Time of flight spectrum of COMPTEL events registered during ground calibration (left) and in orbit (right). One ToF channel corresponds to 0.25 ns. Downward moving photons interacting first in D1 and then in D2 lead to a peak around channel 120, while upward moving photons interacting first in D2 and then in D1 produce a peak around channel 80. In the calibration data the upward moving photons can be clearly distinguished from the downward moving photons, while in the flight data the downward moving photons are blended with the tail of the dominating upward moving photons. The figure is adapted from Knödlseder (1994).