Four satellites are necessary because PUNCH views the entire inner solar system continuously, and Earth blocks approximately half the view from any one location in orbit. The lone Narrow Field Imager (NFI) acquires an annular field of view (FOV) around the Sun. Three Wide Field Imagers (WFIs) acquire data in a trefoil on the sky. As the spacecraft orbit, the trefoil rotates on the sky and builds up the full 90° circular PUNCH FOV. The Narrow Field Imager (NFI) is a compact externally occulted coronagraph with a similar field of view to SOHO/LASCO-C3: from 6 to 32 R☉ on the sky. The Wide Field Imager (WFI) is a wide-field heliospheric imager (HI) design based on STEREO/HI. It images from 18 to 180 R☉ on the sky. HI designs are analogous to coronagraphs, but in linear instead of circular geometry.
NFI images include a large and very dynamic stray light component. The PUNCH team are working on a technique to handle this. For now, we are using a PCA-based approach to fit and remove this stray light, and the F corona is also removed by it. The PUNCH team fit about 50 PCA components to a group of about 500 good images (i.e. not outlier images—no corrupted regions, no heavy saturation). The fitted components are then used to transform and un-transform an image to be subtracted. This reconstructed image, composed of only the leading PCA components, tends to contain the stray light and F corona but little of the K corona, and we subtract it out. The image to be subtracted is excluded from the set of images the PCA is fit to, to reduce the likelihood that dynamic K corona structure is contained in the PCA components. To reduce the impact of planets and the Moon, which can leave prominent artifacts in the leading PCA components, we divide the image plane into quarters. Each quarter is fit and filtered separately, with the fitting only using images that do not contain a planet or the Moon in that quarter. This quartering approach, combined with outlier image rejection, tends to reduce an initial set of 1000 images to about 500 that are actually fit for any one quarter of the image.
Because this PCA approach removes the F corona from the NFI images but we are not yet subtracting the F corona from WFI images, NFI is currently excluded from the CTM mosaics, but this will be changed in the future.