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A chopping mirror or chopper is a mirror within a telescope (possibly within the optics of a particular instrument) which is mounted so that its position can be quickly moved to a slightly different angle, effectively pointing the telescope toward a slightly different point within the celestial sphere. The purpose is acquisition of blank-sky data for sky subtraction, in particular, for a rapidly-changing sky: in the mid infrared, for example, thermal emission from the Earth atmosphere changes rapidly, on the order of a second or less. A usual method of estimating how much of the light from a target is actually from the atmosphere is to repoint the telescope at blank sky and use sky subtraction to isolate the EMR from the target itself. If the amount of such emitted EMR is changing faster than the entire telescope can be repointed, an internal mechanism that can change the aim quickly is useful. The chopping mirror may be automatically rocked back and forth rapidly to alternately collect data on the target and the blank sky. This automatic rocking, which may be as much as ten times per second, is termed chopping.
The term nodding refers to a less-frequent change, e.g., a few times per minute that adjusts the aim so the other chopping position is pointed at the target. This is used for additional subtraction to reduce unwanted signal inherent in the optical paths associated with the two chopping positions, i.e., signal from within the telescope.