TITLE: GCN CIRCULAR NUMBER: 30231 SUBJECT: GRB 210610B: iTelescope optical afterglow observations DATE: 21/06/13 18:36:43 GMT FROM: Arto Oksanen at Nyrola Obs., Finland Markku Nissinen (Taurus Hill Observatory, Varkaus, Finland) and Arto Oksanen (Hankasalmi Observatory, Hankasalmi, Finland) report: We have detected GRB 210610B optical afterglow (Page et al., GCN30170) using iTelescope T18 (0.32-m f/8.0 + CCD in AstroCamp at Nerpio, Spain), iTelescope T21 (0.43-m f/6.8 + f/4.5 focal reducer + CCD in New Mexico Skies at Mayhill, New Mexico, USA) and iTelescope T11 (0.50-m f/6.8 + f/4.5 focal reducer + CCD in New Mexico Skies at Mayhill, New Mexico, USA). We took total of 24 exposures with clear (CR), V and R filters. The afterglow was clearly detected with each filter at the position RA 16:15:40.43 DEC +14:23:57.4. The following magnitudes were measured from comparison of a nearby star (V = 13.97, r'=13.70) from the APASS DR9 catalogue (Henden+, 2016): ------------------------------------------------------------------------------ JD UTC Mag Err Filter AirMass Telescope Exposure(s) ------------------------------------------------------------------------------ 2459376.36806 June 10 20:50 16.79 0.05 CR 1.28 T18 7x60 2459376.37868 June 10 21:05 17.15 0.10 V 1.24 T18 10x60 2459376.64285 June 11 03:25 17.76 0.02 CR 1.29 T21 2x300 2459376.65108 June 11 03:37 17.95 0.12 R 1.26 T11 5x300 ------------------------------------------------------------------------------ We used r' reference magnitude for the clear filter and the R filter measurements.