Astrometry and photometry

[als ik op deze pagina kom dan verwacht ik eerst een alinea over astrometrie en fotometrie. kun je dat hier toevoegen? ik zou zoiets doen: ‘de helderheid en exacte posities van astronomische objecten zoals sterren en planeten kan veel vertellen over hun eigenschappen. via astrometrie proberen onderzoekers van LEOPARD […] en fotometrie levert informatie op over bijvoorbeeld weerpatronen op exoplaneten.]

Probing the weather on exoplanets 

Like the planets in the solar system, exoplanets are expected to have clouds in their atmospheres. Detecting and studying these clouds will improve our understanding of the atmospheric composition and weather systems on these other extrasolar worlds. 

Thus far, variability has only been observed with the extremely stable Hubble Space Telescope for the exoplanet 2M1207b, which has a photometric [brightness?] variation between 0.78 and 1.36 percent. [is dit afkomstig van LEOPARD? en wat zeggen die helderheidsvariaties, zijn dat inderdaad weerpatronen?]

Normalized light curves for 2M1207B (upper) and A (lower) with filter F125W (left) and F160W (right). Figure adopted from Zhou et al. (2016).

Furthermore, brown dwarfs that have masses between that of a planet and star have shown to have cloud structures, that express themselves in brightness variations while the object rotates.

Exoplanet orbits

Via astrometric monitoring of exoplanets the orbits and masses of exoplanets can be directly measured, which constraints their formation pathways. A study of the dynamical environment of exoplanetary systems becomes possible. [het lijkt mij goed om hier al heel kort te vertellen hoe dat ongeveer gaat en hoe je relevante informatie uit het licht haalt. je vangt dus direct licht op van de exoplaneet? hoe haal je daar de massa uit? en de baan?]

Keplerian orbit fit of HR 8799 e. Figure adopted from Lacour et al. (2019) [kun je hier kort uitleggen wat je ziet? ik zie de 'offset' van een ster? en dat komt doordat er een planeet omheen draait?]

Exoplanet astrometry and photometry

Photometry and astrometry of exoplanets is challenging, because the direct reference for both measurements, the host star, is occulted by the coronagraph. This makes it impossible to use the stellar light to correct the image for seeing and transmission changes in the Earth’s atmosphere.

To overcome this problem, diffractive methods have been developed to generate artificial speckles [on the sensor? pupil plane? en hoe doe je dat: met het liquid crystal?] that serve as photometric and astrometric references. These methods apply static phase or amplitude modulations in the pupil plane before the coronagraph’s focal-plane optic. [deze zin vind ik lastig… dit is dus de correctie die nodig is?] The position of the artificial speckles is such that they are not occulted by the coronagraph.

The speckle grid implemented by GPI. Image adopted from Wang et al. (2014)

Current high contrast imaging instruments implement these artificial speckles either with a square grid that acts as an amplitude grating, or with a static DM modulation [?]. However, the limiting factor of these solutions is their coherency with the time-varying speckle background, which results in interference that dynamically distorts the shape and brightness of the artificial speckles. This in turn ultimately limits their astrometric and photometric precision. [dit is een lastige passage, maar de speckles worden dus ook weer beïnvloed door de variërende achtergrond? omdat je ze als het ware optelt? kun je dat nog verhelderen?]

At LEOPARD we have developed the Vector Speckle Grid (VSG), a novel optical element to generate artificial speckles that serve as photometric and astrometric references when studying directly imaged exoplanets.

The VSG imposes opposite amplitude or phase modulation on the opposite polarization states in the pupil plane. [is deze zin nog te verhelderen, dit is lastig… wat is een ‘opposite amplitude’? ten opzichte van de achtergrond/halo? en dat kan de sensor dus uit elkaar houden?] This generates artificial speckles that are incoherent with the underlying speckle halo. This greatly increases the astro- and photometric precision. VSGs can be implemented using liquid-crystal technology to impose the geometric phase on the circular polarization states. 

Comparison of artificial speckles generated by regular speckle grids or the VSG. The speckles generated by the VSG show significantly less distortion. [kun je hier nog expliciet aangeven wat er beter is. die onderste twee afbeeldingen zijn minder vervormd?]
Comparison of artificial speckles generated by regular speckle grids or the VSG. The speckles generated by the VSG show significantly less distortion. [kun je hier nog expliciet aangeven wat er beter is. die onderste twee afbeeldingen zijn minder vervormd?]
Lab data of the VSG implemented with liquid-crystal technology.
Lab data of the VSG implemented with liquid-crystal technology.

Related publications

S.P. Bos, A & A 638, A118 (2020) Vector speckle grid: instantaneous incoherent speckle grid for high-precision astrometry and photometry in high-contrast imaging