Off-axis Point Spread Function Research Articles

Modeling off-axis diffraction with the least-sampling angular spectrum method

Accurately yet efficiently simulating off-axis diffraction is vital to design large-scale computational optics, but existing rigid sampling and modeling schemes fail to address this. Herein, we establish a universal least-sampling angular spectrum method that enables efficient off-axis diffraction modeling with high accuracy. Specifically, by employing the Fourier transform’s shifting property to convert off-axis diffraction to quasi-on-axis, and by linking the angular spectrum to the transfer function, essential sampling requirements can be thoroughly optimized and adaptively determined across computation. Leveraging a flexible matrix-based Fourier transform, we demonstrate the off-axis point spread function of exemplary coded-aperture imaging systems. For the first time, to our knowledge, a significant speed boost of around 36× over the state of the art at 20° is demonstrated, and so is the viability of computing ultra-large angles such as 35° within seconds on a commercial computer. The applicability to high-frequency modulation is further investigated.

Off-axis point spread function reconstruction forsingle conjugate adaptive optics.

Modern giant segmented mirror telescopes (GSMTs) such as the Extremely Large Telescope, which is currently under construction, depend heavily on adaptive optics (AO) systems to correct for atmospheric distortions. However, a residual blur always remains in the astronomical images corrected by single conjugate AO (SCAO) systems due to fitting and bandwidth errors, which can mathematically be described by a convolution of the true image with a point spread function (PSF). Due to the nature of the turbulent atmosphere and its correction, the PSF is spatially varying, which is known as an anisoplanatic effect. The PSF serves, e.g.,as a quality measure for science images and therefore needs to be known as accurately as possible. In this paper, we present an algorithm for PSF reconstruction from pupil-plane data in directions apart from the guide star direction in an SCAO system. Our algorithm is adapted to the needs of GSMTs focused on estimating the contribution of the anisoplanatic and generalized fitting error to the PSF. Results obtained in an end-to-end simulation tool show a qualitatively good reconstruction of the PSF compared to the PSF calculated directly from the simulated incoming wavefront as well as stable performance with respect to imprecise knowledge of atmospheric parameters.

Experimental Verification of a One-dimensional Diffraction-limit Coronagraph

We performed an experimental verification of a coronagraph. As a result, we confirmed that, at the focal region where the planetary point spread function exists, the coronagraph system mitigates the raw contrast of a star-planet system by at least 1 × 10−5 even for the 1-λ/D star-planet separation. In addition, the verified coronagraph keeps the shapes of the off-axis point spread functions when the setup has the source angular separation of 1λ/D. The low-order wave front error and the non-zero extinction ratio of the linear polarizer may affect the currently confirmed contrast. The sharpness of the off-axis point spread function generated by the sub-λ/D separated sources is promising for the fiber-based observation of exoplanets. The coupling efficiency with a single mode fiber exceeds 50% when the angular separation is greater than 3–4×10−1 λ/D. For sub-λ/D separated sources, the peak positions (obtained with Gaussian fitting) of the output point spread functions are different from the angular positions of sources; the peak position moved from about 0.8λ/D to 1.0λ/D as the angular separation of the light source varies from 0.1λ/D to 1.0λ/D. The off-axis throughput including the fiber-coupling efficiency (with respect to no focal plane mask) is about 40% for 1-λ/D separated sources and 10% for 0.5-λ/D separated ones (excluding the factor of the ratio of pupil aperture width and Lyot stop width), where we assumed a linear-polarized-light injection. In addition, because this coronagraph can remove point sources on a line in the sky, it has another promising application for high-contrast imaging of exoplanets in binary systems.

