Long-wavelength and short-wavelength photometry bands cover identical fields of view. The spectrometer field of view is offset in the +z direction. Chopping is done along the y axis (left-right in this view)and also allows observation of the internal calibrators on both sides of the used area in the telescope focal plane. The chopper throw for sky observations is ± 1/2 the width of the photometer field such that object and reference fields can be completely separated (photometer field 1 and 2)

A fixed mirror is used to split the focal plane into the photometry and spectroscopy channels of the instrument. In the photometry section, the two wavelength bands are simultaneously imaged with different magnification to reach full beam sampling in both bands. In the spectroscopy section, an optical image slicer re-arranges the 2-dimensional field along the entrance slit of the grating spectrograph such that, for all spatial elements in the field, spectra are observed simultaneously.