Biomedical Engineering Reference
In-Depth Information
r
0
Piecewise
linear fit
Phase
Φ
Primary
mirror
D
>>
r
0
FIGuRE 8.11
Degrees.of.freedom.for.a.deformable.mirror.used.in.an.astronomical.adaptive.optics.system..he.
piecewise.linear.it.of.the.phase.is.it.for.each.coherence.length.
r
0
..(Credit:.Claire.Max,.Astro.289C,.UCSC.)
Continuous
Piston
Tip-tilt
FIGuRE 8.12
Diferent.types.of.deformable.mirrors..
Top
:.Continuous.facesheet.mirror..
Middle
:.Segmented.mirror.
with.piston-only.mode..
Bottom
:.Segmented.mirror.with.piston.and.tip-tilt..(Credit:.Don.Gavel.)
where.
d
.is.the.subaperture.size.for.the.mirror,.
r
0
.is.the.turbulence.coherence.length,.and.
a
F
.is.the.it-
ting.error.coeicient,.which.depends.on.the.type.of.DM.that.is.being.used..A.physical.interpretation.of.
the.itting.error.is.that.the.mirror.functions.as.a.high-pass.ilter,.correcting.the.low.spatial.frequency.
components.of.the.wavefront.aberration.and.passing.the.high-frequency.components..he.spatial.band-
width.of.the.ilter.is.1/
r
0
.
Some. of. the. diferent. types. of. DMs. include. segmented. mirrors,. where. each. segment. can. be. actu-
ated.independently,.and.continuous.facesheet.mirrors,.where.a.thin.continuous.mirror.is.bonded.to.an.
array.of.actuators.(Figure.8.12)..For.the.segmented.mirrors,.the.segments.can.be.actuated.in.piston-only.
mode,.where.each.mirror.segment.can.be.displaced.in.the.direction.normal.to.the.segment.only.(i.e.,.
up.and.down.like.a.piston),.or.with.the.addition.of.tilting.the.mirror.in.addition.to.the.piston.motion.
he.segments.can.also.be.of.various. shapes.(e.g.,.square,.circular)..he.itting.coeicients.for.these.
various.conigurations.are.given.in.
Table.8.1
.(Hardy.1998).
In.addition.to.diferent.itting.coeicients,.segmented.mirror.can.lose.optical.intensity.through.the.
gaps.between.segments.if.they.are.large.and.reduction.of.the.Strehl.ratio.from.difraction.by.the.edges.
of.the.segments..he.number.of.actuators.
N
.that.are.required.to.make.the.same.degree.of.correction.will.
also.vary.depending.on.the.coniguration:
6 5
2
=
=
a
a
N
N
d
d
.
1
2
F
1
2
1
F
2
So.that.a.piston-only.DM.with.square.segments.will.require.(1.26/0.28)
6/5
.=.6.2.more.actuators.than.
a.continuous.facesheet.mirror.to.achieve.the.same.amount.of.correction..A.piston.plus.tilt.mirror.with.
