The magnitude value measured (or predicted) for a comet's nuclear condensation. Note that the true comet nucleus is rarely, if ever, directly observed from the earth because of the large amount of gas and dust that is ever-present in the inner coma close to the nucleus, serving to hide the true nucleus' surface. So-called "nuclear magnitudes" are therefore fraught with problems as to true meaning, especially because such nuclear magnitudes are extremely dependent upon instrumentation (aperture, focal-ratio, magnification) and wavelength. Nuclear magnitudes are chiefly used for astrometric purposes, in which predictions are made for the brightness of the comet's nuclear condensation so that astrometrists can gauge how faint the condensation is likely to be and thus how long an exposure is needed to get a good, measurable image. (Astrometrists are only concerned about measuring the nuclear condensation, which is considered to be the site of the main mass of any comet. ) See also definition for "Magnitude", below.