Spatial Linear Dark Field Control: Stabilizing Deep Contrast for Exoplanet Imaging Using Bright Speckles [IMA]

Direct imaging of exoplanets requires the ability to build and maintain a high contrast dark hole (DH) within the science image to a high degree of precision. Current techniques, such as electric field conjugation (EFC), have been demonstrated in the lab and have shown that they are capable of generating a DH with high contrast. To do so, such techniques require continuous wavefront estimate updates that are acquired by interrupting the DH, thereby competing with the science measurement. In this paper, we introduce and demonstrate spatial linear dark field control (LDFC) as a new technique by which the DH contrast can be controlled and maintained without any disruption to the science image. Instead of rebuilding the DH using EFC after it degrades over time, spatial LDFC locks the high contrast dark field (DF) after EFC using the bright field (BF) that responds linearly to wavefront variations that modify both the BF and the DH.

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K. Miller, O. Guyon and J. Males
Tue, 14 Mar 17

Comments: 9 pages, 11 images

The EBEX Balloon Borne Experiment – Optics, Receiver, and Polarimetry [IMA]

The E and B Experiment (EBEX) was a long-duration balloon-borne cosmic microwave background polarimeter that flew over Antarctica in 2013. We describe the experiment’s optical system, receiver, and polarimetric approach, and report on their in-flight performance. EBEX had three frequency bands centered on 150, 250, and 410~GHz. To make efficient use of limited mass and space we designed a 115~cm$^{2}$sr high throughput optical system that had two ambient temperature mirrors and four anti-reflection coated polyethylene lenses per focal plane. All frequency bands shared the same optical train. Polarimetry was achieved with a continuously rotating achromatic half-wave plate (AHWP) that was levitated with a superconducting magnetic bearing (SMB). Rotation stability was 0.45~\% over a period of 10~hours, and angular position accuracy was 0.01~degrees. This is the first use of a SMB in astrophysics. The measured modulation efficiency was above 90~\% for all bands. To our knowledge the 109~\% fractional bandwidth of the AHWP was the broadest implemented to date. The receiver that contained one lens and the AHWP at a temperature of 4~K, the polarizing grid and other lenses at 1~K, and the two focal planes at 0.25~K performed according to specifications giving focal plane temperature stability with fluctuation power spectrum that had $1/f$ knee at 2~mHz. EBEX was the first balloon-borne instrument to implement technologies characteristic of modern CMB polarimeters including high throughput optical systems, and large arrays of transition edge sensor bolometric detectors with mutiplexed readouts.

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EBEX. Collaboration, A. Aboobaker, P. Ade, et. al.
Tue, 14 Mar 17

Comments: 49 pages, 32 figures, to be submitted to The Astrophysical Journal Supplement

AGILIS: Agile Guided Interferometer for Longbaseline Imaging Synthesis – Demonstration and concepts of reconfigurable optical imaging interferometers [IMA]

In comparison to the radio and sub-millimetric domains, imaging with optical interferometry is still in its infancy. Due to the limited number of telescopes in existing arrays, image generation is a demanding process that relies on time-consuming reconfiguration of the interferometer array and super-synthesis. Using single mode optical fibres for the coherent transport of light from the collecting telescopes to the focal plane, a new generation of interferometers optimized for imaging can be designed. To support this claim, we report on the successful completion of the `OHANA Iki project: an end-to-end, on-sky demonstration of a two-telescope interferometer, built around near-infrared single mode fibres, carried out as part of the `OHANA project. Having demonstrated that coherent transport by single-mode fibres is feasible, we explore the concepts, performances, and limitations of a new imaging facility with single mode fibres at its heart: Agile Guided Interferometer for Longbaseline Imaging Synthesis (AGILIS). AGILIS has the potential of becoming a next generation facility or a precursor to a much larger project like the Planet Formation Imager (PFI).

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J. Woillez, O. Lai, G. Perrin, et. al.
Tue, 14 Mar 17

Comments: 16 pages, 10 figures, 2 tables, accepted in A&A

Polynomial Apodizers for Centrally Obscured Vortex Coronagraphs [IMA]

Several coronagraph designs have been proposed over the last two decades to directly image exoplanets. Among these designs, the vector vortex coronagraphs provide theoretically perfect starlight cancellation along with small inner working angles when deployed on telescopes with unobstructed pupils. However, current and planned space missions and ground-based extremely large telescopes present complex pupil geometries, including secondary mirror central obscurations, that prevent vortex coronagraphs from rejecting on-axis sources entirely. Recent solutions combining the vortex phase mask with a ring-apodized pupil have been proposed to circumvent this issue, but provide a limited throughput for vortex charges $>2$. We present a family of pupil plane apodizations that compensate for pupil geometries with circularly symmetric central obstructions caused by on-axis secondary mirrors for charge 2, 4, and 6 vector vortex coronagraphs. These apodizations are derived analytically and allow the vortex coronagraph to retain theoretically perfect nulling in the presence of central obscurations. For a charge 4 vortex, we design polynomial apodization functions assuming a greyscale apodizing filter that represent a substantial gain in throughput over the ring-apodized vortex coronagraph design, while for a charge 6 vortex, we design polynomial apodized vortex coronagraphs that have $\gtrsim 70\%$ total energy throughput for the entire range of central obscuration sizes studied. We propose methods for optimizing apodizations produced with either greyscale apodizing filters or shaped mirrors. We conclude by demonstrating how this design may be combined with apodizations numerically optimized for struts and segment gaps in telescope pupils to design terrestrial exoplanet imagers for complex pupils.

