Coronography

The techniques discussed previously permit the resolution of objects at small angular separation by improving the angular resolution of the system. However, the contrast of the two objects is also an important consideration - it is more difficult to reliably detect and characterise a faint object adjacent to a bright one than a pair of objects of similar brightness. While PSF modelling and subtraction techniques would ideally allow for the full removal of light from one star, leaving fainter objects easily detectable, uncertainties in the modelling and noise in the recorded image can leave residuals that mask or can be confused with the fainter objects of interest. Furthermore, scattered starlight caused by imperfections in the optical system can add a significant amount of background noise. Finally, a practical consideration is that the presence of a bright star within the field of view limits the maximum possible exposure time that can be achieved before causing effects such as detector saturation and persistence; these short exposure times can significantly compromise the chances of detecting a faint companion at all.

A coronograph is a device designed to block a significant amount of light from a source at the centre of the image before it reaches the detector, while light from other objects in the field of view passes through with minimal attenuation and aberration. The technique was first applied successfully by Lyot (1939), after whom the classical Lyot coronograph is named. Two important concepts that are common to all corono- graphic techniques are the pupil plane, in which the incoming rays of light are parallel and the telescope pupil is imaged, and the image or focal plane, in which the rays of light are focus and the astronomical objects are imaged. A lens allows light to be transformed from one plane to the other, and the intensity of light in one plane is the

Fourier transform of the intensity in the other.

In general, the incoming light is transformed from the pupil plane to the image plane, in which a focal plane mask is placed. The remaining light is then transformed back to the pupil plane, in which a Lyot stop is placed, which rejects additional un- wanted light that remains after the focal plane mask. Finally, the light is once again transformed back to the image plane on the detector. In a classical Lyot coronograph, the focal plane mask takes the form of a small occulting spot, which for solar obser- vations blocks the disc of the Sun, and for observations of more distant (unresolved) sources, blocks the core of the star’s diffraction pattern. The Lyot stop in such a system is a baffle that removes light at the edges of the image plane, this light having leaked past and been diffracted by the occulting spot.

A large number of alternative coronograph designs exist, each attempting to optimise one or more of the characteristics of the device. Guyon et al. (2006) provides a review of the numerous competing designs that were being considered for the current generation of exoplanet direct imaging instruments such as SPHERE (Beuzit et al. 2008), GPI (Macintosh et al. 2008), and SCExAO (Jovanovic et al. 2015), all of which include coronographs as a fundamental component. Kasdin et al. (2003) details four parameters that influence the design of coronographs:

• The contrast of the object of interest relative to the diffuse background in the image,

• The transmission of the system for light from object of interest (and hence the required exposure time),

• The inner working angle (IWA) or inner working distance (IWD), the angu- lar separation below an object of interest becomes strongly affected by the coronograph,

• The discovery space, the fraction of the field of view in which an object of interest can be found.

For a classical Lyot coronograph, the contrast, transmission, and IWA are closely linked. A larger baffle forming the Lyot stop improves contrast by reducing scattered starlight, but reduces transmission by blocking some light from the object of interest too. A larger occulting spot will block a larger fraction of the star’s PSF, but as the spot grows larger, objects of interest at larger angular separation are also blocked, increasing the IWA. The discovery space of the classical Lyot coronograph is one of its advantages, as the full field of view is available – a notable example of a design in which there is a strong angular dependence is the Four-Quadrant Phase-Mask, which significantly attenuates companions that fall near the boundaries between the mask’s four quadrants (Rouan et al. 2000). Fig. 1 of Guyon et al. (2006) provides a detailed visual comparison of the behaviour of various coronograph designs in relation to the four parameters listed above.

3

Lucky imaging of transiting exoplanet

host stars

The main work covered in this thesis is a multi-year campaign to obtain lucky imaging observations of transiting exoplanet systems. The main objective of this work has been to constrain the existence of contaminating stars at small angular separation from these systems, and to analyse and catalogue any detected ‘companion’ stars. For each of these, relative photometry was obtained in at least one photometric band, with simultaneous observations in two bands becoming routine from September 2014 onwards. In addition, astrometric measurements (i.e. separation and position angle) relative to the planet host star were derived for each companion star.

Where photometry was obtained in both bands, the temperature of each detected companion star was determined, which was further used to determine if the companion star’s photometric distance was consistent with the distance of the planet host star, and hence that the two stars potentially form a bound binary system. For systems identified as potential binaries, a literature search was performed for additional photo- metric, astrometric, or spectroscopic data which supported (or rejected) the binary star hypothesis. Various features of the population of confirmed or plausible planet-hosting binaries derived from both this work and others were studied, such as the proportion of hot Jupiters in stellar binaries, and the distribution of companion star masses.

The work in this chapter has been published in two separate refereed papers, with Evans et al. (2016) covering the analysis of observations during 2014, and Evans et al. (2018) covering those results from 2015 and 2016.