Lecture 6 Flashcards
Actuators
are transducers which convert an electric signal into a physical property
Active Optics counteract
effect of gravity and other deformations of the telescope mirrors
Adaptive optics correct for
the effects of the Earth’s atmosphere
Active Optics actuate on
large primary mirror
Adaptive Optics actuate on
smaller mirror: secondary mirror
Active optics operate on
tens of seconds
Adaptive optics operate on
hundredths of a second
Wavefront sensing
light from reference star picked off by dichroic beamsplitter
focussed on wavefront sensor
wavefront sensing measures
direction of propagation of the wavefront rather than its phase
gradient or curvature within array of zones covering the telescope aperture
Shack-Hartmann Wavefront Sensing (WFS)
sub-divide the wavefront and measure the tilt of each section through the displacement of an array of spots on a CCD
The overall centroid of the image is
estimated from the average of the displacement within each sub-aperture
error in determining the angle corresponding to the centre of the image
σ(a) = α(SA) / √n
phase error due to photon counting
<ε^2(ph)> = 4π^2/n ((α(SA)d)/λ)^2
The Strehl ratio is often used to quantify
the performance of an AO system
The Marechal Approximation
S ≈ e^-(<ε>^2)</ε>
total residual phase variance is
equal to the sum of variance from all sources
<ε^2(tot)> = <ε^2(1)> + <ε^2(2)> + …
α(SA) =
λ/r0 = λ/2d
Shack-Hartmann WFS senses
slope
applies corrections to segmented mirror that produces variable tilt in response to a control signal
Curvature WFS senses
curvature
applies corrections to bimorph mirror that produces variable curvature in response to a control signal
local curvature within sub-aperture of area a0
C(W) = 1/r(W)
r(W) = local radius of curvature
Common adaptive optics
Segmented Mirror
Deformable Mirror
Bimorph Mirror
Piezoelectric actuators are made from
a ceramic (PZT) that changes length upon the application of an electric field
Bimorph mirror
Two thin plates of a Piezoelectric material bonded over a configuration of electrodes
The primary source of error is due to
Photon Counting
The Marechal Approximation
allows us to express the Strehl ratio in terms of phase variance
Wavefront correction is carried out by
segmented or deformable mirrors
