Lecture 6

Question 1

Actuators

Answer 1

are transducers which convert an electric signal into a physical property

Question 2

Active Optics counteract

Answer 2

effect of gravity and other deformations of the telescope mirrors

Question 3

Adaptive optics correct for

Answer 3

the effects of the Earth’s atmosphere

Question 4

Active Optics actuate on

Answer 4

large primary mirror

Question 5

Adaptive Optics actuate on

Answer 5

smaller mirror: secondary mirror

Question 6

Active optics operate on

Answer 6

tens of seconds

Question 7

Adaptive optics operate on

Answer 7

hundredths of a second

Question 8

Wavefront sensing

Answer 8

light from reference star picked off by dichroic beamsplitter

focussed on wavefront sensor

Question 9

wavefront sensing measures

Answer 9

direction of propagation of the wavefront rather than its phase

gradient or curvature within array of zones covering the telescope aperture

Question 10

Shack-Hartmann Wavefront Sensing (WFS)

Answer 10

sub-divide the wavefront and measure the tilt of each section through the displacement of an array of spots on a CCD

Question 11

The overall centroid of the image is

Answer 11

estimated from the average of the displacement within each sub-aperture

Question 12

error in determining the angle corresponding to the centre of the image

Answer 12

σ(a) = α(SA) / √n

Question 13

phase error due to photon counting

Answer 13

<ε^2(ph)> = 4π^2/n ((α(SA)d)/λ)^2

Question 14

The Strehl ratio is often used to quantify

Answer 14

the performance of an AO system

Question 15

The Marechal Approximation

Answer 15

S ≈ e^-(<ε>^2)</ε>

Question 16

total residual phase variance is

Answer 16

equal to the sum of variance from all sources

<ε^2(tot)> = <ε^2(1)> + <ε^2(2)> + …

Question 17

α(SA) =

Answer 17

λ/r0 = λ/2d

Question 18

Shack-Hartmann WFS senses

Answer 18

slope

applies corrections to segmented mirror that produces variable tilt in response to a control signal

Question 19

Curvature WFS senses

Answer 19

curvature

applies corrections to bimorph mirror that produces variable curvature in response to a control signal

Question 20

local curvature within sub-aperture of area a0

Answer 20

C(W) = 1/r(W)

r(W) = local radius of curvature

Question 21

Common adaptive optics

Answer 21

Segmented Mirror
Deformable Mirror
Bimorph Mirror

Question 22

Piezoelectric actuators are made from

Answer 22

a ceramic (PZT) that changes length upon the application of an electric field

Question 23

Bimorph mirror

Answer 23

Two thin plates of a Piezoelectric material bonded over a configuration of electrodes

Question 24

The primary source of error is due to

Answer 24

Photon Counting

Question 25

The Marechal Approximation

Answer 25

allows us to express the Strehl ratio in terms of phase variance

Question 26

Wavefront correction is carried out by

Answer 26

segmented or deformable mirrors