18 research outputs found
Focal Plane Wavefront Sensing using Residual Adaptive Optics Speckles
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 non-common-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 6.5m 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.Comment: Astrophysical Journal (accepted). 26 pages, 21 figure
Pupil Plane Phase Apodization
Phase apodization coronagraphs are implemented in a pupil plane to create a
dark hole in the science camera focal plane. They are successfully created as
"Apodizing Phase Plates" (APPs) using classical optical manufacturing, and as
"vector-APPs" using liquid-crystal patterning with essentially achromatic
performance. This type of coronagraph currently delivers excellent broadband
contrast (10) at small angular separations (few ) at
ground-based telescopes, owing to their insensitivity to tip/tilt errors.Comment: Invited chapter, to be published in the "Handbook of Astronomical
Instrumentation", Vol. 3, eds. A. Moore and D. Burrows, WSPC (2018). 9 pages,
1 figur
Polarization dOTF: on-sky focal plane wavefront sensing
The differential Optical Transfer Function (dOTF) is a focal plane wavefront
sensing method that uses a diversity in the pupil plane to generate two
different focal plane images. The difference of their Fourier transforms
recovers the complex amplitude of the pupil down to the spatial scale of the
diversity. We produce two simultaneous PSF images with diversity using a
polarizing filter at the edge of the telescope pupil, and a polarization camera
to simultaneously record the two images. Here we present the first on-sky
demonstration of polarization dOTF at the 1.0m South African Astronomical
Observatory telescope in Sutherland, and our attempt to validate it with
simultaneous Shack-Hartmann wavefront sensor images.Comment: 11 pages, 9 figures, Proc. SPIE Vol. 991
First On-Sky High Contrast Imaging with an Apodizing Phase Plate
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 magnitudes from 2-9 lambda/D over a 180 degree region. Stellar
images were obtained in the M' band (4.85 microns) at the MMTO 6.5m telescope,
with adaptive wavefront correction made with a deformable secondary mirror
designed for low thermal background observations. The measured PSF shows a halo
intensity of 0.1% of the stellar peak at 2 lambda/D (0.36 arcsec), tapering off
as r^{-5/3} out to radius 9 lambda/D. Such a profile is consistent with
residual errors predicted for servo lag in the AO system.
We project a 5 sigma contrast limit, set by residual atmospheric
fluctuations, of 10.2 magnitudes at 0.36 arcsec separation for a one hour
exposure. This can be realised 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'>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 2-5 lambda/D compared to direct imaging
techniques.Comment: 22 pages, 5 figures, 1 table, ApJ accepte
Calibrating a high-resolution wavefront corrector with a static focal-plane camera
We present a method to calibrate a high-resolution wavefront (WF)-correcting device with a single, static camera, located in the focal-plane; no moving of any component is needed. The method is based on a localized diversity and differential optical transfer functions to compute both the phase and amplitude in the pupil plane located upstream of the last imaging optics. An experiment with a spatial light modulator shows that the calibration is sufficient to robustly operate a focal-plane WF sensing algorithm controlling a WF corrector with 40,000 degrees of freedom. We estimate that the locations of identical WF corrector elements are determined with a spatial resolution of 0.3% compared to the pupil diameter
An apodizing phase plate coronagraph for VLT/NACO
We describe a coronagraphic optic for use with CONICA at the VLT that
provides suppression of diffraction from 1.8 to 7 lambda/D at 4.05 microns, an
optimal wavelength for direct imaging of cool extrasolar planets. The optic is
designed to provide 10 magnitudes of contrast at 0.2 arcseconds, over a
D-shaped region in the image plane, without the need for any focal plane
occulting mask.Comment: 9 pages, 5 figures, to appear in Proc. SPIE Vol. 773
Imaging extrasolar planets by stellar halo suppression in separately-corrected color bands
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.Comment: Accepted by ApJ
In vivo imaging of the human rod photoreceptor mosaic,”
Although single cone receptors have been imaged in vivo, to our knowledge there has been no observation of rods in the living normal eye. Using an adaptive optics ophthalmoscope and post processing, evidence of a rod mosaic was observed at 5°and 10°eccentricities in the horizontal temporal retina. For four normal human subjects, small structures were observed between the larger cones and were observed repeatedly at the same locations on different days, and with varying wavelengths. Image analysis gave spacings that agree well with rod measurements from histological data. [6] report imaging of the photoreceptor mosaic in a rod monochromat. Imaging rods will have important applications in the study of retinitis pigmentosa Four normal subjects (denoted by N1, N2, N3, and N4) between the ages of 19 and 26 years were imaged using the Rochester AO ophthalmoscope For the retinal location, 5°and 10°in the horizontal, temporal retina (TR) were chosen for several reasons: (i) the image quality degrades with increasing eccentricity owing to increased scatter from the overlying retinal layers; (ii) the cones can be used as physical landmarks, because the cone size and spacing increase with increasing eccentricity, whereas the rod size and spacing stay constant over small excursions (<15°) [10,11]; and (iii) there are fewer overlying nerve fibers. The contrast of the cone mosaic is wavelength invariant The imaging source was a krypton flashlamp with pulse durations of 4 ms delivered through a 1:5 mm entrance pupil. Single pulse energies for the 650 and 750 nm wavelengths were 0.44 and 0:27 μJ, respectively, a factor of 40 below the safety limit