The shift-and-add method (more recently "image-stacking" method) is a form of speckle imaging commonly used for obtaining high quality images from a number of short exposures with varying image shifts. It has been used in astronomy for several decades, and is the basis for the image stabilisation feature on some cameras. The method involves calculation of the differential shifts of the images. This is easily accomplished in astronomical images since they can be aligned with the stars. Once the images are aligned they are averaged together. It is a basic principle of statistics that variation in a sample can be reduced by averaging together the individual values. In fact, when using an average, the signal-to-noise ratio should be increased by a factor of the square root of the number of images. A number of software packages exist for performing this, including IRAF, RegiStax, Keiths Image Stacker, Hugin, and Iris.
- Baba, N.; Isobe, S.; Norimoto, Y.; Noguchi, M. Stellar speckle image reconstruction by the shift-and-add method, Applied Optics (ISSN 0003-6935), vol. 24, May 15, 1985, p. 1403-1405.
- Christou, J. C., Image quality, tip-tilt correction, and shift-and-add infrared imaging, Publications of the Astronomical Society of the Pacific, vol. 103, Sept.1991, p. 1040-1048.
- Hugin - open source image software with shift-and-add "image-stacking"
- Iris - freeware astronomical images processing software
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