Colour-coded map of the expected number of field-of-view transits experienced by sources at different celestial positions in a 5 year mission. The projection uses equatorial coordinates, with right ascension running from  −180° to  +180° right-to-left. The blue line is the ecliptic. The average number of field transits in this plot is 88, although the value that is normally used for performance evaluation is 72 (accounting for dead time). An over-abundance of transits occurs at 45° away from the ecliptic plane due to the difference between the 45° spin axis angle with respect to the sun and the 90° angle between spin axis and the fields of view.