Exposure selection

In order to apply the Lucky Exposures image selection procedure to observational data taken on astronomical sources, one star in the field can be selected to act as a reference for measurement of the Strehl ratio and position of the brightest speckle. The data reduction software written by the author selected a small rectangular region in each short exposure which surrounded the reference star, but did not include a significant flux contribution from any other sources in the field. This region of each short exposure was then sinc-resampled to have four times as many pixels in each dimension. The Strehl ratio and position of the brightest speckle were then calculated from the resampled image region. The exposures having the highest Strehl ratios were then selected for further processing. A summary of this approach to the data reduction is given in Figure 3.13.

Figure 3.13: Flow chart describing the data reduction method for Lucky Exposures imaging of astronomical targets.

The sinc-resampling process described in panel c) of Figure 3.13 is one of the most computationally intensive parts of the data reduction as it involves two Fast Fourier Transforms (FFTs). By limiting the area of the image which is resampled to a small region around the reference star, the speed of the data reduction process is dramatically improved, allowing near real-time data-reduction while observing using a year 2000 vintage PC. The time-saving became even more significant in later runs using larger image dimensions.

After the position of the brightest speckle and Strehl ratio for each short exposure had been calculated in panel d) of Figure 3.13, the short exposures with the highest Strehl ratios were selected for further processing. The full frame image for each of these short exposures was sinc-resampled, and then re-centred and co-added based on the location of the brightest pixel in the reference star image as calculated in panel d). The process of sinc resampling the full short exposure image was computationally intensive, but this was only applied to the selected exposures (typically $1$--$10$% of the total number of exposures).