Biomedical Engineering Reference
In-Depth Information
.
8.. Because.typical.pinhole.sizes.range.from.several.micrometers.to.100.μm.in.a.PMT,.the.alignment.
of.the.pinhole.is.very.critical..For.a.new.system,.it.is.recommended.to.start.with.a.pinhole.size.
many.times.larger.than.what.is.required.by.the.best.image.quality.and.then.try.several.smaller.
pinhole.sizes.to.reduce.the.pinhole.size.to.what.is.needed..If.the.optical.system.has.residual.aber-
rations.ater.being.compensated.by.the.DM,.the.difraction-limited.pinhole.size.may.not.be.the.
best.choice.as.it.may.cut.of.too.much.of.the.signal.and.reduce.the.intensity.too.much..When.a.
diferent.wavelength.is.used.in.the.system,.the.pinhole.axial.position.should.be.adjusted.and.reop-
timized.for.the.best.images..It.is.a.good.practice.to.check.the.pinhole.alignment.every.day.when.
the.AO.system.is.operated..Finely.adjust.the. x ,. y .positions.until.the.acquired.images.are.brightest.
and.then.perform.the.measurement.
9.10 Understanding Alignment Penalties
When. discussing. the. misalignment,. it. is. important. to. understand. their. efects. and. penalties. on. per-
formance.. First. of. all,. if. the. optical. system. is. perfect. without. aberrations,. what. image. quality. can. be.
expected.from.the.system?
If.the.optical.system.is.aberration-free,.the.resolution.of.the.image.is.limited.by.difraction..he.point.
spread.function.(PSF).describes.the.response.of.an.imaging.system.to.a.point.source..For.an.aberration-
free.optical.system,.the.response.for.a.point.source.is.an.airy.disk.pattern.
he.minimum.resolvable.details.are.deined.by.the.Rayleigh.criterion..Two.objects.can.be.resolved.
as.long.as.the.central.peaks.of.the.images.do.not.overlap..he.imaging.process.is.said.to.be.difraction-
limited.when.the.central.peak.of.the.difraction.pattern.of.one.point.source.coincides.with.the.irst.dark.
ring.of.the.difraction.pattern.of.the.second.point.source.
If. there. are. aberrations. in. the. optical. system,. what. criterion. should. be. used. to. evaluate. the. image.
quality?
he. Strehl. ratio. is. deined. as. the. ratio. of. the. central. peak. intensities. of. the. aberrated. PSF. and. the.
difraction-limited.PSF.
(
)
I
0,0 aberrated
(0,0)
.
S
=
.
(9.9)
I
For.small.aberrations,.the.Strehl.ratio.can.be.approximated.by.the.wavefront.error:
2
π
λ σ .
S = −
2
.
(9.10)
1
where.σ.is.the.wavefront.error.
he.system.is.close.to.difraction.limited.when.the.Strehl.ratio.is.greater.than.80%..his.is.the.Maréchal.
criterion..he.corresponding.rms.wavefront.error.must.be.better.than.1/14.of.a.wavelength..his.means.
that.the.total.system.aberrations.must.be.less.than.1/14.of.wavelength.ater.compensation.by.the.DM.
In. an. AO. system,. the. total. wavefront. error. comes. from. several. sources,. including. common. path.
errors,.non-common.path.errors,.and.phase.registration.errors.
Common.path.wavefront.errors.refer.to.aberrations.that.are.common.to.the.optical.paths.of.both.the.
science.camera.and.the.WFS..Since.the.WFS.can.measure.the.common.path.aberrations,.DMs.can.be.
used.to.compensate.the.aberrations.in.both.the.science.and.WFS.optical.paths..his.is.acceptable.as.long.
as.correcting.the.optical.system.aberrations.does.not.take.away.too.much.phase.compensation.capability.
of.the.DM.for.compensating.speciic.sample.aberrations..Commercially.available.DMs.typically.have.
modulation.capability.from.several.micrometers.up.to.100.μm,.so.using.half.a.micrometer.to.compen-
sate.for.common.path.errors.is.acceptable..However,.these.errors.should.be.minimized.
 
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