TITLE: GCN CIRCULAR NUMBER: 30169 SUBJECT: GRB 210610A: MITSuME Ishigaki optical observation DATE: 21/06/10 19:54:13 GMT FROM: Takashi Horiuchi at Ishigakijima Astronomical Obs Takashi Horiuchi, Hidekazu Hanayama (NAOJ), Katsuhiro L. Murata, Yoichi Yatsu, Nobuyuki Kawai (Tokyo Tech) report on behalf of the MITSuME collaboration: We observed the field of GRB 210610A (K. L. Page et al. GCN Circular #30160) with the optical three color (g', Rc, and Ic) CCD cameras attached to the 105 cm Murikabushi telescope of Ishigakijima Astronomical Observatory, Okinawa, Japan. The observation with a series of 60 sec exposures started on 2021-06-10 15:40:04 UT (37 min after Swift BAT trigger). We detected the point source at the position consistent with the afterglow reported by K. L. Page et al. GCN Circular #30160. Here, we removed bad data (images) with wrong sky conditions and obtained the magnitudes as follows: T0+[min] MID-UT T-EXP[sec] measured magnitudes ------------------------------------------------------------------------------------------------ 37 2021-06-10T15:57:45 900 (60s × 15) g'=19.41+/-0.14, Rc=18.19+/-0.04, Ic=17.73+/-0.10 ------------------------------------------------------------------------------------------------ T0+ : Elapsed time after the burst T-EXP: Total Exposure time The SDSS catalog (DR16) is used for flux calibration.