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Investigating the atmospheres of rocky exoplanets is key to performingcomparative planetology between these worlds and the terrestrial planets thatreside in the inner solar system. Terrestrial exoplanet atmospheres exhibitweak signals, and attempting to detect them pushes at the boundaries of what ispossible for current instrumentation. We focus on the habitable-zoneterrestrial exoplanet LHS 1140b. Given its 25-day orbital period and 2 hrtransit duration, capturing transits of LHS 1140b is challenging. We observedtwo transits of this object, approximately 1 yr apart, which yielded four datasets thanks to our simultaneous use of the IMACS and LDSS3C multiobjectspectrographs mounted on the twin Magellan telescopes at Las CampanasObservatory. We present a jointly fit white light curve, as well as jointly fit20 nm wavelength-binned light curves from which we construct a transmissionspectrum. Binning the joint white light-curve residuals to 3-minute time binsgives an rms of 145 ppm; binning down to 10-minute time bins gives an rms of 77ppm. Our median uncertainty in Rp^2/Rs^2 in the 20 nm wavelength bins is 260ppm, and we achieve an average precision of 1.3x the photon noise when fittingthe wavelength-binned light curves with a Gaussian process regression. Ourprecision on Rp^2/Rs^2 is a factor of four larger than the feature amplitudesof a clear, hydrogen-dominated atmosphere, meaning that we are not able to testrealistic models of LHS 1140b's atmosphere. The techniques and caveatspresented here are applicable to the growing sample of terrestrial worlds inthe Transiting Exoplanet Survey Satellite era, as well as to the upcominggeneration of ground-based giant segmented mirror telescopes.

Simultaneous Optical Transmission Spectroscopy of a Terrestrial, Habitable-Zone Exoplanet with Two Ground-Based Multi-Object Spectrographs