Tilted-secondary mirror (TSM) for forward motion compensation in high resolution earth observation satellite cameras

Abstract

The most important difficulty encountered in obtaining high-resolution and quality images from earth observation satellites is the image motion that occurs in the focal plane of the optical system due to the orbital movement of the satellite. The orbital motion of satellites requires optical systems to perform their tasks on a fast-moving platform. As a result of this, smear occur in the obtained ground images, this reduces the image quality and is resulting in blurred images. By keeping the exposure time short so that images of the desired region can be captured in a short time to reduce blur, however, in this case, the signal-to-noise ratio of the image and its contrast are reduced, resulting in noisy and poor-quality images. Forward Motion Compensation (FMC) methods make it possible to obtain high quality images by limiting the focal plane image motion caused by the forward motion of satellites. In this research, it has been extensively analyzed why the secondary mirror of RC telescope has been chosen to apply tilt motion for the purpose of FMC, and the best location of the pivot point has been analyzed by using Zemax OpticStudio program. When tilting the secondary mirror, it is important to avoid causing de-focus. In addition, the pivot point must be located in the optical axis directly behind the secondary mirror, and results have shown that a pivot point distance of 24 mm is also sufficient for the mechanical design of the piezo tilt stage. Approximately 52 pixels can be shifted by moving the secondary mirror by 0.015 degrees. This movement allows for an increase in the image exposure time by 52 times. TSM to compensate for forward motion allows high-resolution images to be obtained without degrading image quality by limiting focal plane image motion and still maintaining the diffractionlimited performance of the optical system. This research has been proposed to develop new method to increase the exposure time without need of additional optical element to apply tilt motion, such as scanning mirrors or high cost TDI sensors.