Lecture 7: Adaptive Optics Errors & Guide Stars

Question 1

total residual phase variance is equal to

Answer 1

the sum of variance from all sources

Question 2

wavefront reconstruction

Answer 2

individual slopes fitted together

Question 3

wavefront reconstruction limited by

Answer 3

finite spacing between the actuators of the mirror

Question 4

fitting error

Answer 4

small details of wavefront remain uncompensated

Question 5

number of sub-apertures for a mirror diameter D

Answer 5

N=aperture area/grid spacing area

Question 6

control or ‘servo’ system

Answer 6

adjusts shape of deformable mirror to match shape of wavefront

hundreds of adjustments made per second

Question 7

the finite bandwidth of the control system (or servo) leads to

Answer 7

a residual phase error that limits the Strehl ratio

Question 8

servo with bandwdth f3dB has associated time constant

Answer 8

t3dB = 1/f3dB

ie how fast corrections can be made

Question 9

wavefront sensor needs bright point source to

Answer 9

provide sufficient signal to noise in a short exposure time

Question 10

typically light from science target cannot be used as too faint, instead

Answer 10

use light from a reference source within the field of view

Question 11

a suitable reference source should be

Answer 11

1. point like - with small angular radius
2. close proximity to science object - within the isoplanatic patch
3. bright - need photons!

Question 12

natural guide stars

Answer 12

bright stars

Question 13

laser guide star

Answer 13

laser used to generate an artificial star

Question 14

isoplanatic patch

Answer 14

area of sky over which a natural guide star is effective

angular radius of theta0, centered on target star

Question 15

a sufficiently bright natural guide star is not always

Answer 15

available within the isoplanatic patch

artificial may be needed

Question 16

theta0 prop to r0 prop to lambda^6/5 so

Answer 16

theta0 is few arc seconds in optical
few tens of arc seconds in IR

greater availability of natural guide stars when observing in the infrared

Question 17

angular anisoplanatism

Answer 17

for an angle between guide star and target, the residual wavefront error is

<Eiso^2> = (theta / theta0)^5/3

Question 18

errors are smaller the closer

Answer 18

the guide star is to the target

also less when observing in the IR

Question 19

laser guide star - artificial star created

Answer 19

above turbulence

placed within isoplanatic patch

Question 20

laser light is

Answer 20

monochromatic

Question 21

dichroic beamsplitter used to

Answer 21

direct laser light onto wavefront sensor

remaining light passes through beamsplitter to science detector

Question 22

Rayleigh beacon

Answer 22

focused altitude between 10 and 20km

rayleigh back scattering of light from molecules in lower atmosphere

produces artificial star at low altitude

Question 23

rayleigh beacon - beacon observed off axis appearing as

Answer 23

an elongated ‘pencil’ of light

Question 24

rayleigh beacon - use short wavelengths since

Answer 24

rayleigh scattering is proportional to 1/lambda^4

Question 25

sodium beacon

Answer 25

laser runed to sodium-D lines excites sodium atoms in the mesophere sodium atoms deposited by micrometeorites sodium re-emit the light producing artificial star

Question 26

resonant backscatter from sodium atoms at

Answer 26

90km

Question 27

brightness prop to

Answer 27

laser power

Question 28

use pulsed laser

Answer 28

only detect light which has travelled up to sodium layer and back again reject light arriving at shorter timescales

Question 29

cone effect

Answer 29

outer portions of wavefronts not sensed only sense wavefronts within 'cone' residual error 'cone effect'

Question 30

scheme has two actuators

Answer 30

a tip tilt and a wavefront corrector

Question 31

atmosphere effectively has

Answer 31

a wedge component light from star deflected size and orientation of wedge changes over time

Question 32

laser beam deflected as it

Answer 32

passes through the atmosphere on the way up scattered light deflected as it passes back down through the atmosphere

Question 33

delfections

Answer 33

equal and opposite cancel each other out

Question 34

laser guide star does not appear to wander and hence

Answer 34

cannot be used to correct for the effects of tip-tilit anisoplanatism

Question 35

light from natural guide star passes through atmosphere only once so

Answer 35

does appear to wander hence can be used to correct for effects of tip-tilit anisoplanatism

Question 36

centroid determined from

Answer 36

average of displacement within each sub-aperture

Question 37

natural guide star can be

Answer 37

several orders of magnitude dimmer further form the target star

Question 38

finite actuator spacing leads to a

Answer 38

residual fitting error

Question 39

finite servo bandwidth leads to

Answer 39

a residual bandwidth error

Question 40

a laser guide star only samples part of the turbulence leading to

Answer 40

the cone effect

Question 41

tip-tilt anisoplanatism is the

Answer 41

wandering of the centroid of the image

Question 42

natural guide stars have limited coverage but are essential for

Answer 42

the correction of tip-tilit anisoplanatism