Data taken on $\zeta$ Boötis

Two runs were taken on the $0.8$ $as$ binary $\zeta$ Boötis, each comprising $24000$ short exposures. During both of these runs the binary drifted slightly across the detector due to telescope tracking problems. The parts of each of the two runs were identified where both binary components were well within the readout region of the CCD (more than $16$ pixels from the edge), and only these short exposures were used in the following analysis.

Two example short exposures of $\zeta$ Boötis are shown in Figures 3.20a and 3.20b. Figure 3.20a is typical of the data set (with a Strehl ratio close to the median) while Figure 3.20b has an unusually high Strehl ratio of $0.26$. In both of these exposures there is strong similarity between the shapes of the speckle patterns around each binary component, suggesting that the PSF due to the atmosphere is the same for both stars. The small differences which are visible between the images of the two binary companions can partly be explained by the different alignment of the stellar images with respect to the pixel grid of the CCD. Figure 3.20c shows the effect of sinc-resampling the image shown in Figure 3.20b to have four times as many pixels in each dimension, revealing a hint of the first Airy ring. The similarities between the stellar images of the two binary companions is even more pronounced in this resampled image.

Figure 3.20: Example short exposure images of $\zeta$ Boötis: a) a typical exposure, having Strehl ratio of 0.074 (close to median); b) a good exposure with Strehl ratio of 0.26; and c) the same exposure as shown in b) but sinc-resampled to have four times as many pixels in each dimension.

The Lucky Exposures image selection procedure was applied to the data using the approach described in Chapter 3.3.2. The left-hand brighter component of $\zeta$ Boötis was initially used as the reference star.