Aperture increase options for the Dutch Open Telescope

This paper is an invitation to the international community to participate in the usage and a substantial upgrade of the Dutch Open Telescope on La Palma (DOT, http://dot.astro.uu.nl). We first give a brief overview of the approach, design, and current science capabilities of the DOT. It became a successful 0.2-arcsec-resolution solar movie producer through its combination of (i) an excellent site, (ii) effective wind flushing through the fully open design and construction of both the 45-cm telescope and the 15-m support tower, (iii) special designs which produce extraordinary pointing stability of the tower, equatorial mount, and telescope, (iv) simple and excellent optics with minimum wavefront distortion, and (v) largevolume speckle reconstruction including narrow-band processing. The DOT’s multi-camera multi-wavelength speckle imaging system samples the solar photosphere and chromosphere simultaneously in various optical continua, the G band, Ca IIH (tunable throughout the blue wing), and Hα (tunable throughout the line). The resulting DOT data sets are all public. The DOT database (http://dotdb.phys.uu.nl/DOT) now contains many tomographic image sequences with 0.2-0.3 arcsec resolution and up to multi-hour duration. You are welcome to pull them over for analysis. The main part of this contribution outlines DOT upgrade designs implementing larger aperture. The motivation for aperture increase is the recognition that optical solar physics needs the substantially larger telescope apertures that became useful with the advent of adaptive optics and viable through the DOT’s open principle, both for photospheric polarimetry at high resolution and high sensitivity and for chromospheric fine-structure diagnosis at high cadence and full spectral sampling. Our upgrade designs for the DOT are presented in an incremental sequence of five options of which the simplest (Option I) achieves 1.4 m aperture using the present tower, mount, fold-away canopy, and multi-wavelength speckle imaging and processing systems. The most advanced (Option V) offers unblocked 2.5 m aperture in an off-axis design with a large canopy, a wide 30-m high support tower, and image transfer to a groundbased optics lab for advanced instrumentation. All five designs employ adaptive optics. The important advantages of fully open, wind-transparent and wind-flushed structure, polarimetric constancy, and absence of primary-image rotation remain. All designs are relatively cheap through re-using as much of the existing DOT hardware as possible. Realization of an upgrade requires external partnership(s). This report about DOT upgrade options therefore serves also as initial documentation for potential partners.