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A systematic error (plural: systematic errors, often shortened to systematics) of a measurement (e.g., a measurement made by a telescope instrument), is an error in the measurement that is not random, but rather a measurement error inherent in the design of the device. A very simple example is a sensor with a duty cycle, i.e., it is sensing a certain percent of the time but is sensing during the periodic times that data is being unloaded and the resulting measurement of received EMR is skewed. Another simple example is a non-flat sensitivity function of a receiving instrument: the amount received at different wavelengths varies. (These cases are so straightforward that they hardly deserve the name systematic error, but instruments have more-complicated and less-clear instances.) The term may sometimes be used for natural causes, e.g., airmass, and/or may sometimes be known issues with mathematics used. Systematic errors are in contrast to the random errors due to small imperfections in the equipment, or due to over-precise quantification, in effect adding random digits to the end of the quantity. If the systematics are known, the measurements can be culled to avoid them, adjusted in a way that compensates for them, and/or the sources of error may be noted.