Lecture 6: Wavefront Sensing & Correction

Question 1

active optics compensates for other long-term dirfts in the telescope such as

Answer 1

mechanical relaxation of flexures

thermal expansion of truss

Question 2

active optics counteracts effect of

Answer 2

gravity and other deformations of the telescope mirros

Question 3

active optics acute on

Answer 3

large primary mirror (plus tip-tilit of secondary mirror)

Question 4

active optics operate on timescales of

Answer 4

tens of seconds

rate about 0.05Hz

Question 5

common adaptive optics

Answer 5

segemented mirror
deformable mirror
bimorph mirror

Question 6

piezoelectic / electrostrictive actuators

Answer 6

displacement/curvature proportional to applied voltage

Question 7

bimorph mirror

Answer 7

two thin plates of piezoelectric material bonded over a configuration of electrodes

applied voltage causes mirror to bend in response

Question 8

wavefront sensing

Answer 8

light from reference star picked off by dichroic beamsplitter

focused on wavefront sensor

info used to adjsut shape of adaptive optic

independent of wavelength

Question 9

most common wavefront sensing concepts

Answer 9

Shack-Hartmann
Curvature

Question 10

Shack-Hartmann WFS senses

Answer 10

SLOPE

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

Question 11

Shack-Hartmann WFS best for

Answer 11

large number of sub-apertures
N>50

Question 12

curvature WFS senses

Answer 12

CURVATURE

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

simple scheme

Question 13

curvature wfs is competitive with S-H for

Answer 13

a relatively small number of sub-apertures

N< or =50

Question 14

S-H WFS - wavefront divided into

Answer 14

portions
one to each sub-aperture

Question 15

S-H WFS - light from each sub-aperture focussed onto

Answer 15

CCD camera/detector array

Question 16

S-H WFS - PLANE WAVEFRONT

Answer 16

regular series of spots in the focal plane

Question 17

S-H WFS - corrugated wavefront

Answer 17

irregularly spaced spots

Question 18

S-H WFS - slope of each portion of the wavefront determined from

Answer 18

the displacement of the spot from the central position

measurements used to reconstruct wavefront

Question 19

S-H WFS - distorted wavefront has

Answer 19

local gradient, a, at a particular lenslet

Question 20

tilt of the wavefront is measured in two directions:

Answer 20

ax and ay

Question 21

with sub-aperture size, d, angular size of image (FWHM) will be

Answer 21

for d<ro - aSA=lambda/d

for d>ro - aSA = lambd/r0

Question 22

the overall centroid of the image is estimated from

Answer 22

the average of the displacement within each sub-aperture

this info is then used for tip-tilit correction

Question 23

photon counting noise leads to

Answer 23

an uncertainty in the measurement of the centre of the spot

Question 24

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

Answer 24

sigma a / aSA = 1/root n

Question 25

When reconstructing the wavefront over all N sub-apertures, the total photon counting error scales with

Answer 25

the log of the number of sub-apertures prop to log(N)

Question 26

the strehl ratio is often used to

Answer 26

quantify the performance of an adaptive optics system

Question 27

in corrected images, where the phase-error is relatively small, what can be used?

Answer 27

the Marechal approximation s = e^-

Question 28

materchal approximation is valid for

Answer 28

phase errors < or = 4 rad^2

Question 29

marechal approximation allows one to

Answer 29

predict the strehl ratio from known errors in wavefront sensing scheme

Question 30

total residual phase variance is equal to

Answer 30

the sum of variance from all sources

Question 31

curvature WFS - measure intensity at

Answer 31

planes P1 and P2

Question 32

curvature WFS - plane wavefront comes to a focus at

Answer 32

Z

Question 33

curvature WFS - local curvature within sub-aperture of area a0

Answer 33

C_W=1/r_W r_W = local radius of curvature

Question 34

curvature WFS - curved wavefront comes to focus at

Answer 34

Zc = Zr_W / Z +r_W

Question 35

curvature WFS - focal shift

Answer 35

deltaz = Z-Z_C = Z^2 / Z+r_w

Question 36

curvature WFS - local wavefront curvatures very small compared to

Answer 36

those produced by the telescope objective thus rw>>Z

Question 37

curvature WFS - signal delta I defined as

Answer 37

normalised difference in intensity at planes P1 and P2

Question 38

curvature WFS - errors in reconstructing the wavefront over all N sub-apertures scales as

Answer 38

N^2 (compared to log N for S-H)

Question 39

curvature WFS VS S-H WFS for wavefront reconstruction error

Answer 39

wavefront reconstruction error much greater for curvature sensing than for S-H