Exposure selection results

In order to assess the imaging performance of the Strehl selection method, the data on V656 Herculis and $\epsilon$ Aquilae listed in Table 3.2 were analysed using the approach described in Figure 3.13. The best $1\%$ of exposures were selected and co-added - the resulting images for V656 Herculis and $\epsilon$ Aquilae are shown in Figures 3.14a and 3.14b. Shown beneath are the average (seeing-limited) images from the same data in Figures 3.14c and 3.14d, representing conventional long exposures. It is possible that telescope tracking errors might have contributed to the asymmetry in the long exposure image of V656 Herculis, but it is difficult to distinguish these errors from the random motion due to the atmosphere.

The image selection method provides images with FWHM of $80\times 94$ $mas$ for V656 Herculis and $79\times 94$ $mas$ for $\epsilon$ Aquilae, a very substantial improvement over the FWHM of the conventional astronomical images ($490\times 600$ $mas$ and $380$ $mas$ respectively). In the Lucky Exposures images the first Airy ring is visible (although it is not uniform around the stars). In both images the total flux beyond the first Airy ring is relatively small. If these PSF were available for imaging complex fields, extremely high image resolution and quality would be obtained.

Figure 3.14: Two stars were observed on the first night at the NOT without saturation - V656 Herculis and $\epsilon$ Aquilae. Panels a) and b) show the best 1% of exposures shifted and added for V656 Herculis and $\epsilon$ Aquilae respectively, processed using the method described in the text. Beneath these panels are the respective averaged images in panels c) and d). These were generated by summing all of the short exposures without re-centring, and represent the conventional astronomical seeing disks at the times of the observations. The Strehl ratios and FWHM for the four images are: a) $0.21$ and $80\times 94$ $mas$, b) $0.26$ and $79\times 94$ $mas$, c) $0.018$ and $490\times 600$ $mas$, d) $0.033$ and $380$ $mas$.