Keck Interferometer Requirements and
Implications for NGAO
Christopher Neyman
W. M. Keck Observatory
Keck NGAO Team Meeting #2
November 14, 2006
012 Robinson Laboratory, Caltech
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NGAO requirements for interferometry status and plans
• Compiling requirements for NGAO and IF
• Draft report on IF requirements complete
•
•
Under review (Adkins, Colavita, Wizinowich)
Future KAON
• Collate NGAO/IF requirements into larger NGAO requirements doc
• Today only discussing a subset of the NGAO/IF requirements
•
Those that are unique to interferometry
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This talk will cover some of the unique challenges
when using NGAO with the Keck interferometers
1. Fundamental effects
2. Optical and mechanical
connections
3. Intersystem coordination
3
The electric field from each telescope must match for
interference to occur
Orientation
•Identical beam trains with
same image flips and
rotations (also pupil)
Differential phase shift
•Identical beam trains with
same ordering of reflections
Coherent wavefront
•AO or AO and
single mode
fibers
Image courtesy European Southern Observatory
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Fringe visibility is the fundamental observable for
interferometer
• Constructive &
destructive interference
of light
• Fringe contrast or
visibility:

Zero
OPD
Noise

I I
V 

I I
I+
I-
Fringe
Envelope
Actual starlight fringes from IOTA -  And
Photo credit: R.R. Thompson
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Metallic coatings introduce phase shifts between
polarizations
Phase shift increases with
larger angle of incidence
Wavelength dependent shifts
Example data for enhanced
silver coating
source Traub, “Michelson
summer school”, 2002
Note: S and P refer to electric vector perpendicular and parallel to plane of incidence
(containing incident and reflected ray)
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Keeping order and orientation of reflections identical in
beam trains minimizes phase shifts
Same orientation but x out of phase
Typical solution: order & angle match
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Differential phase shifts can reduce fringe contrast to
zero!
Consider combining two electric fields

