Monogrid Imaging with Pixelized Detectors: The GRIPS Imaging Concept

We describe the imaging concept used by the Gamma-Ray Imager/Polarimeter for Solar flares (GRIPS) project. In its high-altitude balloon payload form, GRIPS is equipped with a rotating grid (Multi-Pitch Rotating Modulator, MPRM) with 13 pitches ranging from ≈12.5 ≈ 12.5 ” to ≈162.5 ≈ 162.5 ” FWHM angular resolutions. An array of spatially-differentiated (pixelized) germanium strip detectors (GeD) allows the localization of incoming photons from ~20 ∼ 20 keV to $>10$ > 10 MeV, to within the strip pitch of ~0.5 ∼ 0.5 mm or better. At 100 ≳ 100 keV, Compton reconstruction can also be used to reject non-solar photons and determine polarization. In this introductory paper, we concentrate on the optical response (point-spread-function) of the imaging grid from low to high energies. We also introduce several detector effects, which become progressively more important at higher energies.