TITLE: GCN CIRCULAR NUMBER: 9211 SUBJECT: GRB090418A Rc-band photometry DATE: 09/04/23 19:03:44 GMT FROM: Arne A. Henden at AAVSO A. Henden (AAVSO), J. Gross (SRO), B. Denny (DC-3), D. Terrell (SwRI), and W. Cooney (SRO) report: We obtained photometry of the GRB090418A afterglow reported by Chornock et al. (GCN 9148) using the Sonoita Research Observatory (SRO) 35cm telescope in southern Arizona, utilizing an automatic VOevent trigger. The Rc-band exposures began about 2 minutes after the burst and continued until twilight. Ten 60-second and five 180-second exposures were acquired. Astrometry of the afterglow from several images and using UCAC as the reference catalog yields coordinates: 17:57:15.151 +33:24:20.93 J2000 (+/- 50mas) Photometry, assuming that the star at 17:57:42.53 +33:25:47.2 has an R magnitude of 13.12 UT(mid) delT exp Rc err 11.1728 162 60 15.802 0.044 11.1919 231 60 16.185 0.048 11.2111 300 60 16.500 0.043 11.2303 369 60 16.818 0.058 11.2494 438 60 17.019 0.066 11.2686 507 60 17.249 0.067 11.2878 576 60 17.440 0.063 11.3069 645 60 17.605 0.105 11.3261 714 60 17.680 0.089 11.3450 782 60 17.855 0.115 11.3958 965 180 18.007 0.104 11.6522 1888 180 18.778 0.129 11.7067 2084 180 18.919 0.138 11.7592 2273 180 18.960 0.142 11.8114 2461 180 18.891 0.196 Where delT is the time in seconds from the burst (Mangano et al., GCN 9149), and the exposure is in seconds. The optical afterglow candidate is in the filter reflection halo from HD163948, a 6.9mag star about 2arcmin distant, so the last few magnitudes may have a systematic bias. We note that this field was not calibrated by SDSS through DR6. We will perform a BVRI calibration over the next few nights.