Double Cluster with Field Rotation
This is a stack of four images. Field rotation towards the edges of the image is clearly visible (click on the image for a larger version); stars appear as streaks. The degree of rotation and the size of the affected area varies depending on the location of the object(s) in the sky. The only practical solution for amateur astronomers is an equatorial wedge mount that aligns the telescope with the celestial pole. Field derotators are also an option, but they tend to be expensive and have results of variable quality.