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A region 140 square degrees toward the Galactic Centre was searched for monochromatic optical light, both pulses shorter than 1 sec and continuous emission. A novel instrument was constructed that acquires optical spectra of every point within 6 square degrees every second, able to distinguish lasers from astrophysical sources. The system consists of a modified Schmidt telescope, a wedge prism over the 0.28-meter aperture, and a fast CMOS camera with 9500 x 6300 pixels. During 2021, a total of 34800 exposures were obtained and analyzed for monochromatic sources, both sub-second pulses and continuous in time. No monochromatic light was found. A benchmark laser with a 10-meter aperture and located 100 light years away would be detected if it had a power more than ~60 megawatt during 1 sec, and from 1000 light years away, 6000 MW is required. This non-detection of optical lasers adds to previous optical SETI non-detections from more than 5000 nearby stars of all masses, from the Solar gravitational lens focal points of Alpha Centauri, and from all-sky searches for broadband optical pulses. These non-detections, along with those of broadband pulses, constitute a growing SETI desert in the optical domain.