| Astrophysics (Index) | About |
Sky subtraction is a technique that assists in observation of astronomical objects: removing the general background radiation of the sky from the observation data of a specific object or objects. The aim is to reduce the EMR added to the image by the intervening interstellar medium, interplanetary medium, and Earth atmosphere, i.e., that not from the source itself, as well as EMR from behind any unresolved sources. 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., its spectral energy distribution over the same range in the case of spectrography, or through the same filter in the case of photometry. Methods of optimizing this process include smoothing of the observation data of the "empty" region, which undoubtedly is not completely empty, and/or using the average of more than one empty region. In the age of CCDs and digital images, subtraction pixel-by-pixel is typical. A kind of sky subtraction is also very useful in polarimetry.