It is important to note that all the photons that reach HESSI can be considered as traveling on parallel paths since the Sun is so far away (93 million miles or 1.5 x 10^11 m) compared to the diameter of the HESSI grid tray (~1 m). The biggest angle between the directions of photons that could be detected with HESSI is thus:
360/(2 pi 1.5 x 10^11) degrees = 1.2 x 10^-6 arcseconds
This is much less than the 2-arcsecond angular resolution that is achieved with HESSI’s finest grids and so we are justified in assuming that all of the photons from a point source on the Sun arrive at HESSI with parallel paths.