Qualità d'immagine del VLT

Il requisito fondamentale per VLT è la straordinaria qualità dell'immagine. Il VLT dovrebbe sfruttare appieno le condizioni di "seeing" eccezionalmente buone del sito del Paranal, cioè i momenti con un'atmosfera particolarmente stabile sopra il sito, con un minimo di turbolenza dell'aria.

In questo diagramma, la qualità dell'immagine misurata delle immagini astronomiche dell'UT1 del VLT è graficata a confronto del "seeing", così come viene misurato dal Seeing Monitor, un piccolo telescopio situato anch'esso in cima alla montagna del Paranal.

La linea tratteggiata mostra i requisiti di qualità dell'immagine, come specificato per la prima luce del VLT. La linea punteggiata mostra le specifiche per la qualità dell'immagine, tre anni dopo la prima luce, quando il VLT sarà completamente ottimizzato. La linea continua rappresenta il limite fisico, quando il telescopio non aggiunge ulteriore distorsione dell'immagine rispetto a quella introdotta dall'atmosfera.

Il diagramma dimostra che le specifiche della prima luce sono state pienamente raggiunte e, in modo impressionante, che le prestazioni effettive  del VLT a volte rientrano già nelle specifiche più rigorose che si prevede saranno soddisfatte tra tre anni.

Vari effetti contribuiscono a degradare la qualità dell'immagine di un telescopio rispetto al seeing locale ed essi devono essere ridotti al minimo per ottenere i migliori risultati scientifici. Questi includono imperfezioni negli specchi ottici e nel movimento del telescopio per compensare la rotazione terrestre durante un'esposizione, nonché la turbolenza dell'aria generata dal telescopio stesso. Le specifiche rigorose mostrate in questa figura si traducono in requisiti molto rigorosi riguardanti la qualità di tutte le superfici ottiche, il controllo attivo dello specchio da 8,2 m, l'accuratezza dei movimenti del telescopio e, nel prossimo futuro, le rapide compensazioni "tip-tilt" fornito dallo specchio secondario, e infine dal controllo termico del telescopio e dell'intero rivestimento.

L'unico modo per ottenere una qualità dell'immagine "migliore di quella dell'atmosfera" è l'uso di dispositivi di ottica adattiva che compensano le distorsioni atmosferiche. Uno di questi dispositivi sarà operativo sul VLT entro il 2000, consentendo quindi agli astronomi di ottenere immagini di una nitidezza di 0,1 arcsec.

In questo diagramma, sia il seeing che la qualità dell'immagine del telescopio vengono misurati tramite la larghezza massima a metà altezza (FWHM) del profilo di intensità della luce di una sorgente puntiforme. L'incertezza delle misure è indicata dalla croce nell'angolo in basso a destra.

Crediti:

ESO

A proposito dell'immagine

Identificazione:eso9820i
Lingua:it
Tipo:Grafico
Data di pubblicazione:27 Maggio 1998
Notizie relative:eso9820
Dimensione:1298 x 1651 px

A proposito delll'oggetto

Nome:Very Large Telescope
Tipo:Unspecified : Technology : Observatory : Telescope

Formati delle immagini

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