A systematic error (plural: systematic errors, often shortened to systematics) in a measurement, e.g., by the instrument of a telescope, is an error that is not random, such as an error created in the design of the device: a very straight-forward example might be a sensor with a duty cycle, i.e., it is sensing a certain percent of the time but is failing to sense periodically as the data is unloaded: its measurement of received EMR is skewed. Another example is a non-flat sensitivity function of receiving instruments: the amount received at different wavelengths varies. The term may sometimes be used for natural sources, e.g., airmass, or known issues with mathematics. This type of error is in contrast to the random errors due to small imperfections that are unknown or cannot be avoided. If the systematics are known, the measurements can be culled to avoid them, adjusted to compensate, or noted.