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We report the development of MuSCAT3, a four channel simultaneous imager installed on the 2m Faulkes Telescope North at Haleakala Observatory on Maui, Hawai’i. MuSCAT3 has a capability of 4-color simultaneous imaging in g (400–550 nm), r (550–700 nm), i (700–820 nm), and zs (820–920 nm) bands with four independent 2048×2048 pixel CCDs, each having a field of view of 9.1×9.1 arcmin2 with a pixel scale of 0.27 arcsec per pixel. The development of MuSCAT3 started from September 2019, and MuSCAT3 achieved its first light on September 28th, 2020. The Las Cumbres Observatory started science operations of MuSCAT3 since November 4th, 2020, although a part of its capabilities are still limited.

Over four thousands of exoplanets have been found to date. To understand the formation mechanisms of these diverse exoplanets, it is essential to unveil the detailed physical properties of these exoplanets by various methods, including high-precision transit photometry from the ground. So far two observing techniques have succeeded in achieving high-precision photometry from the ground; one is defocusing and the other is using optical diffusers. Diffusers spread light from stars over many pixels and stabilize the top-hat like point spread function (PSF), reducing systematic noises in photometric observations. In addition to it, diffusers enable us to use longer exposure times, which reduces atmospheric scintillation noises. We have developed diffuser units and installed them to the optical three-band imager MuSCAT. Each diffuser unit can store two types of diffusers with different opening angles, which allows us to change the strength of scatter of light from stars. We installed the diffuser units to MuSCAT in July 2019 and carried out on-sky examination. We observed out of transit (oot) phase of 11 planet-hosting stars alternatively with the following three configurations: with diffusers, without diffusers and with onfocus, and without diffusers and with defocus. From these observations, we confirmed that diffusers stabilized PSF and peak counts of stars. Throughput of the diffusers are measured to be about 93%. In addition, with diffusers, we succeeded in observing 55 Cnc, which is too bright (V=5.4) to observe without diffusers. We achieved the photometric precisions of 426, 641, and 783 ppm per 1 minute, or 188, 221, and 196 ppm per 5 minutes, in g', r', and zs-band, respectively.