High-contrast observations in optical and infrared astronomy are defined as any observation requiring a technique to reveal a celestial object of interest that is in such close angular proximity to another source brighter by a factor of at least 10^5 that optical effects hinder or prevent the collection of photons directly from the target of observation. This is a relatively new type of observation that enables research on previously obscured parts of the Universe. In particular, it is most applicable to Comparative Planetary Science, a field that directly attacks such questions as "how common are planetary systems? What types of planets exist, and are there planets other than Earth that are capable of supporting life as we know it?" We survey the scientific motivations for high-contrast observations, provide an overview of the techniques currently being used or developed, and discuss some ideas and studies for future prospects.
Deep Dive into High-Contrast Observations in Optical and Infrared Astronomy.
High-contrast observations in optical and infrared astronomy are defined as any observation requiring a technique to reveal a celestial object of interest that is in such close angular proximity to another source brighter by a factor of at least 10^5 that optical effects hinder or prevent the collection of photons directly from the target of observation. This is a relatively new type of observation that enables research on previously obscured parts of the Universe. In particular, it is most applicable to Comparative Planetary Science, a field that directly attacks such questions as “how common are planetary systems? What types of planets exist, and are there planets other than Earth that are capable of supporting life as we know it?” We survey the scientific motivations for high-contrast observations, provide an overview of the techniques currently being used or developed, and discuss some ideas and studies for future prospects.
In some sense, all of optical and infrared astronomy requires "high-contrast" observations. Indeed, the Sun irradiates the surface of the Earth with about 10 35 photons per second in the wavelength span between 0.5 and 5 µm. In contrast, the full Moon's irradiation of Earth is about a million times smaller. Vega, one of the brightest stars in the sky, irradiates the Earth at a rate that is about another million times smaller, with roughly 10 24 photons per second. This is 10 -11 times the Earth-bound photon flux of the Sun. Beyond that, state-of-the-art, deep observations in optical astronomy have detected objects even 10 -13 times fainter than Vega. Somehow, astronomers have picked one photon from such an object for every 10 24 from the Sun.
Fortunately, photons travel in extremely well-determined directions, and we have a persistent natural eclipse of the Sun with half of the surface of Earth immersed in night at any given moment, vastly reducing, by about 18-20 orders of magnitude, not only the number of photons from the Sun directly incident on a ground-based telescope, but also the number entering such a telescope due to atmospheric Rayleigh scattering (and other less-important sources of sky background). Furthermore, in space there is no atmosphere and only minimal ambient dust in our solar system to scatter solar photons into a telescope. Thus, to study many of the objects in the sky, nothing more than a standard telescope (to select photons from precisely determined directions) and suitable instrumentation (to analyze those photons) is needed to study objects that are not next to the Sun’s position in the sky, or that happen to be in the darkness of the night sky.
The ratio of intensity of light between a brighter and a fainter object.
Nature provides us with the “contrast” we need to study of much of the universe.
Imagine, however, attempting to study Vega when it is just 0.1 arcseconds off the limb of the Sun. Somehow one must filter the light of the Sun from that of Vega. In fact, during the famous solar eclipse of 1919, several bright stars in the Hyades were photographed within a few arcseconds of the Sun’s limb, confirming the prediction of general relativity, in one of the most important observations of the 20 th century, that the apparent positions of these stars would be distorted by almost 2 arcseconds due to the gravitational influence of the Sun (Dyson et al. 1920). These observations, though, required the eclipse, which allowed the stars to shine more brightly than the background of light due to the solar corona and atmospheric scattering. In truth, these stars were at least 10 12 times fainter than, and within a few arcseconds of, the Sun. These observations, along with Lyot’s (1939) coronagraphic observations of the Sun’s corona, possibly qualify as the first “high-contrast” observations in optical astronomy. Close proximity and a vast difference in brightness are the critical elements of what we mean by “high contrast” for the purposes of this article. More precisely, we define “high-contrast observation” as any observation in which the object being studied is detected with another object in the field of view, that is at least 10 5 times brighter, and which is in such close angular proximity to the target object that its light due to scattering or diffraction would prevent the observation without special conditions or methods to suppress its light.
Clearly, high-contrast observations have led to fundamental results in physics, as well as enabled fields such as observational solar physics. Furthermore, as has become increasingly clear, especially over the past two decades, there are fascinating parts of the universe, that we have only just begun to observe, because a bright object, such as a star, obscures the region of interest where objects 10 5 to 10 15 times fainter exist. These regions, the close vicinities of our stellar neighbors, and the objects in them may have important connections and clues to the origins and evolution of stars, life, the Earth, and our solar system, and may also yield answers to some of the most profound questions in this field, such as “How common is life in the universe?” or “Are planets like Earth rare?” At this point in time, high-contrast observing is primarily used in three subfields of astronomy: comparative exoplanetary science and star and planet formation. Such types of observations can also be applied to the study of advanced stages of stellar evolution where significant outflows from aging stars are present, although little has been done in this area. In actuality, the first three areas are intrinsically linked and form what is becoming an increasingly multi-disciplinary field of research in its own right, not merely a subfield of astrophysics. Comparative exoplanetary science-the study of planets in general, not just those in our solar system, how they form and evolve, their apparent diversity and their prevalence around stars-requires in
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