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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.
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