Relative observing speed of lens-coupled absorber focal plane arrays for far-IR imaging and spectroscopy: a PRIMA case study

The PRobe far-Infrared Mission for Astrophysics (PRIMA) is an astrophysics mission concept currently under study. The instrument comprises four focal plane arrays (FPAs), each with over 1000 pixels, consisting of lens-coupled kinetic inductance detectors designed for ultra-high sensitivity spectroscopy and imaging. PRIMA covers the far-infrared band from 24 to 250 μm. We present the fundamental properties of lens-coupled absorbers, including aperture efficiency, throughput, and beam patterns. Compared with bare absorbers, lens-coupled absorbers exhibit reduced sensitivity to noise from both the instrument enclosure and the sky background. We analyze FPA sampling strategies in terms of relative observing speed, considering both detector noise–limited and background noise–limited scenarios. In the background noise–limited case, near-maximum-gain sampling is optimal only when the point spread function (PSF) in the reflector focal plane is aligned with the lens. For arbitrary PSF incidence, a near-Nyquist sampling configuration provides the best average performance. These calculations are based on a computationally efficient quasi-analytical technique and assume a hex-packed array of circular lenses, recently developed for PRIMA.