For several weeks the APT will carry 2 visible images (channels 1 and 2). The infra-red system requires out-gassing before being switched
on and is the reason for the delay (usually about 2/3 weeks) in its operation. My understanding is that the outgassing prevents
precipitation (and contamination) when the coolers are switched on. Thereafter the APT format should follow the norm of Channel 2 (visible)
and channel 4 (IR) during daylight passes, switching automatically to channels 3 (3A) and 4 during darkness.
Douglas Deans