Jonathan D. Freeman

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.

Application of bispectrum analysis for phase recovery from one-dimensional infrared speckle data

We apply bispectrum analysis to one-dimensional astronomical infrared specklegrams and compare the recovered phases and their variances with those obtained by the Knox–Thompson algorithm. A number of averaging techniques are used to combine the multiple object phase estimates from the average bispectrum phases. Analysis of these techniques indicates that only a fraction of the nonredundant support of the bispectrum is necessary for the object phase retrieval. We have calibrated our bispectra for zero-mean additive detector noise. Results are presented for a bright point source and the nearby binary star Ross 614AB along with its point-source comparison. The point-source results indicate a phase structure that may be due to telescope aberrations that do not average out in the bispectrum process. The point-source-calibrated phases obtained for Ross 614AB from the bispectrum analysis show the binary structure out to higher spatial frequencies than do the Knox–Thompson results.

Global high-resolution imaging of hotspots on Io

We present 3.8 μm images of the Jovian satellite Io at an unprecedented ground-based resolution of 0″.18. The images were obtained through two-dimensional speckle interferometry using the 6.86 m aperture of the Multiple Mirror Telescope in November 1989 and March 1990. To process the images we developed new speckle reduction techniques appropriate for objects with a size that is a significant fraction of the field of view. The images show three volcanic hotspots, including Pele, detected here for the first time since the Voyager 1 flyby in 1979

Self-calibrating shift-and-add technique for speckle imaging

An image-reconstruction technique for astronomical speckle interferometric data is described. This variant of the shift-and-add algorithm originally developed by Lynds et al. [ Astrophys. J.207, 174 ( 1976)] utilizes a weighted impulse distribution of speckle positions to extract an average speckle for a data set. This is done by means of a weighted deconvolution procedure, similar in form to a Weiner filter, which deconvolves the specklegram by the impulse distribution. Results show that this method appears to be self-calibrating for seeing effects. It yields point-spread functions, for observations of an unresolved star, that compare quantitatively with computed Airy patterns for both simple apertures and the fully phased multiple mirror telescope array. Images of the resolved object Alpha Orionis show evidence of an extended stellar envelope.

Images From Astronomical Speckle Data: Weighted Shift-And-Add Analysis

We have applied a variant of the shift-and-add algorithm originally developed by Lynds, Worden, and Harvey [Astrophys. J. 207, 174 (1976)] to astronomical speckle interferometric data. A set of impulses corresponding in locations and magnitudes to the local maxima in each specklegram is generated and used to obtain an average speckle by means of a Wiener-type filter deconvolution procedure. This technique yields diffraction-limited images that appear to be self-calibrating for seeing effects. Realistic point spread functions have been obtained for a number of telescopes at different wavelengths, and results are also presented for the resolved red supergiant Alpha Orionis. The limiting signal-to-noise ratio of the technique as indicated by the results presented here suggests a dynamic range of =6 stellar magnitudes, with no evidence of residual seeing effects. A matched filter technique is demonstrated for use in locating the speckles of complicated objects or for objects dominated by photon noise.

Nearby solar-type star with a low-mass companion : new sensitivity limits reached using speckle imaging

We have imaged the low-mass companion to the nearby solar-type star Gliese 67 using two-dimensional infrared speckle imaging techniques. The binary has been resolved at J (1.25 μm), H (1.65 μm), and K (2.2 μm), allowing us to determine the magnitudes and colors of the components. In observations spanning 14 months, the secondary has been found at separations and position angles predicted by the astrometric orbit, and we calculate the component masses to be 0.97 and 0.29 M ⊙ .

Infrared imaging of M Cas B using rapid image motion compensation

The orbital parameters and component masses of the μ Cas binary system can provide a measure of the helium abundance in the early Galaxy. In a continuing effort to measure these characteristics, we have obtained the first direct images of the low mass companion to μ Cas A using techniques for high speed correction of image motion at 1.25 and 1.65 μm. These techniques include real-time tracking of the brightest infrared speckle at 75 Hz and of the visible centroid at 500 Hz. Mu Cas B is easily detected with a magnitude difference relative to the primary of >4 at a separation of ∼1.4 arcsec.

Application Of Bispectrum Analysis To Infrared Speckle Data

We have applied the bispectrum algorithm to one-dimensional infrared speckle data for retrieval of diffraction limited phases. We use a standard weighting averaging technique to combine the multiple phase estimates contained in the bispectrum. We have also used the bispectrum modulus to obtain visibility amplitudes. Results, including images, are presented for three different binary stars. Simulated data has also been used to study the behaviour of the algorithm under different signal-to-noise conditions as well as a study of phase recovery for different defocus conditions. We find that the algorithm is sensitive, at a low level, to focus changes especially for noisy data. Comparison of the bispectrum phases is made with those obtained from Knox-Thompson cross-spectra for all cases.

Statistical Analysis of the Weighted Shift and Add Image Reconstruction Technique

The Weighted Shift-and-Add algorithm (WSA) is an image reconstruction technique whereby diffraction limited images of astronomical objects are obtained from speckle interferometric data. This paper attempts to put the understanding of WSA on a firm mathematical basis by a statistical analysis of the algorithm. The approach follows that of the Hunt, Fright, and Bates study carried out for the Simple Shift-and-Add algorithm. The expected WSA profile is found to be linearly dependent on the square modulus complex coherence function of a speckle pattern. The last section of this paper contains a discussion of how the statistical analysis compares to the results obtained with our new Weighted Shift-and-Add procedures using cross-correlation (WSA/XC) and deconvolution (WSA/WD).

Images From Astronomical Speckle Data: Weighted Shift-and-Add Analysis

We have applied a variant of the Shift-and-Add algorithm originally developed by Lynds, Worden & Harvey to astronomical speckle interferometric data. A set of impulses corresponding in location and magnitude to the local maxima in each specklegram is generated and used to obtain an average speckle by means of a Wiener-type filter deconvolution procedure. This technique yields diffraction limited images which appear to be self calibrating for seeing effects. Realistic point spread functions have been obtained for a number of telescopes at different wavelengths and results are also presented for the resolved red supergiant Alpha Orionis. The limiting signal-to-noise of the technique as indicated by the results presented here suggests a dynamic range of ~6 stellar magnitudes with no evidence of residual seeing effects.