Lecture 5: Adaptive Optics

Question 1

turbulent cells evolve on longer timescales than

Answer 1

time it takes the wind to move a cells by its own size

Question 2

turbulent cells move by its own size in time

Answer 2

t0 = t2-t1 = approx ro/V

Question 3

atmospheric time constant or coherence time

Answer 3

t0=r0/V

Question 4

good observing site, typical night

Answer 4

V=10m/s
r0=10cm (visible spectrum)
t0=10ms

Question 5

exposure time short

Answer 5

t<t0

Question 6

exposure time long

Answer 6

t>t0

Question 7

adaptive optics must operate on timescales

Answer 7

shorter than t0

Question 8

isoplanatic angle

Answer 8

approx r0/h

Question 9

isoplanatic angle is the angle over which

Answer 9

turbulence pattern shifted by distance r0

angular separation at which light from two stars becomes uncorrelated

Question 10

isoplanatic angle, good observing site, typical night

Answer 10

h=10km
ro=10cm
theta 0 2”arc

Question 11

theta0 determines

Answer 11

area of sky over which adaptive optics are effective

Question 12

original wavefront restored by

Answer 12

inserting equal and opposite path length corrections producing corrected image

Question 13

wavefront compensation - correction carried out in

Answer 13

reflection by deformable mirror

adjusted in shape to match wavefront

Question 14

adaptive optic scheme

Answer 14

1. plane wavefront from star corrugated by turbulence in atmos
2. diverging beam beyond telescope made parallel by collimator
3. collimated beam reflects off deformable mirror
4. unwanted light picked off by dichroic beamsplitter
5. focused onto wavefront sensor

Question 15

wavefront sensor needs bright point source to

Answer 15

provide sufficient signal to noise in a short exposure time

Question 16

strehl ratio - effect of

Answer 16

wavefront compensation on the intensity profile of a turbulence-degraded image of a star

Question 17

airy disc - strehl ratio=

Answer 17

1

Question 18

partial compensation - strehl ratio

Answer 18

<1

Question 19

no compensation - strehl ratio

Answer 19

«1

Question 20

compensation of the wavefront transfers energy from

Answer 20

halo of the image (radius lambda/ro) to the core of the image (radius lambda/D)

Question 21

strehl ratio S=

Answer 21

I_PSF(0)/I_airy(0)

S=(r0/D)^2

Question 22

the atmospheric seeing is the

Answer 22

resolution resulting from atmospheric turbulence

it is equivalent to the diffraction limited resolution of telescope with diameter r0

Question 23

the fried paramater is also the

Answer 23

length of the wavefront over which the phase changes by 1 radian

Question 24

the atmospheric time constant is the

Answer 24

timescale over which a turbulence cells moves by r0