CARMENES FP modes (both for VIS and NIR) are as follows:

- Disabled (sin Texp)
- One shot (by default Texp = 3 sec but it can be changed)
- Periodic (by default Texp = 3 sec but it can be changed)
- Carmenes (without Texp)
- Always (without Texp)

This is the explanation for each mode:

- Disabled. The FP is not used at all.

- One shot.
  Only one FP shot is performed with the given
  Texp, *IN THE MIDDLE* of the science exposure, so the
  Autostop could cut it out if the SNR is reached before
  half the Exptime is run.

- Periodic.
  It exposes the FP 10 times, each 1/10 of the given Texp.
  This works also with long exposures. I tested a 1200 sec
  exposure and it took the first FP shot after 1 min, then
  again every 2 min.
  In this case, the Autostop feature would subexpose the FP
  if the Exptime is too long and the SNR is reached
  before the Exptime runs out. The given FP Texp is spread over
  the science exposure duration.

- Carmenes.
  It exposes the FP every ~6 sec and each shot is calculated as
  to sum ~51 sec in total (this gives good FP signal).
  Again, the Autostop feature can cause a subexposed FP signal,
  as the 51 sec are spread over the given science Texp.

- Always.
  The FP keeps exposing during all the science exposure.

In summary, the science Exptime must be reasonably close to the
one needed for the required SNR, because, if not, the Autostop
would take care of getting good science SNR, but the FP might be
spoiled.
Setting a too short Exptime doesn't help either, as the Autostop
could double the Exptime if the SNR is not good and in the end
the FP might saturate.