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E. F. Erickson, M. R. Haas, P. K. Davis (NASA Ames)
Calculations of diffraction phenomena for SOFIA (the Stratospheric Observatory for Infrared Astronomy) are described. The analyses establish the diffraction-limited point-spread function for the planned central obscuration of the telescope, confirm the specification for the oversized primary mirror diameter, evaluate spider diffraction effects, and determine the variation in focal-plane flux with position of the telescope relative to the cavity door. The latter is a concern because motion between the door aperture and the telescope can vary the flux from a point source and the sky background by diffraction (even when the door aperture does not physically obstruct the geometrical beam).
We find all these effects to be acceptable in terms of observatory performance, with the possible exception of fractional background variations ~3E-3 at wavelengths ~1mm. Fractional background variations larger than ~1E-6 can exceed photon shot noise in one second for broad-band, background-limited infrared detectors systems. However, we expect that synchronous signal demodulation using the telescope's chopping secondary mirror will obviate this effect, assuming modulation of the diffracted sky radiation by the relative motion of the door and telescope occurs at frequencies well below the chopoper frequency.
This work is supported by the National Aeronautics and Space Administration.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.