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K. Fogarty, L. Pueyo, J. Mazoyer, et. al.
Fri, 10 Mar 17

Comments: 18 pages, 12 figures, submitted to ApJ

Black Holes and Vacuum Cleaners: Using Metaphor, Relevance, and Inquiry in Labels for Space Images [IMA]

This study extended research on the development of explanatory labels for astronomical images for the non-expert lay public. The research questions addressed how labels with leading questions/metaphors and relevance to everyday life affect comprehension of the intended message for deep space images, the desire to learn more, and the aesthetic appreciation of images. Participants were a convenience sample of 1,921 respondents solicited from a variety of websites and through social media who completed an online survey that used four high-resolution images as stimuli: Sagittarius A*, Solar Flare, Cassiopeia A, and the Pinwheel Galaxy (M101). Participants were randomly assigned initially to 1 of 3 label conditions: the standard label originally written for the image, a label with a leading question containing a metaphor related to the information for the image, or a label that contained a fact about the image relevant to everyday life. Participants were randomly assigned to 1 image and compared all labels for that image. Open-ended items at various points asked participants to pose questions to a hypothetical astronomer. Main findings were that the relevance condition was significantly more likely to increase wanting to learn more; the original label was most likely to increase overall appreciation; and, smart phone users were more likely to want to learn more and report increased levels of appreciation. Results are discussed in terms of the need to examine individual viewer characteristics and goals in creating different labels for different audiences.

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L. Smith, K. Arcand, B. Smith, et. al.
Thu, 9 Mar 17

Comments: 50 pages, 7 tables, 2 figures, accepted by the journal “Psychology of Aesthetics, Creativity, and the Arts”

Multi-GPU maximum entropy image synthesis for radio astronomy [IMA]

The maximum entropy method (MEM) is a well known deconvolution technique in radio-interferometry. This method solves a non-linear optimization problem with an entropy regularization term. Other heuristics such as CLEAN are faster but highly user dependent. Nevertheless, MEM has the following advantages: it is unsupervised, it has an statistical basis, it has a better resolution and better image quality under certain conditions. This work presents a high performance GPU version of non-gridded MEM, which is tested using interferometric and simulated data. We propose a single-GPU and a multi-GPU implementation for single and multi-spectral data, respectively. We also make use of the Peer-to-Peer and Unified Virtual Addressing features of newer GPUs which allows to exploit transparently and efficiently multiple GPUs. Several ALMA data sets are used to demonstrate the effectiveness in imaging and to evaluate GPU performance. The results show that a speedup from 1000 to 5000 times faster than a sequential version can be achieved, depending on data and image size. This has allowed us to reconstruct the HD142527 CO(6-5) short baseline data set in 2.1 minutes, instead of the 2.5 days that takes on CPU.

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M. Carcamo, P. Roman, S. Casassus, et. al.
Thu, 9 Mar 17

Comments: 11 pages, 13 figures

CMU DeepLens: Deep Learning For Automatic Image-based Galaxy-Galaxy Strong Lens Finding [IMA]

Galaxy-scale strong gravitational lensing is not only a valuable probe of the dark matter distribution of massive galaxies, but can also provide valuable cosmological constraints, either by studying the population of strong lenses or by measuring time delays in lensed quasars. Due to the rarity of galaxy-scale strongly lensed systems, fast and reliable automated lens finding methods will be essential in the era of large surveys such as LSST, Euclid, and WFIRST. To tackle this challenge, we introduce CMU DeepLens, a new fully automated galaxy-galaxy lens finding method based on Deep Learning. This supervised machine learning approach does not require any tuning after the training step which only requires realistic image simulations of strongly lensed systems. We train and validate our model on a set of 20,000 LSST-like mock observations including a range of lensed systems of various sizes and signal-to-noise ratios (S/N). We find on our simulated data set that for a rejection rate of non-lenses of 99%, a completeness of 90% can be achieved for lenses with Einstein radii larger than 1.4″ and S/N larger than 20 on individual $g$-band LSST exposures. Finally, we emphasize the importance of realistically complex simulations for training such machine learning methods by demonstrating that the performance of models of significantly different complexities cannot be distinguished on simpler simulations. We make our code publicly available at .

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F. Lanusse, Q. Ma, N. Li, et. al.
Thu, 9 Mar 17

Comments: 12 pages, 9 figures, submitted to MNRAS