TITLE: GCN CIRCULAR NUMBER: 9190 SUBJECT: GRB 090418A: RTT150 optical observations DATE: 09/04/22 11:07:11 GMT FROM: Rodion Burenin at IKI, Moscow I. Bikmaev, R.Zhuchkov, N. Sakhibullin (KSU/AST), I. Khamitov, Z. Eker (TUG), U. Kiziloglu (METU), E. Gogus (Sabanci Uni.), R. Burenin, M. Pavlinsky, R. Sunyaev (IKI) report: The optical afterglow of GRB 090418A (Chornock et al., GCN 9148, Mangano et al., GCN 9149) was observed with Russian-Turkish 1.5-m telescope (RTT150, Bakirlitepe, TUBITAK National Observatory, Turkey) under moderate transparences. We obtained series of 600s exposures in BVRc filters, centered at April 18, 23:40 UT, i.e. 0.52 days after the burst. The afterglow is clearly seen at combined R image, and marginally detected at combined B,V frames. Using the same calibration star USNO-B1.0 1234-0288651 as Pavlenko et al. (GCN 9179), we estimate the following magnitudes of OT: Filter, Exposure, mag., err. (s) B 3x600 23.5 limit V 3x600 22.7 limit R 4x600 22.1 +/- 0.1 We made also series of 3x900 sec exposures in Rc filter centered on April 20, 23:00, i.e. 2.5 days after the burst. The afterglow was not detected on the combined frame with a limit R=23.3 mag RTT150 finding chart (Rc) can be found at: http://hea.iki.rssi.ru/grb/090418a/indexeng.html