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Strawman mission concept

This science program requires measuring the sky brightness and polarization at high angular resolution and in many frequency bands across a wide spectral range. It also requires measuring the absolute spectrum of the sky background with moderate angular and spectral resolution. As a baseline, we hence propose to perform the best possible spectropolarimetric sky survey in the 30-6000 GHz frequency range with two instruments optimized for best joint performance, sharing a single platform in orbit around the Sun-Earth L2 Lagrange point: a polarimetric imager (PIM) observing with about 30 broad and 300 narrow spectral bands with a diffraction-limited angular resolution and a sensitivity limited by the photon noise of the sky emission itself; an absolute spectro-photometer (ASP) that will measure sky emission spectra with a spectral resolution between 500 MHz and 15 GHz and an angular resolution of about 1.4°. These complementary instruments will map simultaneously the absolute sky intensity and polarization with high sensitivity and with high spectral or spatial resolution. The data from both instruments can be binned (in frequency) and smoothed to get matching observations with δν/ν ≈ 0.25 and 1.4° resolution, allowing on-sky inter-calibration of both on large scales (and hence absolute calibration of the PIM). This will also enable correction of the ASP spectra from foreground contamination, using high resolution component maps extracted from PIM data (e.g. contribution of large clusters to the y-distortion in ASP data, and line emission from emitting regions that are not resolved in the coarse resolution ASP maps). As the scientific outcome of this mission depends on the complementarity of both instruments and on the control of systematic errors, a careful optimization of the ASP and the PIM (number and bandwidth of spectral bands vs. sensitivity) and of the mission (scanning strategy, joint analysis tools) with comprehensive simulations is an essential future phase of the mission study. The focal planes of both instruments will be cooled to 0.1K using a cryogenic system adapted from that of Planck, with continuous recycling of the gases for an improved mission duration of 4 years (baseline) or more.

fig7
The PRISM spacecraft with its two instruments: PIM, with a 3.5-diameter telescope with a FOV at ∼ 30° from the spacecraft spin axis, and ASP, aligned with the spin axis.

fig8
A possible disposition of the two instruments of PRISM , displaying the location of the off-axis telescope with a 3.5 x 4.2 m primary, of the polarimeter focal plane, and of the spectrometer.

fig8
The two satellites of the PRISM mission inside the Ariane-V fairing. The top one is the observatory: the Sun shields in stowed position are not shown. The bottom satellite (displayed in the Ariane-V SYLDA for a possible launch configuration) provides a set of calibrators for the observatory and the high-gain high data rate communication system.