All LW modes can be used with the chopping secondary. Only a subset of these modes, MR spectroscopy and imaging with the NB_3.21 and NB_3.28 filters, can be used without chopping.
The format of data delivered by these modes is different.
For the data that is taken without chopping, the files correspond to the average of NDIT exposures. This is similar to the format delivered by the SW detector.
For the data that is taken with chopping, one gets two sorts of frames: chopped frames and half cycle frames. Chopped frames correspond to the difference between frames taken with the secondary at the two positions (i.e. ON and OFF beams). In these frames, the sky has already been subtracted and one has both positive and negative images. Half cycle frames correspond to images taken at either of the these positions. In these frames, the sky has not been subtracted and images are always positive.
During the first year of operations, half cycle frames were produced for each chopped frame. Currently, the half cycle frames are delivered for the first chopped frame only.
Because of round off errors and because of the order in the way the half cycle and chopped frames are calculated, the chopped frame is not quite the difference between the two half cycle frames. The difference is at the level of 10e-4 and is insignificant.