Optical Performance of Designs for a Large Aperture Far-Infrared Telescope

Details Optical Performance of Designs for a Large Aperture Far-Infrared Telescope Optical Performance of Designs for a Large Aperture Far-Infrared Telescope

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006aas...209.5602g&link_type=abstract

2007 AAS/AAPT Joint Meeting, American Astronomical Society Meeting 209, #56.02; Bulletin of the American Astronomical Society, V

Physics

Optics

Scientific paper

We have developed two designs for a large single aperture far-infrared space telescope. Achieving the ultimate sensitivity in the far infrared (30 to 300 microns wavelength) requires a telescope cooled to 4 K with a large collecting area. The performance of the system can be degraded by radiation from dust in the solar system (“zodi”), from the Milky Way, and from the spacecraft. It is thus critical to understand the response of any telescope design at angles far from the direction of peak response. We have carried out detailed calculations of the radiation pattern from two different telescope designs for a 10m diameter aperture. This is the nominal diameter of the Single Aperture Far InfraRed (SAFIR) telescope, but can easily be extended to the 6m by 4m aperture proposed for the Cryogenic Aperture Large Infrared Space Telescope Observatory (CALISTO). The first design is the on-axis (symmetric) telescope as described in the SAFIR Vision Mission (VM) Report, while the second is off-axis (unobscured). The calculations, which have been carried out at a wavelength of 1mm, utilize a combination of physical optics/physical theory of diffraction (PO/PTD) and geometrical optics/geometrical theory of diffraction (GO/GTD). The on-axis SAFIR VM design has a relatively large subreflector (0.09 fractional area obscuration) and its feed legs obscure 0.027 of the collecting area. Relative to this inefficient geometry, the unobscured design has higher gain by a factor of 1.45 (1.6 dB) with a 12 dB edge taper Gaussian illumination. It is also characterized by a far lower sidelobe level. The off-axis design is preferable in terms of aperture efficiency and its lower scattered radiation at large angles from the main beam, thus minimizing coupling to the sunshield and allowing observation of a large portion of the sky without compromising sensitivity as a result of extraneous pickup.

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