TITLE: GCN CIRCULAR NUMBER: 22198 SUBJECT: GRB 171205A: GROND observations of the optical/NIR transient DATE: 17/12/07 14:42:34 GMT FROM: Sebastian Schmidl at TLS Tautenburg S. Schmidl (TLS Tautenburg) and P. Schady (MPE Garching) report: We observed the field of GRB 171205A (Swift trigger 794972; D'Elia et al., GCN 22177) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPG telescope at ESO La Silla Observatory (Chile). Observations started at 07:30 UT on December 05, 2017, 10 minutes after the GRB trigger. They were performed at an average seeing of 1.5" and at an average airmass of 1.5. We clearly detect the optical/NIR transient of GRB 171205A (Izzo et al., GCN 22180; Emery & D'Elia, GCN 22181; Butler et al., GCN 22182; Mao et al., GCN 22186; de Ugarte Postigo et al., GCN 22187; Choi et al., GCN 22188; Melandri et al., GCN 22189; Steeghs et al., GCN 22190; Cobb et al., GCN 22192; Volnova et al., GCN 22193; Guidorzi et al., GCN 22197). Based on total exposures of 66 seconds in g'r'i'z', 60 seconds in JH and 180 seconds in K at a midtime of 0.36 hours after the burst, we measure the following preliminary magnitudes and upper limits (AB magnitude system): g' = 19.1 +/- 0.1 mag, r' = 18.9 +/- 0.1 mag, i' = 18.9 +/- 0.1 mag, z' = 18.8 +/- 0.1 mag, J = 18.4 +/- 0.2 mag, H = 18.2 +/- 0.3 mag, and K > 17.6 mag. Magnitudes and upper limits are calibrated against PanSTARRS and 2MASS field stars and are not corrected for the Galactic foreground extinction corresponding to a reddening of E_(B-V)= 0.05 mag in the direction of the burst (Schlegel et al. 1998). Observations are continuing.