Transcript Slide 1 - Caltech Optical Observatories

Optical Relay and Field Rotation
(WBS 3.1.2.2.2, 3.1.2.2.3)
Brian Bauman
April 26, 2007
Keck Next Generation Adaptive Optics
Team Meeting 6
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Status
• Designs for narrow-field (30”) and wide
(100”) AO relays
• Designs for MOAO and TTFA arms
• Designs for WFS package
• WFS split/pickoff options explored
• Field rotation options explored
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Narrow field reflective design
• 25 mm MEMS, 30”
field diameter
• Requires ~18° OAP
• Generates sizable
pupil distortion/tilt
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Narrow field on-axis design
• All-refractive design
• Narrow field, 30” diameter
• Passes LGS, enables “closed loop” high-order
WF control
• MEMS DMs (25 mm pupil)
• MCAO option: Second optical space for 2nd
DM.
• TT stars picked off at front focal plane for
sky coverage (wider TT field), => open loop TT
sensing. Steer additional LGS to TTStars on
technical field?
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On-axis refractive relay
380mm
Focal plane
30 arcsec
diameter
MEMS DM
(25 mm)
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On-axis AO field-dependent aberrations
(but pessimistic because there is no refocusing with λ)
Diffraction-limited
in V band
15 arcsec
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Performance over H-band
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On-axis AO field-dependent
aberrations to LGS
• 2.5 l at edge of field, P-V ray
deflection (WF slope) of 0.15”, mostly
astigmatism and coma.
– Known as a function of field and zenith
angle
– Within dynamic range of radial CCD WFS
– Easily correctable with a MEMS, or use
static corrector + WFS offsets
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On-axis AO field-dependent pupil
distortions
• <43 micron displacement of a marginal
ray at l=1.25m the edge of the field,
<0.1 subaps (430 micron act spacing
MEMS)
• <22 micron displacement as a function
of l=0.38-2.4m over all field angles
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Issues with refractive design
• Chromatic aberration across band
• Inhomogeneity
– Spec at 10^-6, glass thickness ~5mm => ~1.3 nm
rms OPD variation/optic
– Probably not a problem
• Transmission/coatings: probably get 0.500.75% per surface for 700-2400nm; need to
check for visible
• Birefringence
– Glass spec birefringence <4nm/cm (precision
annealing). 5mm glass=>0.2nm rms/optic
– Probably not a problem (except possibly ExAO)
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Wide field relay design
• 100 arcsec diameter field
• All refractive design
• Diffraction-limited at R band (<0.07 l) over
the entire field. Assumes correction of
spherical.
• Pupil distortion: Chief ray shifts 770 microns
= 2 subapertures, tolerable?
• LGS sees 15 l of coma and astigmatism, ~1.0”
of ray deflection
– Challenges the WFS dynamic range (knowable, can
be open-loop corrected)
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Wide field relay
390 mm
25 mm
MEMS
80 mm
(100 arcsec)
field
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Wide field relay – field
dependent aberrations
50 arcsec
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Performance over H-band
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Wide field relay design
• More difficult with small DM due to
pupil distortion
• The “slippery slope” of wide field
closed-loop design:
– Is the wider field science driven? Or is it
just to correct TT stars?
– Would need more lasers to reduce
tomography error
– Would need more conjugate DMs to reduce
gen. anisoplanatism
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Generalized anisoplanatism as a
function of field angle
on the KNGAO standard (r0=15.6cm q0=3 arcsec) atmosphere, from KAON 452
a) DM at 0 km
b) DMs at 0,10 km
c) DMs at 0,5,10 km
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Different field
angle
optimizations
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MOAO relay design
• Appropriate to MOS science arms and
LOWFS
• Reflective design, off-axis ellipse
• MEMS
500mm
MEMS
File: “IRMOS, 2 arcsec field, 10mm woofer first, 60mm DM second, up to dewar window, with apertures.SES”
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LGS WFS
• Closed loop architecture: standard Hartmann lenslet
WFS design at AO relay output.
– On-axis AO relay is low enough aberration so no variable
optics needed, other than focus motion.
– Two DM MCAO is needed to correct LGS over the field,
otherwise it’s open-loop sensing (but closed loop on the DM)
• Open loop architecture: use MEMS correctors in
SHWFS.
– Simple refractive or reflective relay
– Pupil reimage to lenslet only; 2nd pupil reimage not needed for
DM because of narrow field
– Mechanical focus motion
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Wide field LGS WFS system
from TMT IRMOS design
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Wide field LGS WFS system
DMs
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Field Rotation – Narrow Field AO
• We have a K-mirror design from the Indian
Wells meeting that is 500 mm behind focus
and handles 1 arcmin field
– Narrow field AO only has 300 mm available, so
need rotator ahead of focus. How much space is
available ahead of Keck focus?
• K mirrors are small on a narrow field
– 30 arcsec field: fits in 200 mm, 65 mm high, sucks
in focus by ~200 mm
• Conclusion: K mirrors may be feasible for
narrow field AO designs
• Option: tumble AO+instrument
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Field Rotation – Wide Field AO
• Recommend MOAO IFU
on a upward or
downward-looking turret
barrel
• At most 250mm
difference between 90
km and infinity focus.
May need relay optics
into WFS (first optic on
a rotating wheel).
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Conclusions
• Small refractive on-axis AO design approach
eliminates pupil distortion and LGS aberration
problems
• Wider field design is hard with 25mm MEMS
limitation. Relaxed with larger (60 mm) DM.
• Consider drivers for wide field system. Is TT star
pickoff the only issue?
• MOAO reflective relay designs carried over from
TMT/IRMOS study
• Several field rotation options presented, depending
on field size and space available ahead of focus
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