Photometric defocus observations of transiting extrasolar planets

TL;DR

This study demonstrates that defocusing a small telescope during photometric observations of transiting exoplanets significantly improves measurement precision, achieving sub-millimagnitude accuracy over several hours.

Contribution

It introduces and validates the use of the defocus technique with a 0.6m telescope for high-precision photometry of transiting exoplanets, enhancing data quality over traditional in-focus methods.

Findings

Achieved sub-millimagnitude precision over hours for bright stars.

Defocus technique increases photon collection, reducing noise.

Longer exposures and stable tracking improve data quality.

Abstract

We have carried out photometric follow-up observations of bright transiting extrasolar planets using the CbNUOJ 0.6m telescope. We have tested the possibility of obtaining high photometric precision by applying the telescope defocus technique allowing the use of several hundred seconds in exposure time for a single measurement. We demonstrate that this technique is capable of obtaining a root-mean-square scatter of order sub-millimagnitude over several hours for a V $\sim$ 10 host star typical for transiting planets detected from ground-based survey facilities. We compare our results with transit observations with the telescope operated in in-focus mode. High photometric precision is obtained due to the collection of a larger amount of photons resulting in a higher signal compared to other random and systematic noise sources. Accurate telescope tracking is likely to further contribute to lowering systematic noise by probing the same pixels on the CCD. Furthermore, a longer exposure time helps reducing the effect of scintillation noise which otherwise has a significant effect for small-aperture telescopes operated in in-focus mode. Finally we present the results of modelling four light-curves for which a root-mean-square scatter of 0.70 to 2.3 milli-magnitudes have been achieved.