Sky Subtraction is a technique that assists in observation of objects: removing the general background radiation of the sky from the observation of a specific object or objects. This can be of use in Photometry and Spectrography. The simplest form is equivalent to using the image of a nearby empty region of sky and subtracting: i.e., the entire Spectral Power Distribution for spectrography, or through the same bandpass filter in the case of photometry. Additional measures would be to first do some smoothing of the "empty" region, which undoubtedly is not completely empty, and/or to use the average of more than one empty region. In the age of CCDs and digital images, subtraction pixel-by-pixel is typical.