elow is a picture representing how Laser light moves through space and how Plasma light propagates through space.Laser photons move in a co-linear motion and do not move much in an up down right left direction from the line of propagation. Plasma light moves through space as an expanding sphere.

Below is a schematic representing the trains. In this experiment the trains are always parallel and moving at the same speed. they are always 186,000 miles apart.It the bottom train ia at A' the top train is at A. B' to B and C' to C. 

This is a Work Area for Stellar Aberration
 

Before you look at the diagrams below if you wish to read a beautiful chronology and description on Stellar Aberration including the careful measurements and thinking of the time go to this site.
 
 

1:  diagram of the Earth in its orbit around the sun. A star in the distance is to the right not depicted .
A: vector of motion pointed towards star.
B: position of earth with motion vector at a right angle to the star.
C: Position of the Earth when the motion vector is directed away from the star.
D: Not depicted but opposite from B with the vector pointing down.

 

2: Here is the star far in the distance. The light rays leave the star in an solid expanding sphere
 

 

3: Composite diagram showing the green vectors that point to and from the star directly. Here there is no relative motion of the telescope to the incoming light rays allowing the incoming rays to do directly down the telescope. The star would be seen in the same apparent position in these cases. The settings of right ascension and declination would be the same as far as the vector of Earth’s motion is concerned.
 

4: This illustrates the incoming ray that is parallel to the ones in diagram 3. Because of the vector of motion the rays do not go down the barrel straight and are not seen. It is as if the barrel since it is in a right angle motion to the star catches the rays on the side of the barrel.
 

5: The light black barrel is the position as in diagram 4. The upper dark barrel is the position the telescope has to be set in order to allow the rays from the star to go down the barrel for visualization. The lower yellow rays are the original rays from the “known” position that can not be visualized.  
Do Not Forget
The telescope is moving upwards and that is why the lower rays get caught and the upper rays get through.Surprisingly the angle of the telescope does not interfere with the light ray that gets through.