TITLE: GCN CIRCULAR NUMBER: 33087 SUBJECT: GRB 221218A: MITSuME Akeno optical upper limits DATE: 22/12/20 10:21:15 GMT FROM: Ichiro Takahashi at Tokyo Tech I. Takahashi, M. Niwano, R. Hosokawa, K. L. Murata, Y. Imai, N. Ito, S. Sato, N. Higuchi, M. Tateda, M. Sasada, Y. Yatsu, and N. Kawai (Tokyo Tech) report on behalf of the MITSuME collaboration: We observed the field of GRB 221218A (S. Mereghetti et al. GCN Circular #33082, WeiKang Zheng et al. GCN Circular #33085, Y.-D. Hu et al. GCN Circular #33086) with the optical three color (g', Rc, and Ic) CCD cameras attached to the MITSuME 50 cm telescope Akeno. The observation with a series of 60 sec exposures started at 2022-12-18 08:37:35 UT (1405 seconds after the INTEGRAL trigger). We stacked the images with good conditions. We did not detect any uncatalogued sources within the error region (S. Mereghetti et al. GCN Circular #33082). We obtained the 5-sigma limits of the stacked images as follows. T0+[sec] | MID-UT | T-EXP[sec] | 5-sigma limits ------------------------------------------------------------------------------------------------ 1405 | 2022-12-18 08:51:23 | 1140.0 | g'>19.1, Rc>19.0, Ic>18.4 ------------------------------------------------------------------------------------------------ T0+ : Elapsed time after the burst T-EXP: Total Exposure time We used PS1 catalog for flux calibration. The magnitudes are expressed in the AB system. The images were processed in real-time through the MITSuME GPU reduction pipeline (Niwano et al. 2021, PASJ, Vol.73, Issue 1, Pages 4-24; https://github.com/MNiwano/Eclaire).