No abstract

Extra-solar planets have not been imaged directly with existing ground or space telescopes because they are too faint to be seen against the halo of the nearby bright star. Most techniques being explored to suppress the halo are achromatic, with separate correction of diffraction and wavefront errors. Residual speckle structure may be subtracted by differencing images taken through narrowband filters, but photon noise remains and ultimately limits sensitivity. Here we describe two ways to take advantage of narrow bands to reduce speckle photon flux and to obtain better control of systematic errors. Multiple images are formed in separate color bands of 5-10% bandwidth, and recorded by coronagraphic interferometers equipped with active control of wavefront phase and/or amplitude. In one method, a single deformable pupil mirror is used to actively correct both diffraction and wavefront components of the halo. This yields good diffraction suppression for complex pupil obscuration, with high throughput over half the focal plane. In a second method, the coronagraphic interferometer is used as a second stage after conventional apodization. The halo from uncontrollable residual errors in the pupil mask or wavefront is removed by destructive interference made directly at the detector focal plane with an "anti-halo", synthesized by spatial light modulators in the reference arm of the interferometer. In this way very deep suppression may be achieved by control elements with greatly relaxed, and thus achievable, tolerances. In both examples, systematic errors are minimized because the planet imaging cameras themselves also provide the error sensing data.

We report on the performance of a vector apodizing phase plate coronagraph that operates over a wavelength range of 2-5 μmand is installed in MagAO/Clio2 at the 6.5 m Magellan Clay telescope at Las Campanas Observatory, Chile. The coronagraph manipulates the phase in the pupil to produce three beams yielding two coronagraphic point-spread functions (PSFs) and one faint leakage PSF. The phase pattern is imposed through the inherently achromatic geometric phase, enabled by liquid crystal technology and polarization techniques. The coronagraphic optic is manufactured using a direct-write technique for precise control of the liquid crystal patternand multitwist retarders for achromatization. By integrating a linear phase ramp to the coronagraphic phase pattern, two separated coronagraphic PSFs are created with a single pupil-plane optic, which makes it robust and easy to install in existing telescopes. The two coronagraphic PSFs contain a 180°dark hole on each side of a star, and these complementary copies of the star are used to correct the seeing halo close to the star. To characterize the coronagraph, we collected a data set of a bright (m L =0-1) nearby star with ∼1.5 hr of observing time. By rotating and optimally scaling one PSFand subtracting it from the other PSF, we see a contrast improvement by 1.46 magnitudes at l D 3.5. With regular angular differential imaging at 3.9 μm, the MagAO vector apodizing phase plate coronagraph delivers a s D 5 mag contrast of 8.3 (= -10 3.3 ) at 2 l Dand 12.2 (= -10 4.8 ) at l D 3.5.

We present the first astronomical observations obtained with an apodizing phase plate (APP). The plate is designed to suppress the stellar diffraction pattern by 5 mag from 2 À 9k/D over a 180 region. Stellar images were obtained in the M 0 band (k c ¼ 4:85 m) at the MMTO 6.5 m telescope, with adaptive wave-front correction made with a deformable secondary mirror designed for low thermal background observations. The measured point-spread function (PSF) shows a halo intensity of 0.1% of the stellar peak at 2k/D (0.36 00 ), tapering off as r À5=3 out to radius 9k/D. Such a profile is consistent with residual errors predicted for servo lag in the AO system. We project a 5 contrast limit, set by residual atmospheric fluctuations, of 10.2 mag at 0.36 00 separation for a 1 hr exposure. This can be realized if static and quasi-static aberrations are removed by differential imaging, and is close to the sensitivity level set by thermal background photon noise for target stars with M 0 > 3. The advantage of using the phase plate is the removal of speckle noise caused by the residuals in the diffraction pattern that remain after PSF subtraction. The APP gives higher sensitivity over the range (2Y5) k/D than direct imaging techniques.

Optical imperfections, misalignments, aberrations, and even dust can significantly limit sensitivity in high-contrast imaging systems such as coronagraphs. An upstream deformable mirror (DM) in the pupil can be used to correct or compensate for these flaws, either to enhance Strehl ratio or suppress residual coronagraphic halo. Measurement of the phase and amplitude of the starlight halo at the science camera is essential for determining the DM shape that compensates for any noncommon-path (NCP) wavefront errors. Using DM displacement ripples to create a series of probe and anti-halo speckles in the focal plane has been proposed for space-based coronagraphs and successfully demonstrated in the lab. We present the theory and first on-sky demonstration of a technique to measure the complex halo using the rapidly-changing residual atmospheric speckles at the 6.5 m MMT telescope using the Clio mid-IR camera. The AO system's wavefront sensor (WFS) measurements are used to estimate the residual wavefront, allowing us to approximately compute the rapidly-evolving phase and amplitude of speckle halo. When combined with relatively-short, synchronized science camera images, the complex speckle estimates can be used to interferometrically analyze the images, leading to an estimate of the static diffraction halo with NCP effects included. In an operational system, this information could be collected continuously and used to iteratively correct quasi-static NCP errors or suppress imperfect coronagraphic halos.