Diffraction-limited Telescope Research Articles

SCExAO/MEC and CHARIS Discovery of a Low-mass, 6 au Separation Companion to HIP 109427 Using Stochastic Speckle Discrimination and High-contrast Spectroscopy*

Abstract We report the direct imaging discovery of a low-mass companion to the nearby accelerating A star, HIP 109427, with the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument coupled with the Microwave Kinetic Inductance Detector Exoplanet Camera (MEC) and CHARIS integral field spectrograph. CHARIS data reduced with reference star point spread function (PSF) subtraction yield 1.1–2.4 μm spectra. MEC reveals the companion in Y and J band at a comparable signal-to-noise ratio using stochastic speckle discrimination, with no PSF subtraction techniques. Combined with complementary follow-up L p photometry from Keck/NIRC2, the SCExAO data favors a spectral type, effective temperature, and luminosity of M4–M5.5, 3000–3200 K, and log 10 ( L / L ⊙ ) = − 2.28 − 0.04 + 0.04 , respectively. Relative astrometry of HIP 109427 B from SCExAO/CHARIS and Keck/NIRC2, and complementary Gaia–Hipparcos absolute astrometry of the primary favor a semimajor axis of 6.55+3.0 −0.48 au, an eccentricity of 0.54 − 0.15 + 0.28 , an inclination of 66.7 − 14 + 8.5 degrees, and a dynamical mass of 0.280 − 0.059 + 0.18 M ⊙. This work shows the potential for extreme AO systems to utilize speckle statistics in addition to widely used postprocessing methods to directly image faint companions to nearby stars near the telescope diffraction limit.

A 20 m wide-field diffraction-limited telescope: 20 m, wide field UVOIR telescope

A 20 m wide-field diffraction-limited telescope: 20 m, wide field UVOIR telescope

The GLINT South testbed for nulling interferometry with photonics: Design and on-sky results at the Anglo-Australian Telescope

AbstractIn 1978, Bracewell suggested the technique of nulling interferometry to directly image exoplanets which would enable characterisation of their surfaces, atmospheres, weather, and possibly determine their capacity to host life. The contrast needed to discriminate starlight reflected by a terrestrial-type planet from the glare of its host star lies at or beyond a forbidding$10^{-10}$for an exo-Earth in the habitable zone around a Sun-like star at near-infrared wavelengths, necessitating instrumentation with extremely precise control of the light. Guided Light Interferometric Nulling Technology (GLINT) is a testbed for new photonic devices conceived to overcome the challenges posed by nulling interferometry. At its heart, GLINT employs a single-mode nulling photonic chip fabricated by direct-write technology to coherently combine starlight from an arbitrarily large telescope at 1 550 nm. It operates in combination with an actuated segmented mirror in a closed-loop control system, to produce and sustain a deep null throughout observations. The GLINT South prototype interfacesthe 3.9-m Anglo-Australian Telescope and was tested on a sample of bright Mira variable stars. Successful and continuous starlight injection into the photonic chip was achieved. A statistical model of the data was constructed, enabling a data reduction algorithm to retrieve contrast ratios of about$10^{-3}$. As a byproduct of this analysis, stellar angular diameters that were below the telescope diffraction limit($\sim$100 mas) were recovered with1$\sigma$accuracy and shown to be in agreement with literature values despite working in the seeing-limited regime. GLINT South serves as a demonstration of the capability of direct-write photonic technology for achieving coherent, stable nulling of starlight, which will encourage further technological developments towards the goal of directly imaging exoplanets with future large ground based and space telescopes.

A 20 m wide-field diffraction-limited telescope.

A 20 m space telescope is described with an unvignetted 1° field of view—a hundred times larger in area than fields of existing space telescopes. Its diffraction-limited images are a hundred times sharper than from wide-field ground-based telescopes and extend over much if not all the field, 40 arcmin diameter at 500 nm wavelength, for example. The optical system yielding a 1°, 1.36 m diameter image at f/3.9 has relatively small central obscuration, 9% by area on axis, and is fully baffled. Several carousel-mounted instruments can each access directly the full image. The initial instrument complement includes a 400 gigapixel silicon imager with 2 µm pixels (0.005 arcsec), and a 60 gigapixel HgCdTe imager with 5 µm pixels (0.012 arcsec). A multi-object spectrograph with 10 000 fibres will allow spectroscopy with 0.02 arcsec resolution. Direct imaging and spectroscopy of exoplanets can take advantage of the un-aberrated, on-axis image (5 nm RMS wavefront error). While this telescope could be built for operation in free space, a site accessible to a human outpost at the Moon's south pole would be advantageous, for assembly and repairs. The lunar site would allow also for the installation of new instruments to keep up with evolving scientific priorities and advancing technology. Cooling to less than 100E K would be achieved with a surrounding cylindrical thermal shield.This article is part of a discussion meeting issue ‘Astronomy from the Moon: the next decades’.

