C. Adami, P. Amram, S. Basa, K. Parra-Ramos (LAM), T. Adami (ENS Paris-Saclay), B. Schneider (MIT), A. Saccardi, S. D. Vergani, (GEPI, Obs. de Paris), S. Antier, A. de Ugarte Postigo (OCA),
E. Le Floc'h, D. Götz, F. Schüssler, D. Turpin (CEA-Saclay), report, on behalf of a larger collaboration:
We observed the field of GRB 230812B (Fermi GBM Team GCN, 34386 and all subsequent GCNs) using the MISTRAL spectro-imager of Observatoire de Haute Provence (OHP) in imaging mode. We obtained
during the 2023 09 17 night 1x1200s exposure in the r'-band with a mid-epoch of 19:20 UT, during the 2023 09 18 night 1x600s and 2x900s exposures in the r'-band with a mid-epoch of 21:06 UT, and during the 2023 09 19
night 8x600sec and 1x700sec in the r'-band with a mid epoch of 19:58 UT.
We derive the following photometry, calibrated against the Pan-STARRS catalog, not corrected for the underlying galaxy, and not corrected for Galactic dust reddening:
2023 09 17 19:20 UT r' = 22.36 +/- 0.16
2023 09 18 21:06 UT r' = 22.37 +/- 0.14
2023 09 19 19:58 UT r' = 22.45 +/- 0.10
We acknowledge the excellent support from Jerome Schmitt, Stephane Favard and Jean Balcaen (Observatoire de Haute Provence).