BackgroundModelization

Abstract

Uneven Field Illumination (UFI) is a common problem shared by virtually all astrophotographs. Vignetting—where the corners of an image are significantly darker than its bright center is a classic example. Vignetting is caused by a multiplicative gradient, inherent in most optical systems. Multiplicative gradients multiply pixel values based on their location. Pixels further away from the center of an image are multiplied by smaller values than those that are on-axis, making them dimmer. This type of gradient is generally predictable and therefore, relatively easy to remove. The method for correcting a multiplicative gradient is its opposite-division, and specifically division by a master flat frame during preprocessing. Despite flat-fielding however, there is often some residual unevenness remaining in the illumination pattern of the image (Fig. 9.1).