Surface Imaging of Proxima b and Other Exoplanets: Albedo Maps, Biosignatures, and Technosignatures

Abstract Seeing oceans, continents, quasi-static weather, and other surface features on exoplanets may allow for detecting and characterizing life outside the solar system. The Proxima b exoplanet resides within the stellar habitable zone, possibly allowing for liquid water on its surface, as on Earth. However, even the largest planned telescopes will not be able to resolve its surface features directly. Here we demonstrate an inversion technique to indirectly image exoplanet surfaces using observed unresolved reflected light variations over the course of the exoplanet’s orbital and axial rotation: ExoPlanet Surface Imaging (EPSI). We show that the reflected light curve contains enough information to detect both longitudinal and latitudinal structures and to map exoplanet surface features. We demonstrate this using examples of solar system planets and moons, as well as simulated planets with Earth-like life and artificial structures. We also describe how it is possible to infer the planet and orbit geometry from light curves. Then, we show how albedo maps of Proxima b can be successfully reconstructed for tidally locked, resonance, and unlocked axial and orbital rotation. Such albedo maps obtained in different wavelength passbands can provide “photographic” views of distant exoplanets. We estimate the signal-to-noise ratio necessary for successful inversions and analyze telescope and detector requirements necessary for the first surface image reconstructions of Proxima b and other nearby exoplanets using EPSI. This is a significant challenge, but the success of such measurements depends heavily on large-aperture diffraction-limited telescope performance—a feat that may be achieved on the ground before it is in space.

Enhancing SDO/HMI images using deep learning

Context.The Helioseismic and Magnetic Imager (HMI) provides continuum images and magnetograms with a cadence better than one per minute. It has been continuously observing the Sun 24 h a day for the past 7 yr. The trade-off between full disk observations and spatial resolution means that HMI is not adequate for analyzing the smallest-scale events in the solar atmosphere.Aims.Our aim is to develop a new method to enhance HMI data, simultaneously deconvolving and super-resolving images and magnetograms. The resulting images will mimic observations with a diffraction-limited telescope twice the diameter of HMI.Methods.Our method, which we callEnhance, is based on two deep, fully convolutional neural networks that input patches of HMI observations and output deconvolved and super-resolved data. The neural networks are trained on synthetic data obtained from simulations of the emergence of solar active regions.Results.We have obtained deconvolved and super-resolved HMI images. To solve this ill-defined problem with infinite solutions we have used a neural network approach to add prior information from the simulations. We testEnhanceagainst Hinode data that has been degraded to a 28 cm diameter telescope showing very good consistency. The code is open source.

CLUSTER GLIMPSES WITH RAVEN: AO-CORRECTED NEAR AND MID-INFRARED IMAGES OF GLIMPSE C01 AND GLIMPSE C02∗ †

ABSTRACT We discuss images of the star clusters GLIMPSE C01 (GC01) and GLIMPSE C02 (GC02) that were recorded with the Subaru IRCS. Distortions in the wavefront were corrected with the RAVEN adaptive optics (AO) science demonstrator, allowing individual stars in the central regions of both clusters—where the fractional contamination from non-cluster objects is lowest—to be imaged. In addition to J, H, and K′ images, both clusters were observed through a narrow-band filter centered near 3.05 μm; GC01 was also observed through two other narrow-band filters that sample longer wavelengths. Stars in the narrow-band images have an FWHM that is close to the telescope diffraction limit, demonstrating that open-loop AO systems like RAVEN can deliver exceptional image quality. The near-infrared color–magnitude diagram of GC01 is smeared by non-uniform extinction with a 1σ dispersion ΔA K = ±0.13 mag. Spatial variations in A K are not related in a systematic way to location in the field. The Red Clump is identified in the K luminosity function (LF) of GC01, and a distance modulus of 13.6 is found. The K LF of GC01 is consistent with a system that is dominated by stars with an age >1 Gyr. As for GC02, the K LF is flat for K > 16, and the absence of a sub-giant branch argues against an old age if the cluster is at a distance of ∼7 kpc. Archival SPITZER [3.6] and [4.5] images of the clusters are also examined, and the red giant branch-tip is identified. It is demonstrated in the Appendix that the [3.6] surface brightness profiles of both clusters can be traced out to radii of at least 100 arcsec.

