Abstract
The Gran Telescopio Canarias (GTC) is a 10.4-meter segmented telescope currently being built at the Observatorio del Roque de los Muchachos in La Palma, Spain (Alvarez & Rodríguez-Espinosa 2004). The GTC is a partnership between Spain, Mexico and the University of Florida. The Commissioning Instrument (CI) is the first instrument for the GTC. The CI is an optical instrument for imaging, pupil imaging, curvature wavefront sensing, and high-resolution Shack-Hartmann wavefront sensing. Neutral density and BVRI filters can be used in each mode. The CI is designed to verify the optical quality of the telescope, both at the level of individual segments and as a whole, and thereby aid the GTC in achieving its goal of excellent image quality. In particular, it is able to measure relative piston and tilt between primary mirror segments. The CI can also be used to measure stray light.
