TIME-DELAY INTEGRATION CCD READ-OUT TECHNIQUE - IMAGE DEFORMATION

The introduction of electro-optical tracking with CCDs has revitalized the research potential of the transit telescope. Termed the time-delay integration (TDI) read-out technique (or drift scanning), this method offers exceedingly good flat-fielding performance. While this asset is not disputed, there has been no quantitative analysis of potential drawbacks of the TDI technique. The work presented here is the first study of the two main sources of image deformation intrinsic to TDI data: (i) the discrete shifting of CCD pixels to match a continuously moving object leads to a symmetrical elongation of the object's east-west profile, and (ii) the curvature of objects across the CCD columns results in an asymmetrical image deformation and shift of the peak intensity in the north-south direction. It is found that undersampled data (i.e., Gaussian point spread function standard deviations sigma less than or similar 0.85 pixels) suffer severely due to the first source, while star trail curvature deviations greater than or similar to 0.8 pixels can lead to images susceptible to the second. A comparison between three systems which utilize the TDI mode is also resented.