Schematic of the NEAT formation flying spacecraft. NEAT is composed of a mirror spacecraft and a detector spacecraft. The optical configuration is an off-axis parabola so there is no vignetting of the FoV. Sun shades prevent direct or reflected Sun light from reaching the CCD. A metrology system with laser beams launched from fibers located on the mirror projects dynamic Young fringes on the detector. The fringes allow a very precise calibration of the CCD. All three aspects (metrology, no vignetting, and Sun shades) are critical to reach micro arcsecond accuracy.