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Adaptive Optics in the VLT and ELT eraAdaptive Optics in the VLT and ELT era
Wavefront sensors, Wavefront sensors, correctorscorrectors
François WildiObservatoire de Genève
Issues for designer of AO systemsIssues for designer of AO systems
• Performance goals:– Sky coverage fraction, observing wavelength,
degree of compensation needed for science program
• Parameters of the observatory:– Turbulence characteristics (mean and variability),
telescope and instrument optical errors, availability of laser guide stars
• AO parameters chosen in the design phase:– Number of actuators, wavefront sensor type and
sample rate, servo bandwidth, laser characteristics
• AO parameters adjusted by user: integration time on wavefront sensor, wavelength, guide star mag. & offset
Dependence of Strehl on Dependence of Strehl on and and number of DM degrees of freedomnumber of DM degrees of freedom
• Assume bright natural guide star
• No meas’t error or iso-planatism or bandwidth error
S exp 2 exp 0.28 d / r0 5 /3
r0 r0 0.5m / 0.5m 6 /5
S exp 0.28d
r0 0.5m
5 /30.5m
2
Deformable mirror fitting error only
Decreasing fitting error
• Assume bright natural guide star
• No meas’t error or iso-planatism or bandwidth error
Deformable mirror fitting error only
Reminder #1: Dependence of Strehl on Reminder #1: Dependence of Strehl on and number of DM degrees of and number of DM degrees of freedom (fitting)freedom (fitting)
Classical PIEZO actuatorsClassical PIEZO actuators
Large DM’s are on every ELT Large DM’s are on every ELT technological roadmaptechnological roadmap
7
Existing MEMS mirror Existing MEMS mirror (sufficient for Hybrid-MOAO)(sufficient for Hybrid-MOAO)
Boston Micromachines 32x32 actuator, 1.5 um MEMS device.(In Stock)
Basics of wavefront sensingBasics of wavefront sensing
• Measure phase by measuring intensity variations
• Difference between various wavefront sensor schemes is the way in which phase differences are turned into intensity differences
• General box diagram:
Guidestar
Turbulence
Telescope Optics Detector Recon-structor
Wavefront sensor
Transforms aberrations into intensity variations
Computer
Types of wavefront sensorsTypes of wavefront sensors
• “Direct” in pupil plane: split pupil up into subapertures in some way, then use intensity in each subaperture to deduce phase of wavefront. REAL TIME– Slope sensing: Shack-Hartmann, pyramid
sensing– Curvature sensing
• “Indirect” in focal plane: wavefront properties are deduced from whole-aperture intensity measurements made at or near the focal plane. Iterative methods - take a lot of time.– Image sharpening, multi-dither– Phase diversity
Shack-Hartmann wavefront sensor Shack-Hartmann wavefront sensor concept - measure subaperture concept - measure subaperture tiltstilts
CCD CCD
f
Pupil plane Image plane
WFS implementationWFS implementation
• Compact
• Time-invariant
Page 12
CCD rapideCCD rapide
• CCD design complete
• 64 pins– 256x256 pixels– 1200 trames/s– < 1e bruit– Refroidissment Peltier
01 Dec 2005
PF Vis AO WFS 12
01 Dec 2005 PF Vis AO WFS 13
StoreArea
Image Area
240x12024□µm
StoreArea
Image Area
240x12024□µm
OP 1
OP 2 GainRegisters
OP 3
OP 4 GainRegisters
OP 8GainRegisters
OP 7
OP 6GainRegisters
OP 5
Metal ButtressedMetal Buttressed22ΦΦ 10 Mhz Clocks 10 Mhz Clocksfor fast image to for fast image to store store transfer rates.transfer rates.
8 L3Vision Gain 8 L3Vision Gain Registers/OutputsRegisters/Outputs..Each 15Mpix./s.Each 15Mpix./s.
Store slantedStore slanted to allow room for to allow room for multiple outputsmultiple outputs..
Split frame transfer 8-output Split frame transfer 8-output back-illuminated e2v L3Vision CCD for WFS.back-illuminated e2v L3Vision CCD for WFS.
TMT.IAO.PRE.06.030.REL02
14
3. NGS WFS 3. NGS WFS
• Radial+Linear stages with encoders offer flexile design with min. vignetting
• 6 probe arms operating in “Meatlocker” just before focal plane
• 2x2 lenslets
• 6” FOV - 60x60 0.1” pix
EEV CCD60
Flamingos2 OIWFS