Optics studies of the planned SOFIA instrument SPICA/LISA

The proposed Spectral-Photometric Far-Infrared Camera (SPICA) will use the 2.5m SOFIA telescope to image astronomical objects at wavelengths between 20 and 220 mm. A low-resolution spectrometer channel will record simultaneously individual diffraction limited spectra from several tens to several hundreds of spatial positions on the sky. For both observing modes large format far-infrared arrays will be required. Their physical size would be in the range of 40 × 40 mm² to 120 × 120 mm². This extended geometry and the large fields-of-view are the major design drivers for the instrument optics. A concept study with a common collimator and three individual imagers showed that geometrical aberrations remain small compared to diffraction effects, even when standard manufacturing and alignment tolerances are included. An additional study also taking diffraction effects into account, showed the feasibility of a 40 - 70 mm spectrometer using a 64 × 64 element array and an image slicer to image a 26 × 46 square arcseconds sky field at a spectral resolution of ~30. The overall transmission of the spectrometer will be at the order of 20%. While the camera will study the morphology of objects, namely of their coldest matter components at 10 to 20 K, the spectrometer will determine the spectral energy distribution as well as broad band spectral features in galactic and extragalactic objects at the highest sensitivity.