GCN Circular 16165
D. Harbeck (WIYN Observatory), A. Kaur (Clemson U.), A. Delgado-Navarro (Clemson U.), M. Orio (INAF, UW Madison), D. H. Hartmann (Clemson U.) report on behalf of a larger collaboration: We imaged the location of the Swift detected GRB 140423A (Sonbas et al., GCN 16142) at two epochs with the pODI instrument on the 3.5 m WIYN telescope at KPNO (http://www.wiyn.org/ODI/About/wiynpODI.html). Observations in the sdss r' band on 23 April 2014 consist of a single 300s exposure, and on 24 April 2014 we performed two 600s exposures. The optical counterpart of the burst (Elenin et al., GCN 16143; Kaur et al., GCN 16144; Ferrante et al., GCN 16145; Chester et al., GCN 16147) was detected under fair seeing conditions (~ 1 arcsec) in all single exposures. Date Exp. Start Time Magnitude 2014-04-23 UT 09:52:50.400 r' = 18.85 2014-04-24 UT 08:29:12.393 r' = 21.90 2014-04-24 UT 08:41:12.901 r' = 21.85 The photometric error is dominated by calibration and is 0.07 mag (in r') rms for a secondary reference star ensemble (but not accounting for color terms or galactic foreground extinction). The photometric calibration is based on (i) cross-correlating with the sdss dr9 star catalog in the r' band over the entire field of view (24' X 24') (ii) consistency check on exemplary star SDSS J130915.57+494948.5 at sdss r' = 19.98. The photometric error based on this reference star is < 0.02 mag rms.