Biomedical Engineering Reference
In-Depth Information
Relecting.telescopes.have.a.number.of.advantages.over.transmissive.telescopes..hey.are.not.subject.
to.chromatic.aberrations.because.the.relected.light.does.not.disperse.according.to.wavelength..hey.do.
not.create.ghost.images.due.to.surface.relection.of.transmissive.lenses,.and.because.of.this,.the.back-
ground.noise.is.typically.less.
In.a.transmissive.relay.telescope,.the.entrance.pupil.plane.and.the.exit.pupil.plane.are.generally.nor-
mal.to.the.light.beam,.and.thus.they.are.exactly.conjugate.to.each.other..However,.in.an.of-axis.relective.
relay.telescope,.the.mirrors.are.tilted.at.non-normal.incident.angles.to.the.incoming.beam,.thus.making.
the.entrance.pupil.plane.and.the.exit.pupil.plane.only.“quasi”.conjugate.to.each.other..his.may.result.
in.some.errors.in.terms.of.phase.corrections..If.the.ield.of.view.is.small,.the.error.is.oten.negligible.
9.3 optical Aberrations
Optical.systems.with.smaller.focal-length.optics.are.more.compact.than.those.with.longer.focal-length.optics..
However,.the.optical.system.aberrations.are.typically.higher.in.a.compact.optical.system..he.most.common.
aberrations.for.any.optical.systems.are.spherical.aberration,.coma,.astigmatism,.ield.curvature,.distortion,.
and.chromatic.aberration..hese.aberrations.tend.to.degrade.image.quality.as.shown.in.Figure.9.4.
Spherical.aberration.is.quite.oten.the.dominant.error..he.optical.rays.at.the.edge.of.the.lens.do.not.
focus.to.the.same.spot.as.the.rays.at.the.center.of.the.lens.do..As.a.result,.the.image.is.blurred.across.the.
image.plane..he.image.is.uniformly.degraded.across.the.whole.image.by.spherical.aberrations.as.shown.
in.Figure.9.4b..As.the.focus.shits,.the.image.plane.shits.with.it..here.is.a.position.for.which.the.circular.
image.is.at.the.minimum..his.minimum.image.is.called.the.circle.of.least.confusion..he.detector.should.
(b)
(c)
or
(a)
(d)
(g)
or
(e)
or
(f )
FIGuRE 9.4
Illustration.of.images.with.aberrations..(a).Object..(b).Image.with.spherical.aberration..(c).Image.
with.coma..(d).Image.with.astigmatism..(e).Image.with.ield.curvature..(f).Image.with.distortion..(g).Image.with.
chromatic.aberration.