Diffraction-limited step-zoom telescope by image restoration.

The design of a step-zoom telescope and its ability to achieve a diffraction-limited performance is explored. The basic idea is to include digital postprocessing to compensate for changes in the modulation transfer function of the system, assuming the knowledge of the range to the object. The instrument is conformed of a two-mirror telescope, two lenses, and a detector. High-quality images and a zoom telescope that ranges from 22 to 61 f-number is achieved by moving the primary mirror and two lenses. The preliminary calculations for the design process and a simulation that shows the performance of the step-zoom telescope are described.

Development, fabrication, and testing of an anamorphic imaging snap-together freeform telescope

The fabrication chain for the development of an afocal all aluminum telescope using four anamorphic aspherical mirrors is described. The optical and mechanical design are intended to achieve an enhanced system integration with reduced alignment effort by arranging two optical surfaces monolithically on common mirror bodies. Freeform machining is carried out by a hybrid fabrication approach combining diamond turning and diamond milling in the same machine setup. A direct figure correction of diamond turned aluminum mirrors by magnetorheological finishing is presented, resulting in high-precision athermal mirror modules with excellent figure properties. The interferometric system test highlights the diffraction limited telescope performance and the feasibility of the chosen approaches for freeform machining and mechanical integration.

The VAMPIRES instrument: imaging the innermost regions of protoplanetary discs with polarimetric interferometry

Direct imaging of protoplanetary disks promises to provide key insight into the complex sequence of processes by which planets are formed. However imaging the innermost region of such disks (a zone critical to planet formation) is challenging for traditional observational techniques (such as near-IR imaging and coronagraphy) due to the relatively long wavelengths involved and the area occulted by the coronagraphic mask. Here we introduce a new instrument -- VAMPIRES -- which combines non-redundant aperture-masking interferometry with differential polarimetry to directly image this previously inaccessible innermost region. By using the polarisation of light scattered by dust in the disk to provide precise differential calibration of interferometric visibilities and closure phases, VAMPIRES allows direct imaging at and beyond the telescope diffraction limit. Integrated into the SCExAO system at the Subaru telescope, VAMPIRES operates at visible wavelengths (where polarisation is high) while allowing simultaneous infrared observations conducted by HICIAO. Here we describe the instrumental design and unique observing technique and present the results of the first on-sky commissioning observations, validating the excellent visibility and closure phase precision which are then used to project expected science performance metrics.

Speckle Camera Imaging of the Planet Pluto11Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the National Science Foundation on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council

ABSTRACTWe have obtained optical wavelength (692 nm and 880 nm) speckle imaging of the planet Pluto and its largest moon Charon. Using our DSSI speckle camera attached to the Gemini North 8 m telescope, we collected high resolution imaging with an angular resolution of ∼20 mas, a value at the Gemini-N telescope diffraction limit. We have produced for this binary system the first speckle reconstructed images, from which we can measure not only the orbital separation and position angle for Charon, but also the diameters of the two bodies. Our measurements of these parameters agree, within the uncertainties, with the current best values for Pluto and Charon. The Gemini-N speckle observations of Pluto are presented to illustrate the capabilities of our instrument and the robust production of high accuracy, high spatial resolution reconstructed images. We hope our results will suggest additional applications of high resolution speckle imaging for other objects within our solar system and beyond.

Apodized apertures for solar coronagraphy

Aims. We propose the principle of a new solar telescope that makes it possible to observe the solar corona very close to the solar limb, without the help of a Lyot coronagraph. The result is obtained using a strongly apodized aperture. Methods. We obtain the theoretical form of the diffraction halo produced by the solar disk at the level of the corona for a perfect diffraction-limited telescope, for raw and apodized apertures. The problem is first solved at one dimension for which a complete set of analytical expressions can be derived, including the effect of the center-to-limb solar variation. Formal equations are written for the two-dimensional case, and it is shown that the expression may take the form of a 1D integral. Nevertheless, the problem is difficult to solve. An analytic expression can be worked out using the line spread function, which is shown to give a valid approximation of the problem, in excellent agreement with a numerical computation that uses the exact integral. Results. We show for the raw aperture that the diffraction halo is very strong and decreases slowly as p -1 . We propose as a solution to this problem an apodized aperture based on the generalized prolate spheroidal functions (GPSF). Such an apodized aperture may reduce the diffraction halo enough to permit a direct observation of the solar corona very close to the solar limb. A signal-to-noise ratio analysis is given. Conclusions. Different strengths of apodization may be used, but very strong apodizations are indeed mandatory. A good choice seems to be a GPSF aperture with the prolate coefficient c on the order of 10. It could reduce the halo of diffraction by a factor 10 5 (at the cost of an intensity throughput of 10% and a reduction in the classical resolution by a factor of about 1.6) and permit observation of the corona very close to the solar limb.

