TITLE: GCN CIRCULAR NUMBER: 29603 SUBJECT: GRB 210306A: iTelescope T24 optical observations DATE: 21/03/06 14:23:19 GMT FROM: Veli-Pekka Hentunen at Taurus Hill Obs,A95 Veli-Pekka Hentunen and Markku Nissinen (Taurus Hill Observatory, Varkaus, Finland) report: We have detected GRB 210306A optical afterglow using iTelescope T24 (0.61-m f/6.5 + CCD) telescope in Sierra Remote Observatory (Auberry, California, USA). The observations were started at 2021-03-06 05:02:41 (UT). We used Luminance filter and 300 sec exposures. The afterglow was detected at the position RA 08 39 50.11 DEC +60 12 18.8. The following magnitudes were measured from comparison of a nearby star (r = 16.974) from Pan-STARRS DR1 catalogue (Chambers et al., 2016): Tmid (h:min:sec)+T0 Mag Mag_err 01:11:12 18.08 0.08 01:16:55 17.97 0.06 01:22:47 18.13 0.07 02:06:55 18.41 0.10 02:14:53 18.44 0.09 02:18:53 18.52 0.10 02:24:37 18.63 0.10 03:03:34 18.82 0.11 03:09:13 18.85 0.11 03:20:45 18.87 0.12 03:26:56 18.91 0.12 03:32:55 18.84 0.13 04:33:44 19.33 0.19 04:39:39 19.22 0.18 05:03:57 19.33 0.21 05:27:40 19.61 0.38 Magnitudes were not corrected for galactic extinction. URL link for the L filter 300s image: https://www.kassiopeia.net/2021/03/06/grb-210306a-havainto/