Atmospheric dispersion and $\beta$ Delphini

Fig. 3.35 shows the result of a selection of the $1\%$ of images with the best Strehl ratios from a dataset of $7000$ short-exposure CCD images of the binary $\beta$ Delphini. In this case the zenith angle of the observation was $50^{\circ}$ and the images are blurred by $100$ $mas$ due to atmospheric dispersion over the $125$ $nm$ bandpass of the filter, reducing the Strehl ratio of the final image to $0.20$. This dispersion results from the variation in the refractive index of air with wavelength, which causes a change in the atmospheric refraction angle as a function of wavelength. It can be corrected using glass prisms with an appropriate wedge angle.

Figure 3.35: The Lucky Exposures method as applied to the spectroscopic binary $\beta$ Delphini. The best $1\%$ of exposures of $\beta$ Delphini were selected, shifted and added to produce this image. The blurring evident in the image is thought to result from atmospheric refraction.

The magnitude difference between the components is $\Delta m=1.070\pm0.005$. This value is in good agreement with those of Barnaby et al. (2000) of $1.071\pm0.004$ at $798$ $nm$ and $1.052\pm0.010$ at $884$ $nm$ made using a $1.5$ $m$ telescope.