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Session 102 - Hale Prize Lecture.
Invited session, Saturday, January 10
International Ballroom Center,

[102.01] In Pursuit of High Resolution in Solar Physics

R. B. Dunn (National Solar Observatory)

Current observations of tiny key processes on the Sun, for example, spicules, prominences, granulation, magnetic ropes, sunspots and flares lack the spatial resolution to support present theoretical models. Time series with better than 0.1 arcsecond resolution over a field of view on the order of 50 arcsecond are needed together with a wide range of narrow-band filter and spectrographic observations that define the temperature, magnetic and velocity fields.

It is extremely difficult to achieve the required telescopic resolution for making precision measurements of these tiny structures on the Sun that are so fundamental to Solar Physics. Even without the problems caused by turbulence in the Earth's atmosphere, solar telescopes rarely achieve diffraction-limited performance due to the heating of the optical system. In this talk I describe our attempts over the last 25 years to improve the performance of the vacuum tower telescope at Sac Peak. I discuss problems with the vacuum window, dome-caused turbulence, resolution tests, and active optics (deformable mirrors). With a perfect telescope (Strehl Ratio > 0.6) and short exposures ( 20ms) the "frame selection" technique can produce a promising time series. "Destretching" the selected frames improves the uniformity of the series.

Long exposures, for example, those required for spectroscopy of sub-arcsecond phenomena, require adaptive optics (AO). I have spent the past decade pursuing this field. I will discuss the problems that make solar AO so difficult, and how close we are to realizing a practical system.

Program listing for Saturday