The WISE needs to take a 8.8 second exposure, then quickly move 42' in 2.2 seconds, and then take another 8.8 second exposure. It is difficult to start and stop a large object this quickly, especially in space. Thus a scanning mirror is used to "freeze" the sky on the arrays for 8.8 seconds while the spacecraft and telescope make a smooth rotation around the Earth-Sun line at a constant angular rate equal to the orbital rate. During the 2.2 seconds between exposures, the scan mirror "flys back" to its starting position. The animation shows a simple optical scheme where the secondary mirror of a Cassegrain telescope is used as the scan mirror. This is the technique used by 2MASS. For the larger field-of-view and faster optics used by WISE, the coma introduced by tilting the secondary is too large. Thus the WISE telescope is afocal, and delivers a concentrated beam of parallel light to a scanning "tertiary" mirror. [Actually it is the fifth mirror.] Cameras after the scan mirror refocus the light onto the detector arrays.
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