Julian C. Christou

Restoration of astronomical images by iterative blind deconvolution

We have developed a modified version of the Iterative Blind Deconvolution (IBD) algorithm of Lane, applicable to different types of astronomical data. Besides using positivity, convolution, and support constraints, we have also applied band-limit, multiple image, and Fourier modulus constraints. By using all the available image constraining information, we are able to successfully recover object and point spread function information from noisy data. The algorithms performance under controlled conditions using simulated data for a binary source object, a compact multiple quasi-point source object, and an extended object with low contrast are demonstrated

Near-infrared imaging of the Becklin-Neugebauer-IRc2 region in Orion with subarcsecond resolution

The Becklin-Neugebauer source has been partially resolved by speckle imagery at 3.6 μm with an angular resolution of 0.″2. The images, fluxes, and additional constraints are consistent with a spherical dust shell of inner diameter 0.″1, surrounding a hot star. The surrounding region, including the vicinity of IRc2, has been imaged with short-exposure/deconvolution methods at several wavelengths, at a resolution of 0.″5. IRc2 is resolved into four components, and the fluxes of the detected sources were determined at 2.8 and 4 μm. Accurate astronomy of the infrared sources with respect to the SiO maser emission has been obtained. The centroid of the SiO maser is definitely not located at the center of symmetry between lobes A and B, but may correspond to IRc2 component A

Image quality, tip-tilt correction, and shift-and-add infrared imaging

The image quality obtainable with shift-and-add (SAA) imaging for the recovery of diffraction-limited information is quantitatively investigated using data simulating an 8-m aperture at a site with near-IR seeing conditions as found on Mauna Kea. This is compared to the image quality obtainable from image centroiding and wavefront tip-tilt correction. For good seeing conditions, image centroiding and SAA, which tracks the image peak, show similar performance containing about 30 percent of the image power in a diffraction-limited component. However, as the seeing degrades, SAA consistently yields improved resolution maintaining significantly greater diffraction-limited information. This enhances the detection threshold by about 2 magnitudes over the seeing-limited case for the seeing range studied. By comparison, the gain due to image centroiding decreases to less than 1 magnitude at the same poor seeing limit.

Deconvolution of adaptive optics retinal images

We quantitatively demonstrate the improvement to adaptively corrected retinal images by using deconvolution to remove the residual wave-front aberrations. Qualitatively, deconvolution improves the contrast of the adaptive optics images. In this work we demonstrate that quantitative information is also increased by investigation of the improvement to cone classification due to the reduction in confusion of adjacent cones because of the extended wings of the point-spread function. The results show that the error in classification between the L and M cones is reduced by a factor of 2, thereby reducing the number of images required by a factor of 4.

Stellar Bow Shocks in the Northern Arm of the Galactic Center: More Members and Kinematics of the Massive Star Population

We present new 2.2 μm diffraction-limited images from the W. M. Keck 10 m and Gemini 8 m telescopes of the cool Galactic center sources, IRS 1W, 5, 8, 10W, and 21, along with new proper motions for IRS 1W, 10W, and 21. These observations were carried out to test the bow shock hypothesis presented by Tanner et al. as an alternative to a very recent (104 yr) epoch of star formation within the tidal stream of gas and dust known as the northern arm. Resolved asymmetric structure is detected in all the sources, with bow shock morphologies associated with IRS 1W, 5, 8, and 10W. For IRS 1W and 10W, there is an agreement between the position angle of the asymmetry and that of the inferred relative velocity vector of the near-infrared source with respect to the northern arm gas, strengthening the bow shock hypothesis. We therefore conclude that the observed morphology is indeed a bow shock generated by sources plowing through the northern arm. Furthermore, the large extent of the resolved structures (310-1340 AU), along with their luminosities (~104-105 L☉), suggests that their central sources are Wolf-Rayet stars, comparable to the broad He emission-line stars, which have strong winds on the order of 1000 km s-1. The bow shock geometry, along with the proper motion measurements, provide three-dimensional orbital solutions for this enigmatic class of objects. The orientations of the orbital planes of IRS 1W and 10W are consistent with that of the putative clockwise plane which has been proposed as a solution for the He I emission-line stars. While these observations eliminate the need to invoke star formation within the northern arm, they increase by 10% the total known population of massive, young stars with strong winds, whose origin remains unexplained in the context of the nearby supermassive black hole.

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.

Is that really your Strehl ratio

Strehl ratio is the most commonly used metric for adaptive optics (AO) performance. It is also the most misused metric. Every Strehl ratio measurement algorithm has subtle differences that result in different measured values. This creates problems when comparing different measurements of the same AO system and even more problems when trying to compare results from different systems. To determine how much the various algorithm difference actually impacted the measured values, we created a series of simulated point spread functions (PSF). The simulated PSFs were then sent around to the various members of the project who then measured the Strehl ratio. The measurements were done blindly, with no knowledge of the true Strehl ratio. We then compared the various measurements to the truth values. Each measurement cycle turned up impacts which were further investigated in the next cycle. We present the results of our comparisons showing the scatter in measured Strehl ratios and our best recommendations for computing an accurate Strehl ratio.

