Sensitivity to faint objects

The recent development of CCDs with negligible readout noise (see e.g. Mackay et al. (2001); Robbins & Hadwen (2003)) has almost eliminated the noise penalty for high frame-rate imaging at CCD wavelengths. For the first time this has made shift-and-add imaging competitive with adaptive optics for the imaging of very faint objects at I band.

For unresolved sources, the high resolution in Lucky Exposures images can help to reduce the effect of the sky background contribution on images. However, if a large fraction of the observation data is discarded, this necessarily has an impact on the sensitivity of the technique to faint objects in a fixed period of observing time. Astronomers using the Lucky Exposures method have to make a trade-off between high resolution (obtained using a very small fraction of the exposures) and high sensitivity (the fraction of exposures which should be selected to obtain the maximum sensitivity to a faint source depends on the source geometry and observing conditions, but is typically a large fraction of the total number of exposures).