Earth-based optical imaging of Mercury

In recent years, considerable progress has been achieved in producing resolved images of Mercury electronically with short exposures at Earth-based telescopes. For the purpose of obtaining images of the unknown portion of Mercury, the previously started series of observations of this planet by the short exposure method was continued. About 20,000 electronic images of Mercury have been acquired on 1–2 May 2002 under good meteorological conditions during the evening elongation. The phase angle of Mercury was 95–99° and the observed range of longitudes was 210–285°W. Observations were carried out using Ritchy–Chrétien telescope ( D = 1.29 m, F = 9.86 m) with the KS 19 filter cutting wavelengths shorter than about 700 nm. The planet’s disk was seen, on average, at an angle of 7.7″. A CCD with a pixel size of 7.4 × 7.4 ncm in the regime of short exposures was used. By processing a great number of electronic images, a sufficiently distinct synthesized image of the unknown portion of Mercury’s surface was obtained. The most prominent formation in this region is a giant basin (or cratered “mare”) centered at about 8°N, 280°W, which was given a working name “Skinakas basin” (after the name of the observatory where observations were made). By its size, the interior part of this basin exceeds the largest lunar Mare Imbrium. As opposed to Mare Imbrium, the Skinakas basin is presumably of impact origin. Its relief resembles that of Caloris Planitia but the size is much larger. A series of smaller formations are also seen on synthesized images. The resolution obtained on the surface of Mercury is about 100 km, which is close to the telescope diffraction limit. Also considered is the synthesized image obtained at the Mount Bigelow Observatory, on December 4, 2003 (Ritchy–Chrétien telescope, D = 1.54 m, F = 20.79 m, using the same CCD camera).

Stereoscopic Spectroscopy for Efficient Spectral Imaging and Magnetography

Stereoscopic imaging spectroscopy is a novel technique that uses multiple spectral orders and stereoscopic reconstruction to eliminate the need for a slit in a dispersive spectrograph, potentially increasing photon efficiency by 1-2 orders of magnitude compared with conventional spectral imaging. We present advances in stereoscopic reconstruction, allowing detection of the first moments of a spectral line simultaneously over a complete two-dimensional image plane. Using the Advanced Stokes Polarimeter on the Dunn Solar Telescope at the National Solar Observatory, we have created the first solar magnetograms made with this technique. With a simple prototype optical layout, we were able to generate Stokes V magnetograms using the Fe I photospheric absorption line at 617.34 nm (6173.4 A). We calculate that an optimized instrument using this technique could acquire solar magnetograms with ~10 G rms shot noise in 150 ms using this spectral line at the host telescope's diffraction limit. Stereoscopic techniques are useful not only for the solar photosphere but also for photon-starved imaging spectroscopy in general.

Mercury: The Image of the Planet in the 210°–285° W Longitude Range Obtained by the Short-Exposure Method

For the purpose of obtaining images of the unknown portion of Mercury, we continued the previously started series of observations of this planet by the short exposure method. Several thousand electronic images of Mercury have been acquired on 1–2 May 2002 under good meteorological conditions at the high-altitude Skinakas Astrophysical Observatory of Iraklion University (Crete, Greece, 35°13′ E, 24°54′ N) during the evening elongation. The phase angle of Mercury was 95°–99° and the observed range of longitudes was 210°–285° W. Observations were carried out using Ritchy–Chretien telescope (D = 1.29 m, F = 9.857 m) with the KS 19 filter cutting wavelengths shorter than about 700 nm. The planet's disk was seen, on average, at an angle of 7.75″ arcsec. The image scale was equal to 47.8 μm/arcsec. We used a CCD with a pixel size of 7.4 × 7.4 μm in the regime of short exposures. By processing a great number of electronic images, we succeeded in obtaining a sufficiently distinct synthesized image of the unknown portion of Mercury's surface. The most prominent formation in this region is a giant basin (or cratered “mare”) centered at about 8° N, 280° W, which was given a working name “Skinakas basin” (after the name of the observatory where observations were made). By its size, the interior part of this basin exceeds the largest lunar Mare Imbrium. As opposed to Mare Imbrium, the Skinakas basin is presumably of impact origin. Its relief resembles that of Caloris Planitia but the size is much larger. A series of smaller formations are also seen on synthesized images. The resolution obtained on the surface of Mercury is about 100 km, which is close to the telescope diffraction limit. Also considered are the published theoretical estimations of the possible advantages offered by the short exposure method. Some results obtained by other research groups are discussed.

