L. Izzo, D. A. Kann, A. de Ugarte Postigo, C.C. Thoene (all HETH/IAA-CSIC), and F. Aceituno (IAA-CSIC) report:
We observed the field of GRB 180904A (D���Ai et al., GCN #23194) with the 1.5-m OSN telescope in Granada, Spain. We obtained 12 x 300 s images in the Ic band centered at 1.23 hours after the GRB.
At the enhanced XRT position of the GRB (Osborne et al., GCN #23196), we find no source inside the Swift-XRT error circle down to a limit of Ic (AB) > 22.4. We note, however, that the object reported in Malesani et al., GCN #23195, lies just outside the XRT position reported by Osborne et al., and may therefore be unrelated to the GRB.
The calibration was performed using i-band magnitudes of nearby stars in the Pan-STARRS catalog, transformed into Ic magnitudes according to the Lupton (2005) transformations.