Biomedical Engineering Reference
In-Depth Information
F
Field lens
Tip-tilt correction
C
Science
camera
Electron
gun
S
Dichroic beam splitter
P
Rotating
knife edge
K
Electron beam
steering
Image orthicon
wavefront sensor
Eidophor
Feedback
FIGuRE 8.9
Schematic. diagram. of.“seeing. compensator”. using. an.Eidophor. spatial. light.modulator.. he.light.
from.the.telescope's.objective.is.brought.to.a.focus.at.F..A.ield.lens.images.the.light.onto.the.Eidophor,.the.surface.
of.which.is.controlled.by.a.patterned.layer.of.charge.from.an.electron.gun.that.is.raster-scanned.by.an.electron.beam.
steering.system.under.feedback.control.from.a.wavefront.sensor..he.light.is.then.split.by.a.dichroic.beam.splitter P,.
with.some.of.the.light.going.to.a.science.camera.S.and.some.of.the.light.going.to.the.wavefront.sensor.formed.by.a.
rotating.knife.edge.K.at.the.focal.point.of.the.light.and.the.image.orthicon..his.article.originally.appeared.in.the.
Publications.of.the.Astronomical.Society.of.the.Paciic..(Reproduced.with.permission.from.Babcock,.H..W.,.
Publ
Astron Soc Pac.
,.65,.386,.229-236,.1953..Copyright.2004.Astronomical.Society.of.the.Paciic.)
parabolic.mirror..he.schlieren.image.formed.on.the.image.orthicon.wavefront.sensor.is.used.to.modu-
late.the.intensity.of.an.electron.beam.that.is.raster-scanned.across.the.surface.of.the.Eidophor,.creating.
a.charge.pattern.that.deforms.the.surface.of.the.oil,.forming.a.spatial.light.modulator.that.corrects.the.
wavefront.aberrations..A.tip-tilt.corrector.under.feedback.control.from.the.image.sensor.keeps.the.focal.
point.centered.on.the.rotating.knife.edge.
Babcock's.Eidophor-based.seeing.compensator.idea.is.not.too.diferent.from.how.modern.AO.systems.
work,.the.main.diference.being.of.course.the.much.better.technology.now.used.for.light.modulation.
and.wavefront.sensing..A.modern.AO.system.for.astronomy.is.shown.in.
Figure.8.10
.
.Here.the.light.from.
a.telescope.with.a.distorted.wavefront.is.relected.of.the.surface.of.an.adaptive.mirror..he.shape.of.the.
mirror.is.the.opposite.of.the.distorted.wavefront,.as.measured.by.the.wavefront.sensor..he.wavefront.
sensor.and.adaptive.mirror.form.a.closed-loop.feedback.control.system..Ater.relecting.of.the.mirror,.
the.corrected.wavefront.is.recorded.by.a.high-resolution.CCD.camera.
8.4 Speciications for Deformable Mirrors
he.speciications.for.the.DMs.that.are.used.in.astronomical.imaging.are.determined.by.the.wavefront.
aberrations. that. are. used. to. correct.. Since. the. wavefront. aberrations. in. astronomy. are. better. known.
than.the.wavefront.aberrations.in.biological.imaging,.which.can.vary.widely.between.diferent.biologi-
cal.samples,.it.is.useful.to.examine.how.they.can.be.corrected.irst.
he. optical. efects. of. the. earth's. atmosphere. have. been. well. characterized.. he. turbulence. in. the.
atmosphere.is.usually.described.by.the.Kolmogorov.model.for.the.velocity.of.motion.in.a.luid.medium.
