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We present the results from four stellar occultations by (486958) Arrokoth, the flyby target of the New Horizons extended mission. Three of the four efforts led to positive detections of the body, and all constrained the presence of rings and other debris, finding none. Twenty-five mobile stations were deployed for 2017 June 3 and augmented by fixed telescopes. There were no positive detections from this effort. The event on 2017 July 10 was observed by the Stratospheric Observatory for Infrared Astronomy with one very short chord. Twenty-four deployed stations on 2017 July 17 resulted in five chords that clearly showed a complicated shape consistent with a contact binary with rough dimensions of 20 by 30 km for the overall outline. A visible albedo of 10% was derived from these data. Twenty-two systems were deployed for the fourth event on 2018 August 4 and resulted in two chords. The combination of the occultation data and the flyby results provides a significant refinement of the rotation period, now estimated to be 15.9380 ± 0.0005 hr. The occultation data also provided high-precision astrometric constraints on the position of the object that were crucial for supporting the navigation for the New Horizons flyby. This work demonstrates an effective method for obtaining detailed size and shape information and probing for rings and dust on distant Kuiper Belt objects as well as being an important source of positional data that can aid in spacecraft navigation that is particularly useful for small and distant bodies. © 2020. The American Astronomical Society. All rights reserved., This project would have been impossible without support from many groups and individuals. We had exceptional support from the US State Department (John Fazio, US Embassy in Argentina; James Garry, Heath Bailey, and Cheikh Oumar Dia, US Embassy in Senegal), CONAE (Felix Menicocci and Stan Makarchuk), and SOFIA (William Reach, Anil Dosaj, Paul Newton, James Less, Stephen Koertge, Kimberly Ennico, and Karina Leppik), SOFIA/FPI+ (Enrico Pfuller, Jurgen Wolf, and Manuel Wiedemann). Special thanks go to Tony Barry for rapid turnaround updates to his SEXTA reader for timing verification, QHY for updated camera firmware, and Robin Glover for updates to the SharpCap software. The discovery and subsequent astrometric images were made possible by the Hubble Space Telescope and the wonderful staff at the Space Telescope Science Institute (STScI). Extracting high-precision astrometry from the HST data was made possible by early release of Gaia DR2 data. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC,.https://www.cosmos.esa.int/web/gaia/dpac/consortium).Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. The local logistical support from Argentina, especially from the municipality of Comodoro Rivadavia, was critical to the success of the observations on 2017 July 17. Support from the Government of Senegal and the Ministry of Higher Education, Research and Innovation (MESRI) was critical to the success of the observation on 2018 August 4. J. Desmars acknowledges the funding from the European Research Council under the European Community's H2020 (2014-2020/ERC grant Agreement No. 669416 "LUCKY STAR"). The French participants were supported by the French Space Agency (CNES), the 2014-2020/ERC grant Agreement No. 669416 "LUCKY STAR" and the French National Research Institute for Sustainable Development. S.S. acknowledges support from the Africa Initiative for Planetary and Space Science (http://africapss.org) and the Uranoscope de France (https://uranoscope.org). P.S.S. acknowledges financial support by the European Union's Horizon 2020 Research and Innovation Programme, under grant agreement No. 687378, as part of the project "Small Bodies Near and Far" (SBNAF) and also acknowledges financial support from the State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award for the Instituto de Astrofisica de Andalucia (SEV-2017-0709). This paper is based in part on observations obtained at the Southern Astrophysical Research (SOAR) telescope, which is a joint project of the Ministerio da Ciencia, Tecnologia, Inovacaos e Comunicacaoes (MCTIC) do Brasil, the U.S. National Optical Astronomy Observatory (NOAO), the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU). At SOAR, data acquisition was performed with a Raptor camera (visitor instrument) funded by the Observatorio Nacional/MCTIC. Based in part on observations made with the NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA). SOFIA is jointly operated by the Universities Space Research Association, Inc. (USRA), under NASA contract NNA17BF53C, and the Deutsches SOFIA Institut (DSI) under DLR contract 50 OK 0901 to the University of Stuttgart. Financial support for the SOFIA observations was provided by NASA through award NAS297001 issued by USRA. This paper utilizes public domain data obtained by the MACHO Project, jointly funded by the US Department of Energy through the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48, by the National Science Foundation through the Center for Particle Astrophysics of the University of California under cooperative agreement AST-8809616, and by the Mount Stromlo and Siding Spring Observatory, part of the Australian National University. Overall funding for this project was from NASA's New Horizons project via contracts NASW-02008 and NAS5-97271/TaskOrder30 and special appreciation is due to Dr. James Green and Dr. Lori Glaze at NASA Headquarters for their support.

The efficient coding hypothesis posits that sensory circuits and animal behavior should be adapted to the statistical structure of natural signals. Here, we show that the perception of visual textures is adapted to the spatial distribution of light signals at a striking level of detail. We first identify a parametrized 66-dimensional space of grayscale textures defined by local spatial correlations between discrete light intensities. We then devise a method of measuring the contribution of each of these textures to the spatial structure of scenes. Efficient coding predicts that the perceptual salience of a complex sensory signal should be related to its variability in natural settings. Based on an analysis of contrast-equalized natural scenes, this theory predicts that textures involving two-point correlations will be most salient, and it further predicts the relative salience among second-order correlations of different types. We test our predictions by asking observers to locate a briefly-flashed texture strip in a background of white noise. The behavior of individual subjects is highly consistent: correlations beyond the second order are hard to distinguish from random noise, and detection thresholds within this salient second-order subspace are quantitatively predicted across 26 two-dimensional texture planes. We additionally find non-trivial symmetries of natural images that leave psychophysical thresholds unchanged, further supporting our hypothesis. These findings significantly extend previous results obtained for binary images, and they provide deeper insights into the relationship between natural signal statistics and human perception.

The contrast sensitivity function (CSF), how sensitivity varies with the spatial frequency of the stimulus, is a fundamental assessment of visual performance. The CSF is generally assumed to be determined by low-level sensory processes. However, the sensitivities of neurons in the early visual pathways, as measured in experiments with immobilized eyes, diverge from psychophysical CSF measurements in primates. Under natural viewing conditions, as in typical psychophysical measurements, humans continually move their eyes, drifting in a seemingly erratic manner even when looking at a fixed point. Here, we show that the resulting transformation of the visual scene into a spatiotemporal flow on the retina constitutes a processing stage that reconciles human CSF and the response characteristics of retinal ganglion cells under a broad range of conditions. Our findings suggest a fundamental integration between perception and action: eye movements work synergistically with the sensitivities of retinal neurons to encode spatial information.

Many questions remain unanswered regarding the specific cues and mechanisms for emmetropization, the process by which, during development, the eye adjusts itself so that distant objects are in focus. Research has so far primarily focused on the spatial cues present in the image on the retina, such as the degree of blur. However, eye movements incessantly transform a mostly static scene into temporal modulations, so that the input to the retina is not an image, but a spatiotemporal flow of luminance. Models of retinal input signals indicate that this space-time reformatting caused by eye movements yields additional cues to the ones traditionally considered for emmetropization. These cues are implicit in the temporal modulations impinging onto retinal receptors and depend on the optics and shape of the eye and the spatial statistics of the visual scene. Here we examine the characteristics of these signals in the presence of normal eye movements and model the possible consequences of abnormal oculomotor behavior in the development of myopia and hyperopia.