High Resolution Astronomical Imaging Research Articles

A novel hybrid algorithm for lucky imaging

Lucky imaging is a high-resolution astronomical image recovery technique with two classic implementation algorithms, i.e. image selecting, shifting and adding in image space, and data selecting and image synthesizing in Fourier space. This paper proposes a novel lucky imaging algorithm where with space-domain and frequency-domain selection rates as a link, the two classic algorithms are combined successfully, making each algorithm a proper subset of the novel hybrid algorithm. Experimental results show that with the same experiment dataset and platform, the high-resolution image obtained by the proposed algorithm is superior to that obtained by the two classic algorithms. This paper also proposes a new lucky image selection and storage scheme, which can greatly save computer memory and enable lucky imaging algorithm to be implemented in a common desktop or laptop with small memory and to process astronomical images with more frames and larger size. In addition, through simulation analysis, this paper discusses the binary star detection limits of the novel lucky imaging algorithm and traditional ones under different atmospheric conditions.

Towards high-resolution astronomical imaging

This paper is a report from a recent meeting on "the Future of high-resolution imaging in the visible and infrared", reviewing the astronomical drivers for development and the technological advances that might boost performance. Each of the authors listed contributed a section themselves.

High noise astronomical image denoising via 2G-bandelet denoising compressed sensing

In deep space exploration, high resolution astronomical image captured is often contaminated by various cosmic noise signals during its shooting and long distance transmission, which has brought inconvenience to astronomical image analysis. The famous compressed sensing (CS) proposed by Candes et al. can successfully solve the problem of high resolution astronomical image compression and low noise reconstruction. In this paper, we further concern how to reconstruct a high quality image from a high resolution and high noise astronomical image. A 2G-bandelet denoising compressed sensing (BDCS) is first proposed based on the advantage of CS in image denoising and the superior ability of 2G-bandelet in sparse representation of astronomical images, then iterative bandelet thresholding (IBT-BTCS) algorithm based on BDCS is proposed for high resolution and high noise astronomical image reconstruction. Firstly, an iterative bandelet thresholding method is designed to obtain optimal approximation of original image; Secondly, to further improve the reconstructed image quality, group sparse total variation with stepsize constraints (GSTV-SC) method is proposed to adjust the reconstructed astronomical image in each iteration. The simulation results show that the proposed algorithm can quickly reconstruct a high quality astronomical image only using a few observations, preserve more astronomical image details and effectively solve high noise astronomical image denoising problem.

Super-resolution from unregistered aliased astronomical images

Images captured from astronomical telescopes always have aliasing artifacts due to atmospheric turbulence, which typically conveys additional high-frequency content containing extra details of the astronomical scene. Super-resolution (SR) reconstruction can exploit the information existing in the aliasing to create a higher-resolution astronomical image, where the precise registration of input observations is generally regarded as a necessary premise. We consider SR reconstruction as a multichannel sampling scheme with uncertain deviations mathematically and present a frequency domain technique to achieve accurate alignment of aliased astronomical images by utilizing their low-frequency and aliasing-free part of the spectrum. Moreover, a planar motion model including three parameters is allowed to describe the relative uncontrolled motions between the acquisition system and the astronomical scene. Finally, a sharp high-resolution astronomical image can be reconstructed by utilizing the adaptive normalized convolution algorithm. The technique proposed is tested in simulations and practical experiments and shows very excellent SR reconstruction capabilities in the case of aliased astronomical images.

Long-distance telecom-fiber transfer of a radio-frequency reference for radio astronomy

Very-long-baseline interferometry (VLBI) for high-resolution astronomical imaging requires phase-stable frequency references at widely separated radio-telescope antennas. For the first time to our knowledge, we have disseminated a suitable radio-frequency (RF) reference for VLBI over a “real-world” telecom optical-fiber link between radio telescopes that are >100 km apart, by means of an innovative phase-conjugation technique. Bidirectional optical amplification is used in parallel with live traffic, and phase perturbations in the effective optical-fiber path length are compensated. This RF-over-fiber approach obviates the need for separate hydrogen masers at each antenna, offering significant advantages for radio-astronomy facilities such as the Square Kilometer Array.

In-laboratory ALOHA mid-infrared up-conversion interferometer in the photon counting regime at λ = 3.39 μm

Astronomical Light Optical Hybrid Analysis (ALOHA@3.39 µm) is a new instrumental concept designed for high angular resolution astronomical imaging in the L Band. The originality of our experimental device is to include a frequency conversion process in each arm of the interferometer that frequency shifts the mid infrared signal to the near infrared domain. This method relaxes the stringent limitations due to the thermal noise background radiated by the instrument itself. The aim of this paper is to estimate the current sensitivity limit of the instrument and validate the ability to provide calibrated contrasts with a quasi monochromatic signal. We demonstrate the possibility to measure fringe contrasts greater than 94.5% with a signal-to-noise ratio close to 7. The flux level at the input of each arm of the interferometer is then equal to 100 fW. This would correspond to a zero magnitude star in the L Band for a 1 m 2 telescope surface and a 3 nm converted spectral bandwidth.

Reconstruction and transmission of astronomical image based on compressed sensing

In the process of image transmission, the famous JPEG and JPEG-2000 compression methods need more transmission time as it is difficult for them to compress the image with a low compression rate. Recently the compressed sensing(CS) theory was proposed, which has earned great concern as it can compress an image with a low compression rate, meanwhile the original image can be perfectly reconstructed from only a few compressed data. The CS theory is used to transmit the high resolution astronomical image and build the simulation environment where there is communication between the satellite and the Earth. Number experimental results show that the CS theory can effectively reduce the image transmission and reconstruction time. Even with a very low compression rate, it still can recover a higher quality astronomical image than JPEG and JPEG-2000 compression methods.

