K correction

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K correction is a correction to an astronomical object's magnitude (or equivalently, its flux) that allow a measurement of a quantity of light from an object at a redshift z to be converted to an equivalent measurement in the rest-frame of the object. This conversion offers a view of what is called the rest-frame. If one could measure all the light from an object at all wavelengths (a "bolometric" flux), a K-correction is not required. If one measures the light emitted in an emission-line, a K-correction is not required. The need for a K-correction arises because an astronomical measurement through a single filter or a single bandpass only sees a fraction of the total spectrum, redshifted into the frame of the observer. So if the observer wants to compare the measurements through a red filter of objects at different redshifts, the observer will have to apply estimates of the K-corrections to these measurements to make a comparison.

The exact nature of the calculation that needs to be applied in order to perform a K correction depends upon the type of filter used to make the observation and the shape of the object's spectrum.