We study the way Lyot coronagraphs with unapodized entrance pupils respond to small, low-order phase aberrations. This study is applicable to ground-based adaptive optics coronagraphs operating at ...90% and higher Strehl ratios, as well as to some space-based coronagraphs with intrinsically higher Strehl ratio imaging. We utilize a second-order expansion of the monochromatic point-spread function (written as a power spectrum of a power series in the phase aberration over clear aperture) to derive analytical expressions for the response of a "band-limited" Lyot coronagraph (BLC) to small, low-order, phase aberrations. The BLC possesses a focal plane mask with an occulting spot whose opacity profile is a spatially band-limited function rather than a hard-edged, opaque disk. The BLC is, to first order, insensitive to tilt and astigmatism. Undersizing the stop in the reimaged pupil plane (the Lyot plane) following the focal plane mask can alleviate second-order effects of astigmatism, at the expense of system throughput and angular resolution. The optimal degree of such undersizing depends on individual instrument designs and goals. Our analytical work engenders physical insight and complements existing numerical work on this subject. Our methods can be extended to treat the passage of higher order aberrations through band-limited Lyot coronagraphs by using our polynomial decomposition or an analogous Fourier approach.
The Challenges of Coronagraphic Astrometry Digby, Andrew P; Hinkley, Sasha; Oppenheimer, Ben. R ...
The Astrophysical journal,
10/2006, Letnik:
650, Številka:
1
Journal Article
Recenzirano
A coronagraph in conjunction with adaptive optics provides an effective means to image faint companions of nearby stars from the ground. The images from such a system are complex, however, and need ...to be fully characterized and understood before planets or disks can be detected against the glare from the host star. Using data from the Lyot Project coronagraph, we investigate the difficulties of astrometric measurements in diffraction-limited coronagraphic images and consider the principal problem of determining the precise location of the occulted star. We demonstrate how the image structure varies when the star is decentered from the optical axis and show how even small offsets (0.05 l/D) or 5 mas) give rise to false sources in the image. We consider methods of determining the star position from centroiding, instrument feedback, and analysis of point-spread function symmetry and conclude that internal metrology is the most effective technique.
Aims.We seek to produce apodized apertures for application in stellar coronagraphy to help in direct detections of exoplanets. We show that chromatic apodized apertures of any shape in transmission ...can be obtained with a specific MZI and we demonstrate this capability in two cases. Methods.The method takes advantage of the capabilities of the MZI, in which the two outputs correspond to the addition and subtraction of the two wave amplitudes in both arms. The result is obtained by re-imaging the entrance aperture of the telescope in the arms of the MZI where two complementary phase masks ± $\varphi (x,y)$ are set. At the two outputs of the MZI, the re-imaged apertures interfere, and their transmissions are multiplied respectively by a factor of the form $ \cos\varphi(x,y)$ and $i \sin\varphi(x,y)$. They correspond to the apodized and anti-apodized complementary outputs. Results.We present the results obtained for two types of apodization. A 1D cosine apodization for a square aperture is obtained by introducing a thin wedge-shaped air film, slightly tilting one of the mirrors of the MZI. A 2D circular symmetric apodization of the form $\cosx^{2}+y^{2}$ is obtained for a circular aperture using two complementary convergent and divergent lenses as phase masks. Aperture transmissions (in intensity) and corresponding point spread functions (PSFs) are given in each case and compared to the theoretical expectations. Conclusions.We have demonstrated the capability of the MZI to produce an apodized aperture. This result is obtained with no loss of photons, considering the fact that there are two complementary outputs. Considerations are given on the wavelength dependence of this technique.
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