instrumented in the world. In addition, the patrol telescope at the Hilltop Facility provides important diachronic observations of the solar disk.

Probing the motions and magnetic fields of the Sun at the smallest scale allowed by atmospheric seeing, the Sacramento Peak Vacuum Tower Telescope (Sac Peak VTT) is the premier U.S. instrument for solar high-resolution imaging and spectroscopy. Put into operation in 1969, the VTT has a telescope aperture of 0.76 m stopped down from the main mirror of 1.6 m in diameter to allow using all the sunlight through a window at the upper end of an evacuated telescopic column. The evacuated interior of the VTT eliminates internal seeing problems, but at the cost of the polarization limitations imposed by a window under fairly heavy stress. The VTT has the largest aperture and best intrinsic resolution of any vacuum telescope in the world: 0.14 arc-seconds (″) at 0.5 microns (µm) and 0.4″ at 1.5 µm.2 However, one should also note that sites such as Big Bear Lake in California and the Canary Islands enjoy more hours of good-quality seeing than Sacramento Peak.

For many years, the Sac Peak VTT has been a leading facility in high-resolution studies of small-scale activity. The first pictures of the “filigree,” representing magnetic fibrils, were produced there. The VTT is also host to the High Altitude Observatory (HAO) Advanced Stokes Polarimeter (ASP). The ASP measures the full-vector magnetic field at several heights in the solar atmosphere with high-angular resolution and is the first instrument to deliver quantitative information on the vector magnetic field that is limited primarily by the ability to resolve solar features observed through Earth's atmosphere. Its vector magnetograms and coincident Dopplergrams have a sensitivity that will not be substantially surpassed until the deployment of the recently approved SOLIS instrument (discussed below). The task group believes that the crucial role of the VTT in the immediate future, besides continuing its ongoing studies, will be the development of adaptive optics for an Advanced Solar Telescope (AST). As discussed in Chapter 3, the goal of preliminary adaptive optics work at the VTT would be to demonstrate diffraction-limited operation when the atmospheric seeing is in the 0.5-arc-second range.

NSO Kitt Peak

NSO/KP is the site for the Vacuum Telescope and the McMath-Pierce telescope. As discussed below, NSO/KP has unique capabilities to conduct high-resolution observations in the infrared (IR).

Vacuum Telescope: The Vacuum Telescope at Kitt Peak is dedicated to obtaining daily full-disk, seeing-limited observations of solar magnetic fields (magnetograms) and daily full-disk observations of the helium chromosphere (He I 1083 nm spectroheliograms), which are widely distributed in near-real time and archived in a permanent record.

McMath-Pierce Telescope: The McMath-Pierce Telescope at Kitt Peak, put into operation in 1961, is the world's largest solar telescope, with an aperture of 1.5 m. Its


In terms of operational vacuum telescopes, the Sac Peak/VTT is the largest and best high-resolution telescope in the world. THEMIS, on the Canary Islands, is a 90-cm vacuum telescope, but its current image quality is poor as something is apparently wrong with the figure of the mirror.

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