Astronomy & Celestial Mechanics
the sky, computed
Concept

Nulling interferometry — where it appears

Combining light from two or more telescopes out of phase so that a star on the axis cancels while light from beside it passes. It is used to measure faint emission close to bright stars, such as warm dust and planets.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

A fringe that is dark on the star and bright on the planet. The fraction of a point source's light an interferometric nuller transmits, against its angle from the star along the baseline, in milliarcseconds, for two telescopes 10.3 m apart observing at 10 μm — the baseline that puts the first bright fringe exactly on a planet 1 AU from a star 10 pc away, 100 milliarcseconds out. The solid curve is the two-telescope null, sin²: it is zero on the star and rises as the square of the angle. The dashed curve is a quartic null, sin⁴, made by combining four apertures: it is darker near the star and narrower at the planet. The shaded strip is the star itself, 0.93 milliarcseconds across. It looks negligible at this scale, but the null is not perfect across it, and what leaks from that strip is the quantity the whole technique has to beat.

A null filled in by the star itself

Two telescopes combined out of phase cancel a star on the axis and pass a planet beside it, and the cancellation rises only as the square of the angle away from the axis. A star is not a point, so its own edges sit off the axis — and for the Sun seen with the baseline that finds the Earth, they leak through at a hundred and fifty times the Earth's brightness, at every distance, because the leak depends only on the size of the star compared with the planet's orbit.

exoplanets · Direct imaging Where a ten-micron zodi comes from. How much each logarithmic interval of radius contributes to the ten-micron glow of a Sun-like star's exozodiacal dust, for blackbody grains in the Solar System's radial profile, seen face-on from 10 pc: with no instrument (dashed) and through a 14.4 m nulling interferometer whose fringes suppress the light near the star (solid, with the averaged transmission dotted). Without the null the glow is concentrated far inside the habitable zone at 1 AU (marked), because the grains' emission at ten microns rises steeply with their temperature: half of it comes from inside 0.34 AU. The null removes the innermost part and pushes the sensitivity outward, but half the nulled signal still comes from inside 0.69 AU, peaking at 0.74. A zodi level measured this way is a measurement of the dust between a few tenths of an AU and the habitable zone, extrapolated to the habitable zone with the Sun's profile.

A zodi measured in heat and needed in light

The dust around nearby stars that will limit the search for Earth-like planets in reflected light has been measured — but in its own glow at ten microns, by an interferometer whose fringes hide the dust nearest the star. Converting that measurement into the visible brightness at the habitable zone needs the dust's radial profile and its grains' albedo and temperature, none of which was measured. The same ten-micron zodi can mean a quarter or ten times the visible dust a coronagraph will see.

exoplanets · Direct imaging

Named alongside it

The objects these essays reach for when they reach for this one.

Direct imagingHabitable zoneAlbedoAngular diameterBaselineBlackbodyContrastCoronagraphExozodiacal dustInner working angleMid-infraredOptical path difference

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