This is effectively done when the detector can see all of the four "Christmas lights" at the periphery of the secondary mirror, and that lights on opposite sides give roughly the same signal.
(This; is effectively done; T:when the detector can see all of the four "Christmas lights" at the periphery of the secondary mirror) (the detector; can see; all of the four "Christmas lights" at the periphery of the secondary mirror) (lights on opposite sides; give; roughly the same signal)
This the detector lights on opposite sides
is effectively done can see give
T:when the detector can see all of the four "Christmas lights" at the periphery of the secondary mirror
all of the four "Christmas lights" at the periphery of the secondary mirror
roughly the same signal