We report Gemini-South GMOS observations of the exoplanet system WASP-29 during primary transit as a test case for differential spectrophotometry .
We use the multi-object spectrograph to observe the target star and a comparison star simultaneously to produce multiple light curves at varying wavelengths .
The ‘ white ’ light curve and fifteen ‘ spectral ’ light curves are analysed to refine the system parameters and produce a transmission spectrum from \sim 515 to 720 nm .
All light curves exhibit time-correlated noise , which we model using a variety of techniques .
These include a simple noise rescaling , a Gaussian process model , and a wavelet based method .
These methods all produce consistent results , although with different uncertainties .
The precision of the transmission spectrum is improved by subtracting a common signal from all the spectral light curves , reaching a typical precision of \sim 1 \times 10 ^ { -4 } in transit depth .
The transmission spectrum is free of spectral features , and given the non-detection of a pressure broadened Na feature , we can rule out the presence of a Na rich atmosphere free of clouds or hazes , although we can not rule out a narrow Na core .
This indicates that Na is not present in the atmosphere , and/or that clouds/hazes play a significant role in the atmosphere and mask the broad wings of the Na feature , although the former is a more likely explanation given WASP-29b ’ s equilibrium temperature of \sim 970 K , at which Na can form various compounds .
We also briefly discuss the use of Gaussian process and wavelet methods to account for time correlated noise in transit light curves .