Interferometric hartmann wave-front sensing for active optics at the 6.5-m conversion of the multiple mirror telescope.

A little-used interferometric modification to the classical Hartmann optical test is being used for active optics corrections at the 6.5-m Multiple Mirror Telescope (MMT) Observatory. The technique produces compact interference spots whose positions depend on discrete wave-front phase-difference errors. A diffraction model illustrates the formation of interference spots. The limitations on wave-front sampling that are due to atmospheric seeing are estimated. Two data reduction matrix strategies are given. A single correction of collimation and primary mirror figure produces a nearly diffraction-limited telescope image. Wave-front polynomial coefficients ranging from several nanometers to several micrometers in amplitude are reliably detected.

High-resolution Imaging of the Upper Solar Chromosphere: First Light Performance of the Very-high-Resolution Advanced ULtraviolet Telescope

The Very-high-resolution Advanced ULtraviolet Telescope (VAULT) experiment was successfully launched on 7 May 1999 on a Black Brant sounding rocket vehicle from White Sands Missile Range. The instrument consists of a 30 cm UV diffraction limited telescope followed by a two-grating, zero-dispersion spectroheliograph tuned to isolate the solar Lα emission line. During the flight, the instrument successfully obtained a series of images of the upper chromosphere with a limiting resolution of ∼ 0.33 arc sec. The resulting observations are the highest-resolution images of the solar atmosphere obtained from space to date. The flight demonstrated that sub-arc second ultraviolet images of the solar atmosphere are achievable with a high-quality, moderate-aperture space telescope and associated optics. Herein, we describe the payload and its in-flight performance.

Measurements of Solar Magnetic Element Dispersal

The dispersal of elements in the solar photospheric flow field is studied by tracking individual magnetic bright points (MBPs) identified in a G-band 4305 A filtergram time series obtained at the 50 cm Swedish Vacuum Solar Telescope on La Palma, Spain. The time series spans approximately 70 minutes with a field of view of 29'' × 29'' near disk center. All images in the time series are restored to near the telescope diffraction limit (~02 in the G band) using partitioned phase diverse speckle techniques. Regions of enhanced network and quiet Sun are examined. In the network region, automated tracking of individual MBPs reveals approximately Gaussian diffusion, with indications for slightly superdiffusive dispersal. The inferred Gaussian diffusion coefficient is 60.4 ± 10.9 km2 s-1. In the quiet-Sun region, local correlation tracking velocity measurements show the dispersal of artificial tracers to be non-Gaussian over most of our data set with indications of an asymptotic approach to a 285 km2 s-1 Gaussian diffusion.

HIRAS: High-Resolution IRAS Images

The InfraRed Astronomical Satellite (IRAS) all-sky survey was designed and optimized for the detection of point sources. This allowed the survey to be conducted in the form of narrow strip scans with redundant coverage of the sky, but with non-uniform covering densities. The data shows in addition to point sources many sources of extended emission, which are best analyzed from images. However, the non-uniform coverage now forms a significant obstacle in the image (re-)construction. Low resolution images, such as in the Infrared Sky Survey Atlas, yield spatial resolutions of 5–10 times the IRAS telescope diffraction limit; HIRAS improves this to 1–2 times!

The Spectrum-UV Project/The Spectrum-UV Team

AbstractThe Ultra-Violet Space Telescope “SPECTRUM-UV” consists of a 170-cm diffraction limited telescope for spectroscopy and imaging at λ ≥ 912 Å and a co-aligned assembly of two 50-cm telescopes for imaging and spectroscopy in the 400 to 1200 Å range and of four 20-cm, multilayer coated telescopes for narrow band imaging in the 100 to 400 Å range. The Observatory is an international facility to be launched in a highly eccentric orbit in the mid-1990s.