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(1)

Adaptive Optics Adaptive Optics

(and its implementation on the WHT) (and its implementation on the WHT)

• Overall principle

• Subsystems (DM, WFS, etc.)

• Laser Guide Star

Objective: to understand what are the key

components of the system

(2)

Image quality

Telescope diffraction limit

Angular resolution ~λ/D

for 2.5m telescope in V band this is 0.045 arcsec !

Intrinsic optical performance

for classical telescope of order few times 0.1 arcsec

Atmosphere

typically 1 arcsecond

Conclusion:

for any large telescope the

atmosphere is the limiting factor in setting the spatial resolution

(3)

Atmospheric turbulence and seeing

(4)

A few key concepts #1:

FWHM:

width of the image at half height

Strehl ratio:

ratio of peak intensity of aberrated image compared to perfect image

(5)

Euro50 Adaptive Optics Simulation

K-band

3”

(6)

A few key concepts #2:

C

n2

(h):

refractive index structure function.

Characterizes turbulence profile of the atmosphere Integrated over path gives measure of seeing.

r

0

and t

0

:

characteristic length scale and time scale for the seeing cells. r0 is often called the

“Fried parameter”

r0 is ~10cm in visible on a good site r0 ~ λ6/5 and r0 ~ airmass -3/5

(7)

A few key concepts #3:

Wavelength dependence of seeing:

Everything related to seeing improves towards longer wavelengths

‘Seeing’ FWHM ~ λ/r

0

~ λ

-1/5

Zernike polynomials:

a mathematical description

used to describe wavefront distortions

(8)

Adaptive Optics seeks to correct for the Adaptive Optics seeks to correct for the

atmospheric turbulence in order to recover atmospheric turbulence in order to recover

the theoretical diffraction

the theoretical diffraction- - limited image limited image quality of the telescope.

quality of the telescope.

sense wavefront distortion on star

correct wavefront

for science field corrected Atmospheric image

turbulence

WFS

DM

(9)

The Adaptive Optics cycle The Adaptive Optics cycle

Measure the Calculate optimal

wavefront position of

distortion adaptive mirror few 100Hz

Image Set adaptive

improves mirror

(10)

Wavefront

Wavefront Sensor Sensor

Divide up the primary mirror (‘telescope pupil’) Image star for each segment

Spot centroid positions are a measure for the local wavefront tilt.

The centroids are used to correct the DM.

WHT Shack-Hartman primary wavefront sensor

(11)

Wavefront distortion

A local wavefront tilt causes a star image to slightly shift to away from the centre.

By tilting an element on the deformable mirror the wavefront is flattened again.

Deformable Wavefront sensor

mirror element

(12)

Interferometric view

The DM in its mount With Strain Gauge

Pre-amps

76 element segmented mirror.

Each mirror element is mounted on three piezo elements with strain gauge

(13)

NAOMI

NAOMI

(14)

NAOMI NAOMI

OAP2

OAP1 and fast- steering mirror

Deformable mirror

Part of Wavefront sensor

folding-flat

calibration unit

An image showing NAOMI during laboratory integration.

(15)

When AO works

When AO works … …

(16)

Limitations Limitations

Performance degrades at shorter wavelengths as r0 and t0 both reduce with wavelength.

The result is that at shorter wavelength wavefront sensing and correction must happen on a

finer scale

as well as at

higher frequency

High Strehl ratios are readily obtained in K-band but are still impossible in the optical.

(17)

Limitations Limitations

Needs bright star very close to

target, resulting in sky coverage of ~1%

Improve this using laser guide star – Sodium or Rayleigh laser

90 km sodium layer

5-25km

(18)

GLAS Laser Schematic

GLAS Laser Schematic

(19)

beacon 15 km

turbulence

laser

Laser focussed at of 15km

First expand beam, then project using small telescope

Back scattered light is used for wavefront sensing

GLAS laser schematic

GLAS laser schematic

(20)

Project laser beam on small spot at ~15km Pulsed laser linked to

fast shutter to select height and duration of scatter region

Still needs natural star Two wavefront sensors,

for star and laser

h = c∆t / 2

GLAS principles

GLAS principles

(21)

The laser at work

The laser at work … …

(22)

R = 18 & search field of 2 arcmin diameter

Remko Stuik Leiden

(23)
(24)
(25)

1 arcsecond

Seeing limited 8m + AOHSTE-ELT

(diffraction limit a few milliarcsec in the near-IR)

Spatial Resolution

Spatial Resolution

(26)

Referencias

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