Results with binary stars

On the night of 2000 May 13, a number of binary stars were observed, as listed in Table 3.3. These were used to give an indication of the imaging performance of the Lucky Exposures method for astronomical targets where an off-axis reference star is required for exposure selection and re-centring. One of the stars in the binary is used as the reference star for measurements of the Strehl ratio and relative position of the brightest speckle. After exposure selection, re-centring and co-adding, the image of the binary companion then provides a measure of the imaging PSF.

The correlation between the Strehl ratios measured on one binary component with those measured on the other binary component in individual exposures can provide lower limits on both the atmospheric isoplanatism and the signal-to-noise ratio for the Strehl ratio measurements, as the dominant sources of noise (photon shot noise, stochastic detector readout noise) will not be correlated for the two stellar images. Analyses of this sort will also be presented in this section. If the detector ``pattern noise'' was strongly correlated for large distances across the short exposures this might have given a correlated error to the measured Strehl ratios for the two stars. Measurements of the summed Fourier power spectrum for the short exposure images indicate that the pattern noise should not have made a significant contribution to the Strehl ratios for these observations of $\zeta$ Boötis, however.