\[ t=\frac{(1+\frac{v}{c})t'}{\sqrt{1-\frac{v^{2}}{c^{2}}}}=t'\frac{1+cos\alpha }{sin\alpha }=t'\cdot ctg\frac{\alpha }{2}=t'\sqrt{\frac{c+v}{c-v}}=t'\sqrt{\frac{1+cos\alpha }{1-cos\alpha }}\]