measured as a function of wavelength (e.g. @Copperwheat2013; @Bento2014). The use of frame transfer CCDs allow for high frame rates (up to 300Hz) with little dead time (24ms), which is useful to avoid saturation when observing bright stars and enables many more sky flats to be taken.

The observations were made using SDSS $u'$ ($\lambda_{central}$ = 3557Å, FWHM = 599Å) and $g'$ ($\lambda_{central}$ = 4825Å, FWHM = 1379Å) filters and a filter centred on the NaI doublet ($\lambda_{central}$ = 5912Å, FWHM = 312Å). These filters were selected to probe for Rayleigh scattering by observing the difference in transit depth between the $u'$ and $g'$ bands and to search for the broad wings of the sodium doublet with the NaI filter.

The observations were performed with moderate telescope defocussing ($\sim 3$ arcsec) in windowed mode with exposure times of 0.76 seconds in the red and green channels and a cadence of 0.79 seconds. Due to the reduced photon count in the blue channel multiples of 10 frames were averaged on-chip before readout, leading to a 7.9 second exposure time in this channel.

The moon was at 54% illumination on the night of our observations and we analysed the data with an 