Page "Aberration of light" Paragraph 11

For the simple case where the direction of the light beam along the line SE is perpendicular to the motion of the observer then only light emitted at an angle equal to the arc cosine of the ratio of the observer's speed to the speed of light will reach the telescope.

Consequently, the telescope too must be oriented at this same angle away from the light beam's trajectory in the direction of the motion to capture this emitted light.