Apodizing Phase Plate Coronagraph

Introduction

The coronagraph was invented by Bernard Lyot in 1930 to enable observations of the Sun's outer atmosphere. The faintly glowing extended gas in the sun's corona was only visible during a Solar eclipse by the Moon, and Lyot wanted to carry out observations without waiting years at a time.

The optical layout of his Lyot coronagraph is shown below:

The telescope focuses light from both the star and the planet at the Focal Plane, where a mask then blocks out light from the star, but the planet (which is in a slightly different place on the sky) is not blocked by the mask. A lens takes the light from the FP and forms a Pupil Plane (PP). Here, the planet's pupil is an even disk of light, but for the star, the presence of the metal disk in the FP causes light to be redistributed into a series of bright rings. A mask with a diameter smaller than the full telescope pupil image diameter then blocks most of this scattered star light - this is called a Lyot stop. A second lens then reimages the pupil to form the final sky image on the detector.

The Lyot coronagraph is simple, but it has several disadvantages:

• If the star image is not kept centred on the metal disk, starlight “leaks” around the edge of the disk and into the rest of the coronagraph, increasing unwanted light and decreasing the signal to noise at the location of the planet.
• The Lyot stop blocks the outer edge of the telescope pupil, and so the angular resolution of the telescope is reduced - the planet image gets larger in diameter.
• The total throughput of the coronagraph decreases due to the Lyot stop's smaller diameter, and there is a trade-off between throughput and contrast.

Many coronagraphs are sensitive to pointing errors, and so require fast closed tip-tilt loops to ensure that the target star stays in the same position in the FP and so that the suppression works. By designing an optic that modifies only the phase in a pupil plane of a telescope, we remove this constraint. This Phase Apodization Coronagraphy (Codona and Angel 2004) is realised in the form of an Apodizing Phase Plate (APP) (Kenworthy et al. 2007) coronagraph:

By having no FP mask, there is no spurious diffraction in the PP. Instead there is the APP Coronagraph, which modifies the point spread function (PSF) of all objects in the field of view of the coronagraph.

• The APP is completely insensitive to vibrations in the telescope and the AO system tip-tilt correction.
• It is a very simple coronagraph, consisting of only one optic.
• The phase pattern is a smoothly varying function of position, with no discontinuities or vortex singularities. Mathematically ideal coronagraphs with these types of discontinuity are challenging to manufacture and can result in degradation of the coronagraphic null and reduced imaging performance.

• The APP PSF has suppression for only half the field of view in a D shaped pattern.
• The suppression in the D and its Inner Working Angle and the encircled energy transmission are related, and have to be traded off against each other. For most designs we keep 60 to 70% of the encircled energy within the Airy disk of the PSF.

The advantages of the APP outweigh its disadvantages, most significantly the insensitivity of the APP to tip-tilt errors, which has limited the adoption of coronagraphs on ground based telescopes. The robustness of its design means that it has been adopted for use on several large telescopes and is being considered for several more.

Implementation

We introduce variations in phase by varying the thickness of a transmissive plate at the pupil stop location in the science camera. We use Zinc Selenide (with an approximate refractive index of 2.4) and adjust its thickness by using a diamond turning process. Variations in the thickness $\delta t$ lead to variations in phase $=\delta \phi$ for a given wavelength $\lambda$ and refractive index $n$:

$$\delta \phi = \delta t .(n-1).(2\pi/\lambda)$$

The resultant final optic is shown below:

On-sky performance

The APP has been tested and used on the 6.5m MMTO telescope with the Clio camera, and with NaCo on the 8.4m Very Large Telescope. The image below shows the difference of two successive images taken with NaCo at 4.05 microns on the VLT, scaled logarithmically to show the faint diffraction ring details. The left hand image shows the PSF of the telescope as formed by the APP, and the right hand image is the subtracted beamswitched pair. As can be seen, the APP coronagraph suppression works for any object on the sky and position on the science camera.

The performance is shown in data from Meshkat et al. (2014) with the graph for the $7 \sigma$ contrast curve for the star Fomalhaut. We see that we reach the background limit for this star at 0.5 arcseconds of 12.0 magnitudes in a narrowband filter at $4.05\mu m$ with a stellar magnitude of $L=1.03$ mag.

Literature

• “Imaging Extrasolar Planets by Stellar Halo Suppression in Separately Corrected Color Bands” Codona and Angel (2004) ApJL 604, 117
• “A high-contrast coronagraph for the MMT using phase apodization…”[Codona et al. (2006) SPIE 6269]
• “First on-sky high contrast imaging with an apodizing phase plate” [Kenworthy et al. (2007) ApJ 660, 762]
• “A new coronagraph for NAOS-CONICA - the Apodizing Phase Plate” [Kenworthy et al. (2010), Messenger 141, 2]
• “First results from VLT NACO APP: 4 micron images of the exoplanet beta Pictoris b” [Quanz et al. (2010) ApJL 722 49]
• “Searching for gas giant planets on Solar System scales: VLT NACO/APP observations of the debris disk host stars HD 172555 and HD 115892”[Quanz et al. (2011) ApJL 736, 32]