AIROPA IV: Validating point spread function reconstruction on various science cases

We present an analysis of six independent on-sky datasets taken with the Keck-II/NIRC2 instrument. Using the off-axis point spread function (PSF) reconstruction software AIROPA, we extract stellar astrometry, photometry, and other fitting metrics to characterize the performance of this package. We test the effectiveness of AIROPA to reconstruct the PSF across the field of view in varying atmospheric conditions, number and location of PSF reference stars, stellar crowding, and telescope position angle (PA). We compare the astrometric precision and fitting residuals between a static PSF model and a spatially varying PSF model that incorporates instrumental aberrations and atmospheric turbulence during exposures. Most of the fitting residuals we measure show little to no improvement in the variable-PSF mode over the single-PSF mode. For one of the data sets, we find photometric performance is significantly improved (by ∼10 × ) by measuring the trend seen in photometry as a function of off-axis location. For nearly all other metrics we find comparable astrometric and photometric precision across both PSF modes, with a ∼13 % smaller astrometric uncertainty in variable-PSF mode in the best case. We largely confirm that the spatially variable PSF does not significantly improve the astrometric and other PSF fitting residuals over the static PSF for on-sky observations. We attribute this to unaccounted instrumental aberrations that are not characterized through afternoon adaptive optics (AO) bench calibrations.

Off-axis point spread function characterization in laser guide star adaptive optics systems

Adaptive optics (AO) restore the angular resolution of ground-based telescopes, but at the cost of delivering a time-and space-varying point spread function (PSF) with a complex shape. PSF knowledge is crucial for breaking existing limits on the measured accuracy of photometry and astrometry in science observations. In this paper, we concentrate our analyses on anisoplanatism signature only onto PSF: for large-field observations (20) with single-conjugated AO, PSFs are strongly elongated due to anisoplanatism that manifests itself as three different terms for Laser-guide star (LGS) systems: angular, focal and tilt. We propose a generalized model that relies on a point-wise decomposition of the phase and encompasses the non-stationarity of LGS systems. We demonstrate it is more accurate and less computationally demanding than existing models: it agrees with end-to-end physical-optics simulations to within 0.1% of PSF measurables, such as Strehl-ratio, FWHM and fraction of variance unexplained. Secondly, we study off-axis PSF modelling is with respect to C 2 n (h) profile (heights and fractional weights). For 10 m class telescope, PSF morphology is estimated at 1%-level as long as we model the atmosphere with at least 7 layers whose heights and weights are known respectively with 200m and 10%-precision. As a verification test we used the Canada's NRC-Herzberg HeNOS testbed data, featuring four lasers. We highlight capability of retrieving off-axis PSF characteristics within 10% of fraction of variance unexplained, which complies with the expected range from the sensitivity analysis. Our new off-axis PSF modelling method lays the groundwork for testing on-sky in the near future.

Absolute stellar photometry on moderate-resolution FPA images

An extensive database of star (and Moon) images has been collected by the ground-based RObotic Lunar Observatory (ROLO) as part of the US Geological Survey program for lunar calibration. The stellar data are used to derive nightly atmospheric corrections for the observations from extinction measurements, and absolute calibration of the ROLO sensors is based on observations of Vega and published reference flux and spectrum data. The ROLO telescopes were designed for imaging the Moon at moderate resolution, thus imposing some limitations for the stellar photometry. Attaining accurate stellar photometry with the ROLO image data has required development of specialized processing techniques. A key consideration is consistency in discriminating the star core signal from the off-axis point spread function. The analysis and processing methods applied to the ROLO stellar image database are described.

Aspects of the Theoretical Performance of Modal Adaptive Optics

This paper describes early results of a theoretical investigation of the performance of an adaptive optics system in which a limited number of Karhunen-Loeve modes of the wavefront distortion are corrected. We have determined the on-axis point spread function for an idealised adaptive optics system in Kolmogorov seeing, whose performance is defined in terms of radii of included energy and Strehl ratios for a range of examples. We discuss extensions of the analysis for the off-axis point spread function, and for non-Kolmogorov seeing.

Colour in the Polychromatic Off-axis Point-spread Function in the Presence of Primary Coma and Astigmatism

The effects of primary coma and astigmatism in the colour distribution of the polychromatic point-spread function are studied for diverse values of these aberrations. Dispersive wave-aberration coefficients, more significant in the chromaticity distribution than the constant ones, have also been studied in this paper. Moreover, the relative influence of the aberrations in the colour of the off-axis image is shown, so that the colour distribution for every optical system can be predicted from its aberrations. Finally the tolerances for the monochromatic aberrations when the illuminance reaches 0·8 and their corresponding colour distributions are obtained. The influence of the spectral distribution of the source is shown simultaneously.