Biomedical Engineering Reference
In-Depth Information
FIGuRE 8.6
Refraction.of.light.at.an.air-water.interface..Bending.of.light.rays.due.to.refraction.makes.the.straw.
appear.to.be.displaced.at.the.air-water.interface.
Star
Galaxy
Star
Turbulence
Turbulence
Deformable
mirror
Telescope
Telescope
Detector
Fuzzy blob
(a)
(b)
FIGuRE 8.7
(a).Optical.aberrations.in.astronomy.induced.by.light.from.a.star.passing.through.turbulence.in.the.
earth's.atmosphere..he.refraction.of.the.light.through.the.time.varying.index.of.refraction.causes.the.focus.of.the.
light.from.the.telescope.to.move.over.time,.causing.the.image,.which.is.collected.over.time.in.a.CCD..camera,.to.
appear.as.a.fuzzy.blob..(b).he.solution.proposed.by.Horace.Babcock.was.to.relect.the.starlight.of.of.an.Eidophore,.
an.early.form.of.a.deformable.mirror,.and.to.use.the.light.from.a.single.star.as.a.reference.beacon.or.“guide.star.”.
he.mirror.is.deformed.to.bring.the.light.from.the.star.back.into.focus.as.a.single.point.of.light..A.nearby.galaxy,.
which.passes.through.the.same.portion.of.the.atmosphere.and.thus.has.the.same.aberration,.is.relected.of.of.the.
deformable.mirror,.and.the.image.is.corrected..(Credit:.Lawrence.Livermore.National.Laboratory.and.NSF.Center.
for.Adaptive.Optics.)
turbulent.gases..Winds.alot.move.these.density.patterns.at.high.speeds,.depending.on.altitude,.which.
cause.additional.rapid.variations.in.light.paths..In.addition.to.shiting.the.image.of.the.star,.the.density.
variation.changes.cause.the.intensity.of.the.starlight.to.vary.as.rays.cross.and.interfere.constructively.
and.destructively,.making.the.star.appear.to.twinkle.or.scintillate..he.net.efect.of.the.refractive.ray.
bending.and.the.scintillation.is.to.change.the.sharp.point.of.light.from.a.star.into.a.fuzzy.blob,.as.shown.
in.Figure.8.7a.
