The Large Earth-based Solar Telescope (LEST)

Existing telescopes cannot provide the 0.1'' resolution needed to measure directly such basic quantities as the strength of the highly concentrated and fragmented photospheric magnetic field. Both sufficient aperture and adaptive optics, in combination, are required to reach this goal from even the best ground-based sites. The United States, through a partnership between the National Solar Observatory, industry, and the Defense Department, has developed a world leadership position in the field of solar adaptive optics. Recently we have seen a significant advance in our ability to image the solar surface at close to the diffraction limit of moderate-aperture telescopes using this technique. This work, currently underway at a low level at the National Solar Observatory and Lockheed, should be vigorously pressed to an operational system at existing telescopes. Funding at a level of about $1M per annum for a few years should allow the speedy completion of an adaptive optics system on an existing telescope which is demonstrably adequate for a large-aperture solar telescope.

Following the successful completion of these developments, the United States should develop a moderate to large-aperture high-resolution telescope, jointly with other countries. The United States is a participant in the Large Earth-based Solar Telescope (LEST) consortium, which plans to build such a telescope starting in 1992, and pending the results of the adaptive optics program and of the LEST final design study, the United States should plan to contribute up to 1/3 of the cost of that telescope (expected cost for 1/3 participation: $1M design, $15M construction, $1M annual operation).

The LEST will be an ideal complement and long-lived extension to the Orbiting Solar Laboratory, a space mission for high-resolution solar studies in the optical, ultraviolet, and X-ray regions. (OSL is discussed in Section 3.3.1 below.) Whereas the hallmarks of OSL are uniquely high and uniform angular resolution, and unique access to the ultraviolet and X-ray regions, the hallmarks of LEST are large aperture, precision polarimetric capability, longevity, access to the near infrared (routinely out to 2.2 microns), and flexibility of instrumentation. The OSL will obtain visible light images and spectra in selected spectral lines, and continua with angular resolution and stability that could not be matched by any conceivable ground-based telescope. Because OSL is a free-flying telescope in a polar orbit, however, it will have no flexibility for changing its observing capabilities as new scientific and instrumental opportunities arise, and its lifetime (nominally three years, possibly longer) will of course not approach the 25-or-more year useful life of LEST. OSL's design precludes the aperture and extremely low polarization required for the most sensitive measurements of three-dimensional magnetic field structures: although OSL will obtain uniquely highly resolved data on relatively strong magnetic fields, a telescope like LEST, with its large aperture (2.4 times the aperture of OSL) and extremely low polarization, is necessary to measure the full vector magnetic field of weak features. The LEST will build upon results from OSL characterizing the fine-scaled magnetic flux distribution and evolution at the solar surface, to investigate the full (non-potential) vector magnetic fields and their specific implications for energy transport and heating of the outer atmosphere. With the capability to probe deep in the photosphere at the 1.6 micron opacity minimum, LEST will also provide complementary data to OSL about the nature of convection and its overshoot, and the magnetic fields entrained therein. It may be able to provide important helioseismology information as a follow-on to the GONG (and in space, SoHO experiments), using adaptive optics to provide high-resolution over small fields of view for tomography of subsurface structure. Other key research areas abound, and will evolve over the long lifetime of LEST.

LEST will be the only new large solar telescope built with US participation since the mid-70's. It will complement and extend the aging US national facilities (McMath telescope complex on Kitt Peak, built in 1960, vacuum telescope feed added in 1973, and the Sac Peak facilities, the most modern of which dates to the mid '60's). Aside from these national facilities, there are few large university solar telescopes, so that the LEST will be of great importance across the community. In particular, LEST is expected to play a significant role in the training of advanced students in solar research, with great benefits to the vitality of the U.S. solar research community.

The LEST telescope will be the premier solar telescope on the face of the Earth. As befits such an endeavor, it is receiving scientific and financial support from around the world. The LEST foundation contains nine member countries: Australia, Germany, Israel, Italy, Norway, Spain, Sweden, Switzerland, and the United States. The international community of scientists represented by these nations provides a critical number of capable solar researchers able to utilize fully the capabilities of LEST. At the same time, the capital and operating costs will be jointly borne by the member nations. Thus, the United States will gain access to this unique instrument by providing no more than one-third of the total costs. As already

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