The 2-metre Liverpool Telescope will soon be assisted by the New Robotic Telescope (NRT), a 4-metre-class telescope, robotic and fully autonomous, at the Roque de los Muchachos Observatory (ORM) on La Palma, Canary Islands, Spain. The 4-metre primary mirror of the NRT will be comprised of 18 hexagonal segments of 1-meter diameter each. All the individual segments need to be sufficiently aligned to deliver images that can be successfully exploited by the telescope instrumentation. We therefore need an instrument to assist in the alignment of the different segments. To that end, we designed a wavefront sensor that would be not only reliable and robust (indispensable for a robotic telescope), but also economical, and therefore with as many on-the-shelf components as possible. We chose a Shack-Hartmann type of wavefront sensor, that rests on the use of a lenslet array. The assembly of segments can be mapped onto the array imaged at the image plane by the lenslet array. This will allow us not only to detect the misalignment of each segments with respect to the other segments, but also the misalignment of the primary mirror with respect to the secondary mirror. From these information, the position of the primary mirror segments and of the secondary mirror can be altered. A prototype Shack Hartmann wavefront sensor for the NRT has been tested in the laboratory and also on the 2-m Liverpool Telescope. We demonstrate that the basic functionality requirements are met. Detailed analysis of the images is currently underway.
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