, each increase of aperture was limited by the existing technology of the day and ultimately cost concerns. According to Roberto Gilmozzi of the European Southern Observatory (ESO), while the mirror size merely doubled, the cost increased six-fold. Were this rule to hold true today, a telescope with a 100-meter mirror would have a price tag of $20 – 30 billion attached to it. However, Gilmozzi, one of the strongest proponents of the OWL project, believes that he and his colleagues can build one for a mere one billion dollars.

How is this possible? The telescopes constructed in the first half of the 20^th century were of the same general design. For example, the 60", 100" and 200" reflectors built by George Ellery Hale between 1908 and 1949 were monolithic mirrors mounted in similarly constructed tube assemblies that were simply scaled up in size. In contrast, the latest generation of large telescopes have introduced totally new design concepts: Keck’s segmented primary mirror, active optics used by both Gemini and Subaru, and adaptive optics systems in use on the 100" Hooker Telescope, the Canada France Hawaii Telescope (CFHT), and Keck, among others.

Existing Technology

With our existing technology and materials, producing a 100-meter monolithic mirror is impossible. Therefore, the OWL telescope will utilize a segmented primary mirror. The initial proposal calls for 2000 2.3-meter segments. The production of the segments would be reminiscent of an assembly line, with mirror segments continually in the process of being poured, cooled, ceramitized, inspected, cut to size and shaped, then shipped out. To complete production of 2000 segments in 10 years, 1.3 segments must be produced each day, based on a 250 working day year ⁽²⁾. This type of mass production would help to keep costs low. To aid in this assembly line model, a spherical shape will likely be chosen for the segments, as it will make for quicker and easier (and therefore cheaper) production. The disadvantage to spherical mirrors, however, is the introduction of spherical aberration. This distortion of the images produced by the telescope can be corrected for with the use of additional mirrors, although this adds a layer of complexity to the optical system and tends to be problematic.

Regardless of the shape, a segmented mirror introduces another set of challenges. Keeping the individual mirrors in proper alignment with one another requires an extremely complex support structure. The overall figure or shape of the mirror must remain accurate to within a few nanometers. This becomes increasingly difficult as variations in temperature may cause individual segments to expand or contract to different degrees, thereby degrading the figure of the mirror ⁽³⁾. Distortion can also occur due to the effects of gravity as the telescope is moved in altitude. To counteract these negative effects, in addition to an adequate support structure an active optics system must be utilized. With active optics, the mirror segments will be under computer control, and their positions can be independently adjusted as necessary in order to maintain an accurate figure.

A large telescope magnifies not only the image of the observed object, but the blurring effects of atmospheric turbulence as well. When light from a celestial object passes through our atmosphere it is refracted by pockets of warm and cold air and the resulting image is distorted. Adaptive Optics (AO) is a means by which the incoming planar wavefront of light which becomes distorted by its passage through the atmosphere, can be once again made planar ⁽³⁾. An AO system employs a wavefront reconstructor to accomplish this task. The wavefront reconstructor consists of a high speed camera that analyzes the structure of the distorted wavefront. This is used to provide a list of electronic corrections that are passed on to a deformable mirror. The deformable mirror is then commanded into a shape that matches that of the distorted wavefront. In this manner, the distortions in the wavefront are cancelled out, and the wavefront is returned to the planar shape it had before passing through the atmosphere.

The current generation of AO systems are not adequate to handle the corrections that would be needed for an image produced by a 100-meter mirror due to its large size. However, research is commencing on a more advanced technique known as multiconjugate adaptive optics, which uses the light from not one but several stars to produce a three-dimensional map of the turbulence ⁽⁵⁾. In this way, corrections can be applied to a larger area of the sky, comparable in size to the field of view attained by a 100-meter instrument. As an added improvement, these AO systems will employ the use of lasers rather than stars to make the optical corrections. This will allow astronomers to image any area of the sky, rather than being limited to an area of the sky containing some number of appropriate stars in the field of view. A multiconjugate adaptive optics system such as the laser guide star systems currently in development at the Air Force Research Laboratory at Kirtland Air Force Base will be a necessity for a 100-meter telescope.

Conclusion

The construction of a 100-meter optical telescope is feasible using existing technology. Mass production of both optical and structural components will decrease the cost of the instrument to an affordable amount. While many challenges still exist, particularly in the field of adaptive optics, given the current rate of advancements in the field, the technology should be available by the time it would actually be put to use. If ESO realizes their dream, with the construction of OWL the trend of telescope aperture doubling that defined the 20^th century will be broken, and the 21^st century will see the next generation of ground-based optical telescopes grow by an order of magnitude.

References:

(2), (3) Gilmozzi, R., et al, 1998,The Future of Filled Aperture Telescopes: Is a 110m Feasible?, To be published in Advanced Technology Optical/IR Telescopes VI, SPIE 3332