Aktiv Optik

En opfindelse, som rykker

Størrelsen betyder noget, og det gør faconen også. Begge dele er vigtige, ihvertfald når der er tale om konstruktion af kraftigere teleskoper. Større hovedspejle gør det muligt for astronomerne at indfange mere lys, og spejloverflader, som er perfekt slebne er nødvendige for at undgå billedfejl. Jo bedre man kombinerer de to ting, des svagere objekter kan man observere. Desværre er det ikke en nem opgave, og med stadigt større teleskopspejle bliver det at holde dem i den rigtige facon endnu sværere.

Det var i tresserne og i halvfjerdserne, at denne udfordring blev særligt vigtig. Datidens teknologi begrænsede spejldiametrene til omkring 5 meter. Over denne størrelse blev billedkvaliteten stærkt forringet fordi tyngdekraften trak spejlene ud af facon. Med den teknologi, som den gang var til rådighed ville det at bygge spejle over 5 meter have krævet enorme støttestrukturer til at understøtte spejlene, og dermed ville byggeomkostningerne stige, så resultatet ville være en meget tung konstruktion, hvor man alligevel ikke havde mulighed for et bedre teleskop. Der måtte gås nye veje for at sikre optisk kvalitet.

ESOs ingeniør Raymond Wilson kom på det rette tidspunkt med en brilliant og "enkel" ide; nemlig aktiv optik. Et tyndt og bøjeligt hovedspejl kunne kontrolleres af et aktivt system af understøtninger, som skulle trække og skubbe spejlet i facon som modtræk mod de deformationer, som kommer af tyngdekraftens uens træk i spejlet, når teleskopet drejes og vippes (der er mere at læse i den gratis bog "Jewel on the Mountaintop" af Claus Madsen).

Da ESO’s 3.6-metre telescope blev indviet i 1976 var aktiv optik kun en ide i Wilsons hoved. Derfor er det hovedspejl en halv meter tykt, og det har den utrolige vægt af 11 tons.

Den nye ide blev afprøvet i ESOs hovedkvarter med et tyndt 1 meter spejl med aktiv støtte fra 75 "actuators". Disse er motorer, som bevæger sig meget præcist, og som kan kontrolleres præcist. Ved at skubbe på spejlet regulerer de spejlets facon og udligner på den måde de deformationer, som tyngdekraften bevirker.

Under teleskopets bevægelser kan dette aktive system bibeholde spejlets rette facon. De nødvendige korrektioner beregnes løbende med en computer, som er koblet til en billedanalysator. Her opdages selv de mindste afvigelser fra den ideelle spejloverflade. Internt i ESO blev ideen videreudviklet, og efter denne succesfulde afprøvning, blev ideen omsat i stor skala med New Technology Telescope (NTT) Takket være den aktive optik er vægten af NTT hovedspejlet, med en diameter på 3,58 meter og en tykkelse på kun 24 centimeter reduceret til 6 tons.

1-metre telescope supports for the active optics experiment at the ESO headquarters in Garching in 1987

Understøtninger til 1 meter teleskop fra eksperimentet med aktiv optik i ESOs hovedkvarter i Garching i 1987. Kilde: ESO

The New Technology Telescope (NTT) pioneered the Active Optics.

New Technology Telescope (NTT) var pioneren indenfor aktiv optik.

Kilde: ESO/C.Madsen. Bacon

Efter at NTT blev taget i brug i 1990, er aktiv optik blevet brugt i alle større teleskoper; også i ESOs Very Large Telescope (VLT) Wilson har modtaget adskillige priser for sin opfindelse, som har vist sig virkelig at revolutionere astronomien.

Hvert af de fire VLT Unit Telescopes (UT) er udstyret med det bedste aktive optiksystem, som til dato er fremstillet. Systemet kontrollerer både de primære 8,2 m spejle af Zerodur og de sekundære 1,1 m letvægts berylliumspejle i toppen af teleskoperne. Ved sammenligning af signaler fra begge spejle justeres spejlenes facon automatisk med korte intervaller.

Takket være denne teknologi, vejer hvert af de fire UT primærspejle 22 tons, de måler 82 meter i diameter, og de er kun 17 centimeter tykke; samme facon som en enorm pandekage! Hvert spejl hviler på 150 computerkontrollerede understøtninger (aktuatorerne), som så er monteret i en ekstremt stiv celle, med en vægt på omkring 11 tons. Den aktive optik i VLT teleskoperne sikrer, at disse store spejle altid har den bedst mulige facon, og at der dermed altid leverer de bedst mulige billeder fra Universet.

VLT Active Optics System

VLTs Aktive Optik System.

Kilde: ESO

The prestigious Kavli Prize in Astrophysics for 2010 was awarded to Jerry Nelson, Raymond Wilson and Roger Angel for their contribution to the development of giant telescopes.

Den eftertragtede Kavli Prize i Astrofysik for 2010 blev givet til Jerry Nelson, Raymond Wilson og Roger Angel for deres bidrag til udviklingen af kæmpeteleskoper. Kilde: ESO. Bacon

I fremtiden kommer så den næste store udfordring for den aktive optik, med det 39 meter primærspejl, som er planlagt til det nye European Extremely Large Telescope (ELT). Spejlet til ELT kommer til at bestå af 798 individuelle delspejle. Hvert af delspejlene kan bevæges af vippe- og drejestempel, så spejlmosaikken kan fungere som eet kæmpe spejl uanset virkningerne fra temperaturforskelle og tyngdekraft.

Se denne ESOcast om aktiv optik med videre her: Seeing Sharp — Special 50th anniversary episode #3.

Videnskabelige mærkepæle

Aktiv optik installeres på flere af ESOs teleskoper har bidraget til bemærkelsesværdige opdagelser:

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