Lecture 5

Question 1

Fried parameter is an

Answer 1

indication of the strength of phase fluctuations

Question 2

Cn^2 (refractive index structure parameter) describes

Answer 2

the strength of turbulent fluctuations of refractive index

Question 3

Fried parameter describes

Answer 3

length of wavefront over which phase changes by 1 radian

Question 4

turbulent cells evolve on longer timescales than

Answer 4

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

Question 5

wind velocity V(bar) at the altitude of the turbulence determines

Answer 5

temporal variation of the wavefronts entering the telescope

Question 6

turbulent cell moves by its own size in time

Answer 6

t0= t2 - t1

Question 7

atmospheric time constant or coherence time

Answer 7

τ0 = r0/V(bar)

Question 8

the coherence time is the

Answer 8

timescale on which wavefront sensor and deformable mirror of an adaptive optic system must operate

Question 9

exposure time:

Answer 9

τ < τ0 , short
τ > τ0 , long

Question 10

Seeing

Answer 10

is the equivalent resolution one would get by viewing through a diffraction-limited telescope of diameter r0

Question 11

Angular width of seeing-limited image

Answer 11

β = 0.98λ/r0

Question 12

Kolmogorov

Answer 12

r0 ∝ λ^(6/5)

Question 13

better seeing in infrared as

Answer 13

optical path length changes a smaller proportion of the wavelength

Question 14

r0 determines sizes of

Answer 14

individual lenslets and mirror segments in wavefront sensors and deformable mirrors

Question 15

adaptive optics correction in the infrared requires

Answer 15

fewer elements than in the optical

Question 16

Isoplanatic angle

Answer 16

angle over which turbulence pattern shifted by distance r0.

angular separation at which light from two stars becomes uncorrelated

Question 17

Isoplanatic angle =

Answer 17

θ0 = r0/h

Question 18

θ0 determines

Answer 18

area of sky over which adaptive optics are effective

Question 19

wavefront compensation

Answer 19

light from point source becomes distorted as it passed through the atmosphere

wavefront disturbance measured to produce path length error

original wavefront restored by inserting equal and opposite path length corrections producing corrected image

Question 20

adaptive optic scheme

Answer 20

compensate the wavefront by sensing the wavefront then actuating on a deformable mirror

Question 21

Strehl ratio

Answer 21

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

Question 22

no compensation

Answer 22

seeing disc

strehl ratio &laquo_space;1

Question 23

fully compensated

Answer 23

airy disc

strehl ratio = 1

Question 24

partially compensated

Answer 24

core and halo

strehl ratio < 1

Question 25

Compensating the wavefront transfers energy from the halo to the core of the image

Answer 25

increases the peak intensity

Question 26

Strehl ratio derivation

Answer 26

S = I(PSF)(0) / I(Airy)(0)

Intensity = Power/Area

Airy disc ∝ (λ/D)^2
Seeing Disc ∝ (λ/r0)^2

S = (r0/D)^2

Question 27

atmospheric time constant is

Answer 27

the timescale over which a turbulence cell moves by its own size