COALA: a diurnal alignment and phasing instrument for GTC

COALA (COphasing and ALignment Aid) is an instrument aimed to measure the relative alignment and piston of any three adjacent segments of the primary mirror of the GTC during day without using stellar light. The main driver in conceiving COALA has been the increase overall operation efficiency of GTC permitting to recover the telescope alignment after exchanging recoated segments without the constraints of weather and seeing conditions imposed by the current alignment/phasing procedures using focal plane WFS illuminated with stellar light. This instrument is a 6 sub-apertures wavefront sensor built around a Ø350mm telescope. Three sub apertures are used to measured tip-tilt between segments as in a Shack-Hartmann WFS. The other three sub-apertures include chromatic dispersion prisms that permit to measure a chromatically dispersed diffraction image of apertures placed between segments, the analysis of that images permits to measure the relative piston between segments within 8μm range without ambiguity. For using the instrument, a quasi-punctual white light source is placed in the telescope focus to generate a collimated beam. COALA analyze the wavefront of that collimated beam generated by the telescope. COALA is attached to the telescope dome in an alt-azimuthal mount. Combined motion of COALA and telescope mounts permit that the subapertures of COALA can scan the full aperture of the telescope without the need of an intruder mechanical system in front of the primary mirror. COALA has been installed in the GTC in October 2023, and the commissioning tasks are ongoing. This paper describes the design of the instrument and the results obtained during commissioning.