Multi-wavelength imaging system for the Dutch Open Telescope

Abstract

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).

Tunable H-alpha Lyot filter with advanced servo system and image processing: instrument design and new scientific results with the Dutch Open Telescope

“…The cameras observe in the above mentioned G-band, two continuum channels (432 and 655 nm) and Ca II H (396.8 nm). Recently H-alpha (656.3 nm), being the topic of this paper, is added and soon Ba-II (455.4nm) will follow 1, 3,13 . The channels are fed by dichroic splitters and all optical components are diffraction limited and precisely aligned.…”

“…This is corrected within each camera channel separately by use of meniscus lenses. A detailed drawing of the complete multi-wavelength-system is printed in 3 . The present DOT camera field measures 116" over the diagonal, as determined by the effective focal length (19.75 m) and CCD dimensions.…”

“…The DOT optics, described in detail in 3 and schematically shown in Figure 2, consist of a 450mm F/4.44 parabolic mirror with behind the primary focus a lens system which enlarges the primary image 10x and which compensates for coma (as introduced by the parabolic primary). In the primary focus a reflective and water-cooled field-stop is located.…”

“…The telescope is equatorial, built very compact and rigidly, with the primary mirror lifted above the declination axis during observations, with the complete optical light path being above the telescope, in order to be fully exposed to the natural wind flow. The mount contains a novel drive system combining high stiffness with minimal friction and stick-slip 2,3,4,6 . The open concept was new, because all existing high-resolution solar telescopes use evacuation to avoid internal turbulence.…”

“…It became the first solar telescope to regularly achieve images with 0.2" resolution continuously over multiple hours and large fields (80"x60") at high cadence (30 s) thanks to its successful open principle, superb optical performance, exceptional mechanical stability and the homogeneous wind flows which make La Palma a world-class site [1][2][3][4][5] . Figure 1 shows an example of a typical image sequence taken with the DOT.…”