  We report on a pilot study of a novel observing technique, defocussed
transmission spectroscopy, and its application to the study of exoplanet
atmospheres using ground-based platforms. Similar to defocussed photometry,
defocussed transmission spectroscopy has an added advantage over normal
spectroscopy in that it reduces systematic errors due to flat-fielding, PSF
variations, slit-jaw imperfections and other effects associated with
ground-based observations. For one of the planetary systems studied, WASP-12b,
we report a tentative detection of additional Na absorption of
0.12+/-0.03[+0.03]% during transit using a 2A wavelength mask. After
consideration of a systematic that occurs mid-transit, it is likely that the
true depth is actually closer to 0.15%. This is a similar level of absorption
reported in the atmosphere of HD209458b (0.135+/-0.017%, Snellen et al. 2008).
Finally, we outline methods that will improve the technique during future
observations, based on our findings from this pilot study.
