Design aspects of a 30-m G iant I nfrared and S ub m illimeter O bservatory in space ('GISMO'): a new 'flavor' for SAFIR?

Inspired by a paper by Hyde (1999) (H99) we propose a 30-m diffractive Fresnel lens of ultra-high molecular-weight polyethylene (UHMW-PE) as the Primary Lens (L1) of a large cold far-IR and submillimetre space telescope. The design comprises Lens (L1) and Instrument (ISC) spacecraft 3 km apart, orbiting the Sun-Earth second Lagrangian point L2. In the Instrument S/C an off-axis Ritchey-Chretien Field Optical system (FO) re-images L1 onto a Fresnel corrector (FC). Achromatic over a bandpass λ/Δλ ~7.5 at a basic wavelength of 1.2mm and its harmonics, the design offers diffraction-limited performance from ~20 to ~700μm and a 1'x4' FOV. Positional tolerances appear to allow deployment of the lens by very simple means (we suggest using Shape Memory Alloys and pneumatic pressure). The most serious technical challenge may be material homogeneity. Behind an effective sunshade L1 should cool to ~10K by radiation to space in ~1y: its dominant heat source will be the zodiacal emission. GISMO resolves the FIR background at λ ~ 200μm: an all-sky survey to ~100μJy could in principle take