Orthogonal-Transfer CCD

The orthogonal-transfer CCD (OTCCD) is an imager that performs a noiseless shifting of charge in either of the two orthogonal directions in the device plane during image exposure to improve performance in cases where the image is undergoing translational motion. By shifting the already-collected charge to align with the shifted image in real time during exposure, the OTCCD enables an image that is free of the motion blur that a standard imager will experience. One example of an application for which the OTCCD is useful is the case in which there are vibrations from either the camera or the object being imaged or both. Another example involves compensation of atmospheric image distortion in ground-based telescopes. Much of atmospheric distortion for a modest angular field of view consists of translational motion, and its removal by the OTCCD results in a sharper and brighter image of the objects being studied.

Figures 1a (left) and 1b (right) show a four-pixel portion of an OTCCD.

A four-pixel portion of one design to implement an OTCCD is shown in Figure 1. In this design, there are four independent CCD gates in each pixel (numbered 1–4 in Figure 1a). (Note that all type 1 gates are connected together in the imager, and likewise for 2, 3, and 4). To transfer charge, a high positive voltage is applied to adjacent gates consecutively, while all other gates are held to a low voltage. A potential well is formed under the gate with the high potential, and a packet of photoelectrons is held there. As the high potential is moved to neighboring gates, the packet of charge follows.

In Figure 1a, gates of type 4 are held low (therefore forming a vertical barrier to charge flow), while a high voltage is applied consecutively to 1, 2, and 3, causing charge to move in a vertical direction. In Figure 1b, gates of type 1 are held low (now forming horizontal barriers) while a high voltage is applied consecutively to 2, 3, and 4, causing charge to move only in a horizontal direction. Therefore, by using various combinations of these two modes of operation, we can cause charge to move to any adjacent pixel (a diagonal move requires a combination of horizontal and vertical moves).

To demonstrate the improvement of imagery, an OTCCD imager was mounted on a spring and imaged a stationary picture on the wall while the imager was bouncing. A point source of light on the wall was used to determine the motion of this spring-mounted camera.

Figure 2a (left) shows an image taken with the OTCCD feature disabled, and Figure 2b (right) shows the image with the OTCCD enabled. (Click on image to see larger version.)

Figure 2a was taken with the OTCCD feature disabled, so the imager was operating like a conventional CCD. The blurring is caused by the motion of the image across the device during the image-integration period.  Figure 2b is an image taken by the same imager, again mounted on a spring and bouncing, but this time operating as an OTCCD imager with the charge moving in synchronization with the motion of the image across the imager. The improvement in the second image is obvious.

Figure 3. Two surface plots of imagery Figure 3. Two surface plots of imagery from a portion of a star cluster. (Click on image to see larger version.)

The OTCCD has also been used in ground-based astronomy photography to remove most of the jitter caused by atmospheric turbulence. Figure 3 shows two surface plots of imagery from a portion of the star cluster M71. The data in Figure 3 (left-hand image) was taken with no compensation shifting of the OTCCD pixels (that is, the OTCCD was operating like a normal CCD), while the data represented in Figure 3 (right-hand image) was taken with the imager operating as an OTCCD, using a bright guide star to measure the amount of jitter of the image. The star images have an improved signal to noise of 1.5× in the compensated image.


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