Background

During the 1990s the COAST group in Cambridge developed a high frame-rate CCD camera for use in the JOSE seeing monitor based at the William Herschel Telescope on La Palma. The high sensitivity of this camera, and the possibility of adjusting the frame rate and the area of the CCD to be read out made it an ideal choice for experimental observations using the Lucky Exposures method. The unthinned CCD within the camera had a peak sensitivity between $\sim500$ $nm$ and $\sim900$ $nm$ (around the R and I astronomical wavebands).

The optimum aperture size to use for the Lucky Exposures method depends on a compromise between the probability of obtaining a short exposure with a high Strehl ratio and the limiting magnitude of reference star that can be used. The numerical simulations presented in Chapter 2.3.3 suggested that this compromise would be met for telescopes with diameter $d$ seven or eight times greater than the atmospheric coherence length $r_{0}$. The value of $r_{0}$ depends both on the atmospheric conditions and the wavelength at which the observations are undertaken, as described by Equations 2.8 and 2.9. $r_{0}$ is estimated to be $35$ $cm$ at I-band under the median summer seeing conditions at the NOT site (Vernin & Muñoz-Tuñón, 1994; Muñoz-Tuñón et al. , 1997), making the $2.56$ $m$ aperture of this telescope ideally suited to Lucky Exposures experiments in this waveband at this time of year. The exceptionally good astronomical conditions at this site in the summer can be partially attributed to the thermal inversion layer(s) usually being at a lower altitude than the observatory (Figure 3.1).

Figure 3.1: During the summer months the NOT telescope is usually above the thermal inversion layer(s). An inversion layer is typically found at the height of the cloud deck, seen here below the telescope.

In May 2000 high frame-rate imaging observations were undertaken by John Baldwin, Craig Mackay and Graham Cox at the NOT using the JOSE camera. In order to accurately characterise the temporal characteristics of the atmosphere, the observations were made with high frame rates with no autoguider in operation. All of the stars observed were relatively bright (with $I<6$) allowing the shape of the stellar PSF to be recorded with high signal-to-noise in each short exposure.