Due to atmospheric differences, and weather changes during the time the pictures were taken the apparent brightness of the images may differ. Comparing brightness of the images allows us to adjust for these conditions by adding or subtracting sky. The Sky tool measures the amount of brightness counts in the background of the image, which we would like to equal zero, but normally doesn't. After obtaining the brightness counts, we use the Subtraction tool or the Addition tool, to make the number of counts in the refernece image and the subsequent images equivalent. Normalization Normilization is a process which adjusts for brightness differences due to different observing conditions in the reference image and subsequent image.
Normalization consists of multiplyig the subsequent image by the normalizaiton factor.
C2 One uses the multiplication tool to complete this the procedure. Subtraction of the images After alignment, subtraction of sky, and normaliztion have been completed, one is able to subtract the images. Using the Subtraction tool one is able to subtract each subsequent image from the reference image. The resulting image shows the differences between the two original images.