E1  ( E x , E y )  a1eikz 1, ei1
I  E1  E2


E2  ( E x , E y )  a2 eik  z l  1, ei2

2
let    2  1
  2a1a2 



I  I 1   2
coskl   2 cos  2 
2 
a

a
2 
  1

visibility term
modulation term
polarization term
Contrast reduced by polarization term
Lessons learned: AO K mirror recoating!
For  = 180 you have zero fringe contrast
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Differential phase shifts can reduce fringe contrast: a
real world example
Both K1 &
K2 recoat
After recoating only
K1 AO image rotator
significant drop in V2
V2
Fall 2003
returned to nominal
values when K2 was
also recoated
Suspect phase shift
caused by different
protective layers on
top of silver coating
Only K1 recoated
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Interferometry requires NGAO Strehl match with legacy AO
With a direct combination of AO corrected beams
Assume Strehl is the same for all telescopes
Visibility is proportional to Strehl (see ten Brummelar OptLet 1995)
V=Vsystem*SAO
However best AO Strehl still <1.
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In KI use a single mode fiber after beam combiner to
reject incoherent electric field
Visibility loss is now:
V  Vsystem * 2 I1 I 2
I1  I 2 
Where I1 and I2 represent the intensity from K1 and K2 coupled
into the fiber. These will vary as the AO Strehl fluctuates
If the two AO systems produce identical Strehl, the visibility is high
independent of the Strehl absolute value
For NGAO might have to attenuate its beam to match legacy AO
system intensity
Probably easier than Strehl matching
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Summary of fundamental requirements from current Keck
Interferometer
Parameter
Min.
Differential s-p phase shift
Typ.
Max.
Units
-
6
Degrees
2
Degrees
-
22
Percent
-
14.0
µm
Differential image rotation
Intensity ratio (Strehl mismatch)
Wavelength coverage
1.0
Transmission
TBD1
Percent
Residual tilt
0.007
Arc seconds rms
Optical quality
100
Total nm rms
Field of view
30
Arc seconds (radius)
1current
AO system transmission (KI) is ~70%.
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Keck AO supports a diverse set of current and planned
interferometers
Instrument
Observing
mode
Science
wavelength
Current AO
modes
Optical interface with AO
NASA/KI
V-squared
J,H,K
NGS
Coude
Single star
Collimated
NASA/KI
Nulling
N
NGS
Coude
Dual pupil
Collimated
NASA/KI
V-squared
L
NGS
Coude
Single star
Collimated
‘OHANA
V-squared
J,H,K
NGS
Fiber optics
Single star
Focus
MRI/Keck
Astrometry+
phase ref.
J,H,K
LGS/NGS
Coude
Dual star
Focus
After Keck 1 LGS, any mode could use LGS as AO reference source
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Keck interferometers use coude train to send light
to basement (except ‘OHANA)
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NGAO must feed Dual Star Module or replicate its
functionality
•
•
•
Sends collimated beam into coude beam train
Removable cart
Motion control, metrology, alignment aids,
and accelerometers
V2 mode
M2 FULL
DSM table is ~2.1x1.5m
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NGAO must feed Dual Star Module or replicate its
functionality
•
•
•
Sends collimated beam into coude beam train
Removable cart
Motion control, metrology, alignment aids,
and accelerometers
NULLER mode
M2 HALF
DSM table is ~2.1x1.5m
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MRI phase referencing requires two star mode of
DSM
• Light from two stars into coude beam train
• Select on axis star and star up to 30 arc seconds away
• One concept for MRI shown below, details still TBD
OAPs
FSM’s
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NGAO must replicate function of the two current
interferometer feeds
Collimated beam:
•nuller and V2
•112mm beam mapped
to 9m Keck pupil
Focused beam:
•MRI and OHANA
Accelerometers,
metrology, and
alignment aids
DSM: Nuller & V2
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NGAO must replicate function of two interferometer
feeds
Collimated beam:
•nuller and V2
•112mm beam mapped
to 9m Keck pupil
Focused beam:
•MRI and OHANA
Accelerometers,
metrology, and
alignment aids
MRI & ‘OHANA
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Telescope
Pointing
TTO
Secondary
Mirror
Piston
AO Loops
WFO
DCS
Supervisory
Controller
Wavefront
Controller
TTM
TT Loop
DM Loop
WFS
DM
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Telescope
Pointing
TTO
Secondary
Mirror
Piston
IF Offload
(Red)
WFO
DCS
Manual Cent.
Offload
FSM Move
Supervisory
Controller
Wavefront
Controller
TTM
TT Loop
DM Loop
WFS
Cent. offset
KAT Mirror
(Basement)
DM
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Interfaces to synchronize IF sequencer, AO tools, and
telescope control
•
Information about star magnitude and color sent from telescope sequencer to
AO tools
•
•
•
•
•
Used to set AO frame rate, loop gains, close AO loop
Coordination of tracking mirrors between AO, IF, and Tel. Pointing
Nuller uses AO system tip tilt offset for chopping
Currently uses keywords for communication between systems and operators
No attempt to reduce piston errors between K1 and K2
•
•
•
•
Coordinate secondary piston offloads
DM piston, not sensed by AO
Piston of AO tracking mirrors
Future upgrade
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Interfacing NGAO to the Keck interferometers is
challenging
Beam trains and AO Strehl must be carefully
matched between NGAO and legacy AO
NGAO must support several interferometer
modes
NGAO must coordinate with IF software
and hardware
Thanks, Questions?
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References
1. W. A. Traub, “Polarization effects in Stellar Interferometers”, in High Resolution Imaging by
Interferometry, F. Merkle ed, Proc. ESO Conf. 29, 1029-1038 (European Southern Observatory:
Garching Germany, 1988).
2. S.T. Ridgway and W. G. Bagnuolo, “Polarization Revisited” CHARA Technical Report, TR-28,
1996.
3. T. A. ten Brummelaar, W. G. Bagnuolo, and S. T. Ridgway, “Strehl ratio and visibility in longbaseline stellar interferometry”, Optics Letters, vol. 20, num. 6, 1995.
4. S.B. Shaklan, M.M. Colavita, and M. Shao, “Visibility calibration using single-mode fibers in a
long baseline interferometer”, in High Resolution Imaging by Interferometry II, J. M. Beckers
and F. Merkle ed, Proc. ESO Conf. 39, 1271-1283 (European Southern Observatory: Garching
Germany, 1992).
5. Gordon, Colin G. “Generic Criteria for Vibration-Sensitive Equipment”, in Vibration Control in
Microelectronics, Optics, and Metrology, Proceedings of the SPIE Vol. 1619, pp. 71-85,.
Gordon, Colin G. ed. SPIE 1992.
6. E. Johansson, “Summary of External Interfaces in the Current WFC and Implications for the
NGWFC Design”, Keck Adaptive Optics Note 315 (KAON 315).
7. Principles of Long Baseline Stellar Interferometry, Course notes from the 1999 Michelson
summer school, P. Lawson eds. (1999).
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