The Wide Field Imager (WFI) Instruments for the Polarimeter to Unify the Corona and Helliosphere (PUNCH)

TL;DR

The paper details the design, integration, and calibration of the Wide-Field Imager instruments for the PUNCH mission, enabling comprehensive solar corona and heliosphere observations through a trio of specialized heliospheric imagers.

Contribution

It introduces a novel trio of WFI instruments with a symmetric trefoil field of view, enhancing the full solar field coverage for the PUNCH mission.

Findings

Successful in-flight calibration of the WFI instruments.

Effective stray light attenuation enabling faint signal detection.

WFI's combined brightness and polarization measurements support solar wind studies.

Abstract

We describe the design, hardware integration, and calibration performance of the Wide-Field Imager (WFI) instruments for the Polarimeter to Unify the Corona and Heliosphere (PUNCH) mission. The WFI instruments are a trio of visible-light heliospheric imagers that, together, view the outer corona and solar wind from under 3.5{\deg} to over 47{\deg} from the Sun, via sunlight that is Thomson-scattered from free electrons. In flight, the WFIs are arranged so that their collective fields of view form an approximately symmetric trefoil on the sky, comprising three circular-truncated square fields spaced 120{\deg} apart in position angle. The WFIs work with the NFI instrument, described elsewhere, to implement the full PUNCH field spanning all solar position angles, at elongations from 1.5{\deg} to 47{\deg} from disk center. WFI is implemented using dioptric (lens) optics and deep multi-stage baffles that attenuate solar, planetary, and lunar stray light sufficiently for ground processing to reveal the faint signal for the primary science. WFI measures both total brightness (tB) and polarized brightness (pB), via an on-board polarizing filter wheel (PFW) and charge-coupled device (CCD) camera that share a common design with those of the NFI instrument.