High resolution astronomical image restoration system for large ground-based telescope

The resolution of astronomical imaging from large optical telescopes is usually limited by the blurring effects of refractive index fluctuations in the Earth’s atmosphere. In this letter, we develop a lucky imaging system to restore astronomical images through atmosphere turbulence on large telescope. Our system takes very short exposures, on the order of the atmospheric coherence time. The rapidly changing turbulence leads to a very variable point spread function (PSF), and the variability of the PSF leads to some frames having better quality than the rest. Only the best frames are selected, aligned and co-added to give a final image with much improved angular resolution. Our lucky imaging system is successfully applied to restore the astronomical images taken by a 1.23 m telescope. We get clear images of moon surface, Jupiter, and Saturn, and our system can be demonstrated to greatly improve the imaging resolution through atmospheric turbulence. OCIS codes: 100.3020, 010.1290, 010.7060. doi: 10.3788/COL201210.S21004. The resolution of all large ground-based telescopes is severely limited by the effects of atmospheric turbulence. At even the best sites, the resolution of large telescope is degraded by at least a factor of five in the visible. The recovery of the full theoretical resolution of large telescopes in the presence of atmospheric turbulence has been a major goal of technological developments in the field of as

Phase closure retrieval in an infrared-to-visible upconversion interferometer for high resolution astronomical imaging

This paper demonstrates the use of a nonlinear upconversion process to observe an infrared source through a telescope array detecting the interferometric signal in the visible domain. We experimentally demonstrate the possibility to retrieve information on the phase of the object spectrum of an infrared source by using a three-arm upconversion interferometer. We focus our study on the acquisition of phase information of the complex visibility by means of the phase closure technique. In our experimental demonstration, a laboratory binary star with an adjustable photometric ratio is used as a test source. A real time comparison between a standard three-arm interferometer and our new concept using upconversion by sum-frequency generation demonstrates the preservation of phase information which is essential for image reconstruction.

PROBING FINE-SCALE IONOSPHERIC STRUCTURE WITH THE VERY LARGE ARRAY RADIO TELESCOPE

High resolution (~1 arcminute) astronomical imaging at low frequency (below 150 MHz) has only recently become practical with the development of new calibration algorithms for removing ionospheric distortions. In addition to opening a new window in observational astronomy, the process of calibrating the ionospheric distortions also probes ionospheric structure in an unprecedented way. Here we explore one aspect of this new type of ionospheric measurement, the differential refraction of celestial source pairs as a function of their angular separation. This measurement probes variations in the spatial gradient of the line-of-sight total electron content (TEC) to 0.001 TECU/km accuracy over spatial scales of under 10 km to over 100 km. We use data from the VLA Low-frequency Sky Survey (VLSS; Cohen et al. 2007, AJ 134, 1245), a nearly complete 74 MHz survey of the entire sky visible to the Very Large Array (VLA) telescope in Socorro, New Mexico. These data comprise over 500 hours of observations, all calibrated in a standard way. While ionospheric spatial structure varies greatly from one observation to the next, when analyzed over hundreds of hours, statistical patterns become apparent. We present a detailed characterization of how the median differential refraction depends on source pair separation, elevation and time of day. We find that elevation effects are large, but geometrically predictable and can be "removed" analytically using a "thin-shell" model of the ionosphere. We find significantly greater ionospheric spatial variations during the day than at night. These diurnal variations appear to affect the larger angular scales to a greater degree indicating that they come from disturbances on relatively larger spatial scales (100s of km, rather than 10s of km).

Effects of Aberrations on Transfer Functions Used in High Angular Resolution Astronomical Imaging

Abstract In this paper we present the results of the effect of aberrations on the transfer functions used in the high-angular resolution astronomical imaging techniques of speckle interferometry, Knox-Thompson and bispectral imaging. The analyses are based on a computer simulation of imaging through atmospheric turbulence. The results show that as the seeing becomes worse, its effect dominates the behaviour of the transfer functions which tend to be independent of (small) optical aberrations. However, if the wavefront variation due to fixed aberrations is significant over r 0-sized regions in the pupil (where r 0 is the Fried parameter), the above transfer functions do depend on the aberration: in particular, the bispectral transfer function is relatively sensitive to odd aberrations, such as coma.

Deconvolution of spatial-correlation spectra for high-resolution astronomical imaging

A number of techniques used for high-resolution speckle imaging in astronomy generate average spatial-correlation spectra of objects. Here a straightforward method for iteratively deconvolving these noisy spectra to obtain high-resolution images is described.

The Applications of Closure Phase to Astronomical Imaging

Closure phase is a number measured by triplets of Michelson interferometers that is completely independent of certain types of otherwise severe instrumental errors. In the 30 years since closure phase was invented, it has been applied to a diverse number of different problems in astronomical imaging. Methods based on the closure phase now allow imaging of complex objects in the presence of severe aberrations and are vital to the success of modern, high-resolution astronomical imaging both at radio and at optical wavelengths. Over the past 10 to 15 years, the concept of closure phase has been extended and generalized. One of the most important advances has been the development of automatic or self-calibration techniques. This article reviews closure phase methods and some of the many spin-offs and related ideas.