Over an area corresponding to 1.65° diameter on the sky, the image plane has 16 arrays of infrared detectors, each array with 2048x2048 pixels of 20 μm size, corresponding on average to 0.34" on the sky. The focal length of 12.1 m combines with the baffled aperture of 3.7 m to a focal ratio of 3.26. The arrays are separated by 90% of their width in one direction and by just under 50% their width in the other direction. A single exposure therefore corresponds to a patchy "paw print" on the sky. To fill the gaps and obtain a conventional image at least six shifted paw prints have to be combined into a "tile", which then is 1.5° by 1.0°. The image plane of the camera also has wave front detectors used to control the shape of the primary mirror and the position and tip/tilt of the secondary mirror (active optics). This compensates for flexure and ensures a focused image at all altitudes.