Conjugate Plane Photometry: Reducing Scintillation Noise in Ground-Based Astronomical Photometry

Scintillation is the phenomenon that we observe as the 'twinkling' of stars. It results from the optical effect of high-altitude turbulence in the Earth's atmosphere which, at any given time, diverts a fraction of the starlight into or out of the receiving aperture (e.g. telescope or eye) at the ground. This results in a rapid fluctuation of the total light intensity observed. For a large telescope, these fluctuations are averaged over the aperture area, so that the degree of 'twinkling' seen through a large telescope is much less than for the naked eye. However, for some types of telescope observations, these small residual intensity fluctuations are of critical importance: in particular for high-speed measurements of rapidly changing phenomena, such as the reduction in the measured light intensity which results when an extra-solar planet passes behind its parent star, as seen from Earth.

The dip in brightness resulting from such an exoplanet eclipse is usually very small (much less than 0.1%), since the planet contributes only a small fraction of the total light from the system. However, accurate observations of such eclipses can provide a wealth of information about the planet and its orbit around the parent star. For observations from the ground, scintillation noise limits the accuracy with which the eclipse 'light curve' can be determined so that, until now, the best data have typically been obtained with space-based telescopes, above the effects of the Earth's atmosphere.

Here we propose to develop a new technique, known as 'Conjugate Plane Photometry' to effectively 'un-twinkle' photometric observations with ground-based telescopes. In this method, the high-altitude turbulent layer is focused onto a circular mask within the optical system of the instrument. The mask aperture is slightly smaller than the diameter of the telescope itself. This mask rejects that part of the starlight, close to the edge of the telescope aperture, that could have been diverted by the high turbulence into or out of the telescope, causing scintillation. The size of intensity fluctuations due to scintillation are then greatly reduced, so that more accurate photometric observations become possible with ground-based telescopes.

Our proposed research seeks to demonstrate the full potential of the conjugate-plane photometry technique by developing a prototype instrument and deploying it on a large telescope. The ultimate goal of the work will be to demonstrate that photometric observations of phenomena such as exoplanet eclipses can be made from the ground with an accuracy approaching that which is possible with space-based telescopes.

The main long-term science application of the technology to be developed under this proposal is in the area of exoplanet research - specifically, physical characterization of exoplanets and their atmospheres via the observation of eclipse light-curves. Exoplanet science is of enormous and widespread public interest: no astronomy topic engages public imagination more. The development of new astronomical technologies is also of wide interest, and is a good subject on which to base public outreach topics and events - it combines the popular topic of astronomy with the application of new technologies (e.g. adaptive optics) that span a range of industrial applications and other fields. In our experience, this combination of pure science and the application of developing technologies is also very effective in attracting students into physics via astronomy at both the undergraduate and postgraduate levels. In the current economic climate we may also expect favourable public perception of projects such as this, which aim to achieve results via ground-based observations that could previously only be made from space at far greater cost

Historically there are a number of precedents for developments in imaging technology and methods pioneered within astronomy finding applications in other fields, such as medical imaging and surveillance; for example aspects of the development of CCDs, adaptive optics, interferometric and 'lucky' imaging methods. It will be important to achieve suitable dissemination of the theoretical and applied developments in scintillation suppression within the frame of this study, to promote possible adaptation and exploitation in other disciplines.

The PI and Co/I will ensure the wide dissemination of the results of this work by building on their previous experience in public engagement, and via the resources available to them within the host institutions and more widely through the research councils and the HEA. The PI has previously produced astronomy-based outreach material for schools and has initiated and supervised Durham undergraduate projects based on the development of school resources for the teaching of astronomy. The Co/I regularly issues press releases announcing major Ultracam results, e.g. see the Ultracam web pages: http://www.vikdhillon.staff.shef.ac.uk/ultracam/

Specific paths for dissemination will include Departmental and group web pages that are actively maintained at Durham (CfAI) and Sheffield. Public engagement in the topic through public lectures and outreach activities, including department open days and school visits, will be facilitated by the PI and Co/I individually, and in collaboration with departmental Outreach Officers. We anticipate writing an article for the STFC Innovations Club newsletter on the general technique of scintillation suppression and its possible applications.