Analysis of the optical design for the SAFIR telelscope

SAFIR, the Single Aperture Far Infra Red Observatory, is a very powerful space mission that will achieve background-limited sensitivity in the far infrared-submillimeter spectral region. Many processes of enormous interest to astronomers can best be studied in this wavelength range, but require the demanding combination of high sensitivity, good angular resolution, and spectroscopic capability. SAFIR is a 10m class telescope offering good angular resolution, cooled to below 5 K in order to achieve background-limited sensitivity, and equipped with a complement of large-format cameras and broadband spectrometers. Successful operation of such a facility is critically dependent on achieving the level of sensitivity expected, but this is rendered difficult by potential pickup from unwanted sources of radiation. This problem is exacerbated by the fact that the emission from the optical system itself is minimal due to its low temperature, thus emphasizing the importance of minimizing pickup from unwanted astronomical sources of radiation, including the emission from dust in our solar system (analogous to the zodiacal light, hence "zodi"), and the emission from warm dust in the Milky Way (Galactic "cirrus"). The extreme sensitivity of SAFIR to these unwanted sources of radiation makes it essential to understand the relative sensitivity of the telescope/detector system to radiation coming from angles far outside the main beam, and to develop designs which minimize this pickup. In this paper we analyze in some detail the relative telescope sensitivity (referred to as the antenna pattern by microwave engineers) for different designs of SAFIR. These calculations include edge diffraction from the secondary and primary reflector, and also the effect of blockage by the secondary and blockage and scattering by support legs in a symmetric system. By convolving the antenna pattern with the brightness of the sky due to the zodi and cirrus, we can calculate the power received when the antenna is pointed in any specified direction. We can also compare the undesired pickup for different designs, in particular symmetric vs. asymmetric (off-axis or unblocked) antenna configurations. These considerations are vital for achieving the most efficient SAFIR design possible, in terms of achieving maximum sensitivity while being able to observe over a large fraction of the sky.