Conjugate-plane photometry: reducing scintillation in ground-based photometry

High-precision fast photometry from ground-based observatories is a challenge due to intensity fluctuations (scintillation) produced by the Earth's atmosphere. Here we describe a method to reduce the effects of scintillation by a combination of pupil reconjugation and calibration using a comparison star. Because scintillation is produced by high-altitude turbulence, the range of angles over which the scintillation is correlated is small and therefore simple correction by a comparison star is normally impossible. We propose reconjugating the telescope pupil to a high dominant layer of turbulence, then apodizing it before calibration with a comparison star. We find by simulation that given a simple atmosphere with a single high-altitude turbulent layer and a strong surface layer, a reduction in the intensity variance by a factor of similar to 30 is possible. Given a more realistic atmosphere as measured by Scintillation Detection and Ranging (SCIDAR) at San Pedro Martir, we find that on a night with a strong high-altitude layer we can expect the median variance to be reduced by a factor of similar to 11. By reducing the scintillation noise we will be able to detect much smaller changes in brightness. If we assume a 2-m telescope and an exposure time of 30 s, a reduction in the scintillation noise from 0.78 to 0.21 mmag is possible, which will enable the routine detection of, for example, the secondary transits of extrasolar planets from the ground.