Biomedical Engineering Reference
In-Depth Information
8.4.2 Deformable Mirror requirements
To.specify.a.DM.for.a.particular.application,.a.number.of.requirements.must.be.determined..he.pri-
mary.considerations.for.the.DM.requirements.include.the.following:
•.
he maximum displacement or dynamic range (stroke). .his.maximum.stroke.will.determine.the.
magnitude.of.the.wavefront.error.that.can.be.corrected..he.optical.stroke.for.a.mirror.is.twice.
the.mechanical.stroke..In.astronomy,.the.magnitude.of.wavefront.error.depends.on.the.degree.of.
atmospheric.turbulence.and.increases.as.the.diameter.of.the.telescope.increases,.since.it.is.imag-
ing.a.larger.area.of.the.sky..In.biological.imaging,.the.magnitude.of.wavefront.error.will.depend.on.
the.degree.of.refractive.index.variations.and.the.thickness.of.the.specimen.that.is.being.imaged.
•.
he maximum inter-actuator stroke. .he.inter-actuator.stroke.will.determine.the.maximum.gra-
dient.(slope).in.wavefront.error.that.can.be.corrected..his.will.depend.on.the.coupling.between.
adjacent.actuators..A.segmented.mirror.will.have.no.coupling.between.the.actuators,.whereas.a.
continuous.facesheet.may.have.on.the.order.of.20%.coupling.between.the.actuators.since.they.are.
mechanically.linked.through.the.facesheet.
•.
he number of actuators (order). .he.order.determines.the.number.of.degrees.of.freedom.of.cor-
rection.that.can.be.obtained.for.correction.of.the.wavefront.error.
•.
he spacing between the actuators. .he.pitch,.or.distance.between.actuators,.sets.the.highest.spa-
tial. frequency. that. can. be. corrected. by. the. mirror.. As. mentioned. previously,. the. DM. acts. as. a.
high-pass.ilter,.correcting.the.low-spatial-frequency.components.of.the.wavefront.aberration.and.
passing. the. high-frequency. components.. he. highest. spatial. frequency. that. can. be. corrected. is.
where.every.other.actuator.is.up.and.every.intermediate.actuator.is.down.
•.
Range of Zernike coeicients that can be corrected. . he. Zernike. polynomials. (Zernike. 1934),. as.
a. set. of. functions. that. are. orthogonal. on. a. unit. circle,. can. be. used. to. characterize. the. spatial.
response.of.the.mirror..Higher-order.Zernike.polynomials.correspond.to.higher.spatial.frequen-
cies..hese.polynomials.are.convenient.to.use.since.they.can.correspond.to.common.aberrations.
such.as.astigmatism,.focus,.coma,.spherical,.and.trefoil.aberrations.
•.
he pupil diameter (aperture). .he.pupil.diameter.corresponds.to.the.aperture.of.the.DM..Typically.
the.limiting.aperture.of.the.optical.system.that.is.being.corrected.is.projected.onto.the.DM,.so.the.
ratio.of.the.mirror's.aperture.to.the.limiting.aperture.of.the.optical.system.will.determine.how.
much. magniication. is. required.. Typically. the. edge. actuators. of. a. continuous. facesheet. mirror.
will.behave.diferently.than.the.inner.actuators.since.they.have.diferent.boundary.conditions,.so.
they.might.be.excluded.from.the.pupil.of.the.mirror.
•.
he mirror surface quality (RMS roughness). .High-performance.wavefront.correction.requires.a.
high-quality. mirror. surface.. Some. DMs. that. are. being. used. for. high-contrast. imaging. of. dim.
planets.around.bright.stars.call.for.a.surface.igure.of.a.few.nanometers..Some.surface.topography.
resulting.from.mirror.manufacturing.may.have.a.repeat.distance.of.the.actuator.pitch..Examples.
include. the. etch. release. holes. and. support. posts. for. microelectromechanical. systems. (MEMS).
DMs..Although.this.topography.may.be.sizeable,.since.it.has.a.well.deined.pitch,.it.acts.as.a.dif-
fraction.grating.and.higher-order.modes.can.be.iltered.out.with.an.order.sorting.ilter.
•.
Initial surface bow that must be lattened (lost stroke). .Because.of.manufacturing.defects.such.as.
residual.stress,.stress.gradients,.and.other.thin.ilm.defects,.the.initial.mirror.surface.might.not.be.
lat..Some.of.the.mirror's.stroke.may.have.to.be.used.to.latten.the.mirror..On.MEMS.continuous.
facesheet.mirrors,.this.can.be.as.large.as.0.5.μm,.which.can.be.a.signiicant.fraction.of.the.total.
mirror.stroke..Of.course.surface.topography.at.a.spatial.frequency.that.is.higher.than.the.inter-
actuator. spacing. cannot. be. corrected. by. the. mirror,. since. the. mirror. functions. as. a. high-pass.
ilter,.with.the.corner.frequency.determined.by.the.mirror's.pitch.
•.
Mirror thickness. . he. thickness. of. the. mirror. will. determine. how. stif. it. is.. he. stifness. varies.
as.the.third.power.of.the.thickness..For.a.continuous.facesheet.mirror,.the.stifer.the.mirror,.the.
 
Search WWH ::




Custom Search