Abstract. The Dutch Open Telescope (DOT) on La Palma is an innovative optical solar telescope capable of reaching 0.2 arcsec angular resolution over extended durations. The DOT presently progresses from technology testbed to a stable science configuration providing multi-wavelength imaging and multi-camera speckle data acquisition for tomographic mapping of the solar atmosphere. Large-volume speckle processing will soon enable frequent usage and community-wide time allocation, in particular for tandem operation with other solar telescopes pursuing spectropolarimetry and EUV imaging. We summarize the DOT hardware and software in the context of this increasing availability and outline the corresponding "open-DOT" program.

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The Dutch Open Telescope (DOT) is an innovative solar telescope, completely open, on an open steel tower, without a vacuum system. The aim is long-duration high resolution imaging and in order to achieve this the DOT is equipped with a diffraction limited imaging system in combination with a data acquisition system designed for use with the speckle masking reconstruction technique for removing atmospheric aberrations. Currently the DOT is being equipped with a multi-wavelength system forming a high-resolution tomographic imager of magnetic fine structure, topology and dynamics in the photosphere and low-and high chromosphere. Finally the system will contain 6 channels: G-band (430.5 nm), Ca II H (K) (396.8 nm), Hα (656.3 nm), Ba II (455.4 nm), and two continuum channels (432 and 651 nm). Two channels are in full operation now and observations show that the DOT produces real diffraction limited movies (with 0.2" resolution) over hours in G-band (430.5 nm) and continuum (432 nm).

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Double layers of high-tensioned textile membranes were applied to the completely open-foldable dome for the GREGOR telescope for the first time. Simultaneous climate measurements inside and outside the dome have proven the thermalinsulating capability of this double-layer construction. The GREGOR dome is the result of the continuation of the ESO research on open-foldable domes with textile structures, followed by the research for the DOT dome with high-tensioned textile membranes. It cleared the way to extreme stability required for astronomical practice on high mountain sites with heavy storms and ice formation. The storm Delta with 245 km/h 1-minute mean maximum at the location of the GREGOR caused no problems, nor did other storms afterwards. Opening and closing experiences up to wind speeds of 90 km/h were without problems. New technical developments were implemented and tested at the GREGOR dome, opening the way for application to much larger domes up to the 30 m diameter-class range.