Glen Herriot, Laurent Jolissaint, Jeffrey A' Stoesz, JeanPierre Vran, Tim Davidge, NRCHIA

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Wide Field Adaptive Optics---

• Glen Herriot, Laurent Jolissaint, Jeffrey A.
  Stoesz, Jean-Pierre Véran, Tim Davidge, NRC-HIA
• TMT SAC, Tucson, December 17, 2003

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Wide-Field (Ground Layer) AO Mode

• Provide 0.2 to 0.3 over up to 10 arcmin FOV
• Use one DM to correct the ground layer
• Ability to trade-off performance for field
• WFAO can be turned into Classical AO on the fly
• Guide stars under investigation
• Several NGSs
• Several Sodium LGSs
• One or several Rayleigh LGS
• Current work at HIA
• Performance estimation
• Strawman design (relay optics very demanding,
  adaptive M2 ?)
• Development of a Pyramid WFS

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PAOLA Simulations of WFAO
380 masc seeing
7.5
5.0
60 masc Tscope res.
2.5
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Wide Field Adaptive Optics - Introduction

• Classic AO
• high angular resolution,
• small corrected field 10 20 arc seconds
• MCAO
• high angular resolution,
• large corrected field 1 2 arc minutes
• complexity high
• For some topics, field is more critical than
  angular resolution
• The larger the field, the larger the number of
  objects, then the better the statistics
• Large field for Multi Object Spectroscopy
• Integrated Spectra of large samples of Hi-Z
  galaxies

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Wide Field Adaptive Optics - Introduction

• The alternative (F. Rigaut 2000, Christou 1988)
• Keep the field large 5 - 10
• MCAO-like WFS
• Accept medium angular resolution 0.2 - 0.3
• 1 DM conjugated to the main (ground) layer
• DM driven by ltOPDgt within WFS cone
• Corrected field Guide Star cone aperture
• Partial correction high spatial
• frequencies blurred by averaging
• within the WFS cone.
• Tradeoff field vs. performance by optimizing cone
  angle
• Average around surface
• of cone See A. Tokovinin

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Wide Field Adaptive Optics - Simulations

• To estimate WFAO performance, we have built an
  analytical model of the WFAO residual phase
  Spatial Power Spectrum
• Cone Aperture
• DM conjugation altitude
• Field angle
• Assume perfect knowledge of the refractive index
  vertical profile no WFS limitations
  WFAO Upper limit performance
• Implemented into PAOLA (see Bakaskog poster)
• Conditions
• 30 m telescope TMT Band K
• Seeing 0.7 Outer scale 100 m
• Wind velocity 10 m/s Actuator pitch 87 cm, r0
  _at_ K band

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Seeing Profile for Actuator Density Calcs.
13 layer Model with 50 of turbulence lt 500 m
altitude (Mauna Kea type)
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Tradeoff WFAO FOV vs. Image Quality
Natural Seeing
Edge of 6 diameter field
50 EE Radius. arcsec (K band)
6 Cone WFAO
4 Cone WFAO
2 Cone WFAO
Classic AO
Field Radius, Arcminutes
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WFAO Actuator Density DM Altitude
Radius 50EE
Convex AM2 hDM -300 m
0.4
Seeing
Actuators across Pupil 31,62, 93, 124
Ground Conjugate hDM 0 m
Build many actuators in ground layer DM, OR
build modest Gregorian Adaptive Secondary
WFAO
Gregorian AM2 hDM 300 m
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0
Field Radius arcmin
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Spectroscopy of high-Z galaxies
Seeing PSF
WFAO PSF
MCAO PSF
convolve
Galaxy model
Slit
Half light radius
0.2
WFAO image
MCAO image
Seeing image
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Integrated Spectroscopy of Hi-Z galaxies

• Preliminary result
• Save 25 33 of nights using WFAO instead of
  Natural seeing.
• At 1M/night saved, WFAO could be attractive.

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Point Sources are NOT the niche of WFAO
Point Source Integration Time
Time Seconds
Natural Seeing 0.7 WFAO 50 EER0.4 FOV6
diameter MCAO SR0.4 FOV2
diameter SNR3
K band Magnitude
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FWHM is NOT the right Metric for WFAO

• FWHM is NOT the right Metric for WFAO
• The reason is that is misleading.
• It overestimates the gain for spectroscopy.

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WFAO for VLOT Strawman Design

• f/15 Richey-Chretien 20m telescope, f/1 primary
  mirror
• Upward looking instrument on a Nasmyth Platform
• Output field
• 6 arcmin FOV with a goal of 10 arcmin
• no less than f/15 with a goal of f/30 to f/50
  (IFU lenslet gt 0.1mm)
• DM
• One DM conjugated conjugated with M1 or slightly
  higher (TBD)
• Hexagonal actuator geometry to match hexagonal M1
  segments
• Integer number of actuator pitches across segment
  flat
• Image quality
• 50 EE within 0.03 arcsec at the edge of the
  field
• 50 EE within 0.002 arcsec in the central 10
  arcsec

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WFAO design from VLOT book
0.05 Geometric image quality in 4 diameter FOV
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Can we reuse TMT MCAO optics?

• Unfolded MCAO optics are diffraction limited
• (design from Rich Dekany)

2 arcminute dia. FOV
On-Axis
Airy Disc 1 mm
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Folded MCAO with DMs

• Beam footprint on High Altitude DMs grows larger
  with increasing altitude of conjugation

2 MCAO
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Optics may vignette a wide field

• Diameter of High Altitude DMs increases with
  field of view.
• Wide Field mode may vignette beams.

2 MCAO
5 MCAO
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Cross Section of High Alt. DM

• Wide field mode illuminates actuators that are
  not needed for MCAO.

Cant have unsupported face sheet
Cross section of DM
Need additional actuators

• If high altitude mirror is left flat for WFAO,
  extra actuators must still be powered and
  calibrated due to differential expansion vs. T.
• (e.g. 1/3 of DM command range used to flatten
  Altair DM at 0 Celsius, whereas it was flat if
  unpowered at room temperature.)

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Possible Solutions to Vignetting DM size

• Build an independent WFAO system.
• Use an Adaptive Secondary mirror for WFAO
• To acquire guide stars over a wide field, may
  require a special-purpose WFS unit tailored to
  this larger field. Modest upgrade cost.
• Use bypass folds within MCAO

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Bypass flat within MCAO

• Bypassed DM 2-4, OAP 2-3
• Image quality, even at 2 dia. FOV is too poor
  for WFAO.
• Top of OAP1 parent does not match bottom of OAP4
  parent

0.35
Poor Image Quality
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Bypass flat Move OAP4

• Deploy fold mirror
• Rotate Paraboloid 4 by 180 degrees, and translate
  it.
• Image quality remains good.

Airy Disc 1 mm
Translate
Rotate 180
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Wide Field Adaptive Optics - Conclusions

• Conclusions
• Next step Guide star requirements -gt sky
  coverage calculations. Can we use NGS only?
• Optical Design needs a breakthrough, to reuse
  parts of MCAO
• Sensitivity to turbulence profiles at various
  sites, and from day-to-day?
• Is it a good rule of thumb to say that
• WFAO performance is always equivalent to best
  natural seeing image quality?