A transit-like dip can come from the instrument

The science signal is a small fractional decrease in measured brightness. So is a star drifting across a detector whose response is not uniform. So is a focus change, a detector-temperature change, amplifier drift, ADC drift, a change in stray light or background, a change in the calibration state, or a run of samples that went missing. Each of those lowers the recorded number, and the recorded number is all the instrument produces.

I stopped treating those as environmental nuisances to be minimised and wrote each one into the requirements as something that has to be measured. That is where the pointing telemetry requirement comes from, and the temperature sensor near detector, amplifier and converter, and the dark-baseline measurement, and the calibration reference. Every systematic that can imitate the signal needs its own measurement channel, because a systematic you cannot measure is one you cannot subtract.

The numbers attached to that are all forward-looking acceptance criteria: pointing-related brightness error corrected below 0.05 per cent, corrected thermal drift below 0.05 per cent per hour, focus drift below 0.05 per cent per hour, dark uncertainty below 10 per cent of the noise budget.

It reframed what the instrument is. The optical and electronic chain measures brightness; the rest of the payload exists to make the measurement defensible. Most of the requirements are not about detecting the transit at all.

None of it has been measured. There is no detector head, no temperature sensor and no board, so every correction in the design is currently a plan for one.