To get a better sense of what is actually going on we will focus on just one of the nine grid pairs. The animation to the right shows one grid pair and the detector behind them in yellow. This particular image shows what a photon traveling excatly along the HESSI Spin Axis would see. The spin axis is an exact point on the Sun, near its center, where the instrument is pointed at any given time. The fact that the x-ray/gamma ray source is exactly on that reference point results in the fact that the rear grid is covered completely by the front grid. There is no modulation of the visible portion of the detector (yellow). All detectors record a steady rate of photons equal to a fraction of those that pass through the front grid.

If the front and rear grids were “in phase” as they appear in this animation, 50% of the photons from the source on the spin axis would be detected. However, front and rear grids are slightly out of phase in reality. Exactly how far out of phase the grids are will be determined from the first flares detected.