GCN Circular 10666
T. Sakamoto (UMBC/GSFC), D. Donato (ORAU/GSFC), N. Gehrels (GSFC), T. Okajima (JHU/GSFC), Y. Urata (NCU), C.A. Wallace (FGCU) We observed the field of GRB 100420A detected by Swift (trigger #419932; Baumgartner et al., GCN Circ. 10628) with the 14-inch Goddard Robotic Telescope (GRT) located at the Goddard Geophysical and Astronomical Observatory (http://cddisa.gsfc.nasa.gov/ggao/). Total 435 images of 5 sec (200 images), 30 sec (80 images), 60 sec (120 images) and 100 sec (35 images) exposures were taken in the R filter starting from April 20 05:26:53 (UT) about 4.2 minutes after the trigger and stopped on April 20 09:49:15 (UT). We do not detect the optical afterglow both in the individual images and the stucked image, which only combined good quality 400 images, inside the XRT position (Rowlinson et al., GCN #10639). The estimated three sigma upper limit of the combined image (total exposure of 10600 sec) is ~18.9 mag using the USNO-B1 catalog.