Optical captures are a scheduled physical readout after control. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
The exposure schedule has checked time
There are 4 fields at dwell time 5 s, so exposure time is 20 s.
texposure=4⋅5s=20s
Channels multiply the scheduled captures
The same 4 fields through 3 channels produce 12 captures.