Physical properties of the ESA Rosetta target asteroid (21) Lutetia I. The triaxial ellipsoid dimensions, rotational pole, and bulk density

We seek the best size estimates of the asteroid (21) Lutetia, the direction of its spin axis, and its bulk density, assuming its shape is well described by a smooth featureless triaxial ellipsoid, and to evaluate the deviations from this assumption. Methods. We derive these quantities from the outlines of the asteroid in 307 images of its resolved apparent disk obtained with adaptive optics (AO) at Keck II and VLT, and combine these with recent mass determinations to estimate a bulk density. Our best triaxial ellipsoid diameters for Lutetia, based on our AO images alone, are a x b x c = 132 x 101 x 93 km, with uncertainties of 4 x 3 x 13 km including estimated systematics, with a rotational pole within 5 deg. of ECJ2000 [long,lat] = [45, -7], or EQJ2000 [RA, DEC] = [44, +9]. The AO model fit itself has internal precisions of 1 x 1 x 8 km, but it is evident, both from this model derived from limited viewing aspects and the radius vector model given in a companion paper, that Lutetia has significant departures from an idealized ellipsoid. In particular, the long axis may be overestimated from the AO images alone by about 10 km. Therefore, we combine the best aspects of the radius vector and ellipsoid model into a hybrid ellipsoid model, as our final result, of 124 +/- 5 x 101 +/- 4 x 93 +/- 13 km that can be used to estimate volumes, sizes, and projected areas. The adopted pole position is within 5 deg. of [long, lat] = [52, -6] or[RA DEC] = [52, +12]. Using two separately determined masses and the volume of our hybrid model, we estimate a density of 3.5 +/- 1.1 or 4.3 +/- 0.8 g cm-3 . From the density evidence alone, we argue that this favors an enstatite-chondrite composition, although other compositions are formally allowed at the extremes (low-porosity CV/CO carbonaceous chondrite or high-porosity metallic). We discuss this in the context of other evidence.Context. Asteroid (21) Lutetia was the target of the ESA Rosetta mission flyby in 2010 July. Aims. We seek the best size estimates of the asteroid, the direction of its spin axis, and its bulk density, assuming its shape is well described by a smooth featureless triaxial ellipsoid. We also aim to evaluate the deviations from this assumption. Methods. We derive these quantities from the outlines of the asteroid in 307 images of its resolved apparent disk obtained with adaptive optics (AO) at Keck II and VLT, and combine these with recent mass determinations to estimate a bulk density. Results. Our best triaxial ellipsoid diameters for Lutetia, based on our AO images alone, are a × b × c = 132 × 101 × 93 km, with uncertainties of 4 × 3 × 13 km including estimated systematics, with a rotational pole within 5 ◦ of ECJ2000 [ λβ ] = [45 ◦ − 7 ◦ ], or EQJ2000 [RA Dec] = [44 ◦ + 9 ◦ ]. The AO model fit itself has internal precisions of 1 × 1 × 8 km, but it is evident both from this model derived from limited viewing aspects and the radius vector model given in a companion paper, that Lutetia significantly departs from an idealized ellipsoid. In particular, the long axis may be overestimated from the AO images alone by about 10 km. Therefore, we combine the best aspects of the radius vector and ellipsoid model into a hybrid ellipsoid model, as our final result, of diameters 124 ± 5 × 101 ± 4 × 93 ± 13 km that can be used to estimate volumes, sizes, and projected areas. The adopted pole position is within

Effect of wavelength on in vivo images of the human cone mosaic

In images of the human fundus, the fraction of the total returning light that comes from the choroidal layers behind the retina increases with wavelength [Appl. Opt. 28, 1061 (1989); Vision Res. 36, 2229 (1996)]. There is also evidence that light originating behind the receptors is not coupled into the receptor waveguides en route to the pupil [S. A. Burns et al., Noninvasive Assessment of the Visual System, Vol. 11 of 1997 Trends in Optics and Photonics Series, D. Yager, ed. (Optical Society of America, 1997), p. al; Invest. Ophthalmol. Visual Sci. 38, 1657 (1997)]. These observations imply that the contrast of images of the cone mosaic should be greatly reduced with increasing wavelength. This hypothesis was tested by imaging the light distributions in both the planes of the photoreceptors and the pupil at three wavelengths, 550, 650, and 750 nm, with the Rochester adaptive optics ophthalmoscope. Surprisingly, the contrast of the retinal images varied only slightly with wavelength. Furthermore, the ratio of the receptorally guided component to the total reflected light measured in the pupil plane was found to be similar at each wavelength, suggesting that, throughout this wavelength range, the scattered light from the deeper layers in the retina is guided through the receptors on its return path to the pupil.

Speckle interferometry of asteroids II. 532 Herculina

Abstract Speckle interferometry of 532 Herculina performed on January 17 and 18, 1982, yields triaxial ellipsoid dimensions of (263 ± 14) × (218 ± 12) × (215 ± 12) km, and a north pole for the asteroid within 7° of RA = 7 b 47 m and DEC = −39° (ecliptic coordinates γ = 132° β = −59°). In addition, a “spot” some 75% brighter than the rest of the asteroid is inferred from both speckle observations and Herculinas lightcurve history. This bright complex, centered at asterocentric latitude −35°, longitude 145–165°, extends over a diameter of 55° (115 km) of the asteroids surface. No evidence for a satellite is found from the speckle observations, which leads to an upper limit of 50 km for the diameter of any satellite with an albedo the same as or higher than Herculina.