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

Abstract

High precision fast photometry from ground-based observatories is a challenge\ndue to intensity fluctuations (scintillation) produced by the Earth's\natmosphere. Here we describe a method to reduce the effects of scintillation by\na combination of pupil reconjugation and calibration using a comparison star.\nBecause scintillation is produced by high altitude turbulence, the range of\nangles over which the scintillation is correlated is small and therefore simple\ncorrection by a comparison star is normally impossible. We propose\nreconjugating the telescope pupil to a high dominant layer of turbulence, then\napodizing it before calibration with a comparison star. We find by simulation\nthat given a simple atmosphere with a single high altitude turbulent layer and\na strong surface layer a reduction in the intensity variance by a factor of ~30\nis possible. Given a more realistic atmosphere as measured by SCIDAR at San\nPedro M\\'artir we find that on a night with a strong high altitude layer we can\nexpect the median variance to be reduced by a factor of 11. By reducing the\nscintillation noise we will be able to detect much smaller changes in\nbrightness. If we assume a 2 m telescope and an exposure time of 30 seconds a\nreduction in the scintillation noise from 0.78 mmag to 0.21 mmag is possible,\nwhich will enable the routine detection of, for example, the secondary transits\nof extrasolar planets from the ground.\n