Data Reduction Frequently Asked Questions
How do I interpret the SEQ TIME keyword values?
For VISIR, the SEQ TIME keyword is set to the requested exposure time. The actually achieved exposure time will, in most cases, deviate from this value, as only discrete values are allowed for some observational parameters, such as DIT, and others, such as T_chop (time spent in one chopping position), and T_nod (time spent in one nodding position) are pre-defined. The latter is 90 s. Therefore, following page 16 of the VISIR user manual, the number of nodding cycles is derived from the total integration time (user defined SEQ.TIME), but has to be an integer multiple of 2.
In your example, for NDIT to be 3, with NDITSKIP=1, f_chop must have been 0.025, giving T_chop=40 s and N_cycle_chop=2. Then, the total on-source integration time is indeed 240 s (see equation 3 of the VISIR User Manual).