The visible tunable filtergraph for the ATST

The Kiepenheuer-Institut will develop for the Advanced Technology Solar Telescope (ATST) a narrowband tunable filter system (Visible Tunable Filter, VTF) for imaging spectroscopy and spectropolarimetry based on large-format Fabry Perot interferometers. A major challenge for the realization of this instrument is the development of large-format Fabry-Perots with a free aperture of about 250 mm. The instrument will operate in the spectral range between 500 and 900 nm with access to a host of magnetically sensitive lines. The instrument is designed to match the diffraction limit of the 4m-aperture ATST and will be able to observe processes on the sun at spatial scales of 35 km. Its multi-line capability, together with a field of view of one arc minute, and the ability to measure polarization states of the incoming light allow to probe different layers of the solar atmosphere within a couple of seconds. The instrument is capable to vary the spectral sampling, the integration time, and the temporal cadence over a wide range without changing or compromising the opto-mechanical setup. This versatility gives unique possibilities to apply different measurement schemes to a variety of science questions. The ATST is a fully funded US project, with the VTF as the only non-US contribution, and is ready to start construction at the Haleakala summit. The VTF is foreseen as one of the ATST's first-light instruments and should become operational in 2018.