Hi Guys generally when you evaluate the 3 open string tachyon tree level amplitude in CFT, you do a conformal transformation mapping the worldsheet to the upper half of the complex plane and the incoming and outgoing strings become points on the real axis. However in evaluating the string field theory tree level amplitude the problem involves a different procedure where the 3 string worldsheet are conformally transformed in to 3 different worldsheets which are then glued together to form a circle, with the incoming and outgoing strings being the points on the circumference of the circle. I don't understand why the same amplitude is seen in two different ways.
The disk and upper half plane are related by an SL(2,R) transformation. Therefore correlators evaluated on the respective surfaces are identical.