TITLE: GCN CIRCULAR NUMBER: 30316 SUBJECT: GRB 210610B: T-CAT optical observations DATE: 21/06/25 14:40:15 GMT FROM: Denis Marchais at Amateur astronomer I observed the field of GRB 210610B (Page et al., GCN #30170) with the T-CAT telescope 0.4-meter f/4 Newton with a f/3 reducer, IR-cut filter at 630nm and ZWO ASI533MC camera, located in Cintegabelle, France, on June 11, 12 and 13. The camera has a Sony sensor with Bayer filters R,G,B, that fairly match the standard Bc, Vc, and r' filters considering the additional IR-cut filter. Each observation results from stacking 120x 32s exposures. The GRB OT is clearly detected in the stacked frames at the following location, matching other observations (e.g. , Fynho et al. GCN #30182, Osborne et al. GCN #30186) though approaching the detection limit on third observation, June 13th. RA(J2000) = 16h 15m 40.39s Dec(J2000) = +14d 23' 56.3" The following magnitudes were obtained thanks to PixInsight photometric calibration using nearby APASS DR10 stars. Date UT start Filter Exp. (s) OT err 2021-06-11 23:52:21 Bc 120x 32 20.53 +/- 0.05 2021-06-11 23:52:21 Vc 120x 32 20.00 +/- 0.04 2021-06-11 23:52:21 r' 120x 32 19.64 +/- 0.04 2021-06-12 22:48:22 Bc 120x 32 21.79 +/- 0.11 2021-06-12 22:48:22 Vc 120x 32 20.91 +/- 0.07 2021-06-12 22:48:22 r' 120x 32 20.90 +/- 0.10 2021-06-13 22:45:36 Bc 120x 32 21.77 +/- 0.1 2021-06-13 22:45:36 Vc 120x 32 21.42 +/- 0.09 2021-06-13 22:45:36 r' 120x 32 21.76 +/- 0.2 I thank A. Taylor, H.-B. Eggenstein and E. Broens (GCN#30205) for sharing information through the KNCatcher citizen-science programme initiated by S. Antier and A. Klotz as part of GRANDMA initiative (GRANDMA Observations of Advanced LIGO's and Advanced Virgo's Third Observational Campaign, Antier et al. https://arxiv.org/abs/2004.04277v2).