TDI (Time Delay Integration) imaging lets you image large areas of the sky without guiding - so that any telescope will do. In TDI, you point the camera at an area of the sky and turn off the telescope drive. This lets the stars drift across the field of view. The imaging detector is clocked (read out) one column of pixels at a time in sync with the rate of stellar drift. This keeps the accumulating charge underneath each star's image until the star reaches the edge of the CCD, and the readout register, at which point the signal is sent to your computer for storage and later processing. The resulting image has an exposure time equal to the amount of time it takes the stars to drift across the CCD.

Depending on the focal length of the telescope, this can be many minutes. Because the scope itself is not moving, there is no periodic error and no guiding error during the exposure! Furthermore, the image you are recording scrolls as the sky moves past, so the image file can be as many pixels tall as your CCD imaging detector is tall by 1000 pixels in width, or 2000 pixels wide, or 3000, etc.

An SBIG user, Christoph Flohr, has written a very capable program for doing this kind of imaging with SBIG cameras, particularly the ST-7/8/9/10 series. The program can be downloaded for free from www.driftscan.com. For a more complete SBIG .pdf file describing the TDI process, click on this link.