Brent Ellerbroek a , Glen Herriot b , Ryuji Suzuki c , and Matthias Schoeck a
description
Transcript of Brent Ellerbroek a , Glen Herriot b , Ryuji Suzuki c , and Matthias Schoeck a
Evaluation of Astrometry Errorsdue to the
Optical Surface Distortionsin
Adaptive Optics Systems and Science Instruments
Brent Ellerbroeka, Glen Herriotb, Ryuji Suzukic, and Matthias Schoecka
aTMT Observatory Corporation, bNRC Herzberg, cNational Astronomical Observatory of Japan
Adaptive Optics for Extremely Large Telescopes 3Florence, ItalyMay 28, 2013
TMT.AOS.PRE.13.087.REL01AO4ELT3, Florence, 05/28/13
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Presentation Outline
Current astrometry requirements and error budget for TMTObjectives of this exerciseSimplified model for astrometric observationsSimplified modeling assumptionsSummary of analytical resultsApplication to NFIRAOS+IRIS for TMTSummary and future plans
TMT.AOS.PRE.13.087.REL01AO4ELT3, Florence, 05/28/13
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Astrometry Requirements for TMT
[REQ-1-ORD-3650] NFIRAOS shall enable precise differential astrometry measurements,– where one-dimensional time-dependent rms astrometric positional
uncertainties, after fitting distortion measured with field stars, and over a 30 arcsecond field of view
– shall be no larger than 50 µ-arcseconds in the H band for a 100s integration time.
– Errors should fall as t-1/2.– Systematic one-dimensional rms position uncertainties shall be no
more than 10 µas.[REQ-1-ORD-3652] The AO system should provide sufficient calibration information to not degrade the astrometric capabilities beyond the limits set by the atmosphere.
TMT.AOS.PRE.13.087.REL01AO4ELT3, Florence, 05/28/13
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Astrometry Error Budget Organization for TMT
Reference catalogue errorsAtmospheric refraction correctionOther atmospheric effectsOpto-mechanical errors:– Telescope optics calibration– Guide probe position– Imager calibration– Optical surface calibration– Rotator calibration– Quasi-static errors– Stuck actuators, diffraction spikes– Vibrations– Coupling with other effects
Focal plane errors
More than 30 (currently 34) terms grouped in 5 categories– Organization
derived in part from MICADO budget
– Values of many terms are scenario-dependent
– Many terms remain work in progress
Driven by optical surface errors in
IRIS and NFIRAOS
TMT.AOS.PRE.13.087.REL01AO4ELT3, Florence, 05/28/13
Objectives of this Exercise
Develop engineering formulas for estimating astrometry errors due to optical surface errors in instruments and AO systems– Intended as a practical tool to support development of error
budgets and optical surface specifications
Apply to current optical designs and surface specifications for IRIS + NFIRAOS to confirm that TMT astrometry requirements can be metBegin to iterate designs and surface specifications as necessary…
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Observing Sequence Model
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Distortion Calibration w/ Ref. Grid
Distortion Calibration
w/ Stars
Distortion map, post-focal optics
Science Exposure #2
Science Exposure #1
Distortion map, pre-
focal optics
Position Measurement #1
Differential Position
Measurement
Position Measurement #2
• Quasi-static error #1• Boresight error #1
• Quasi-static error #2• Boresight error #2• Field rotation or dither
S S
S S
S
+ ++
+ +-
-
--
-
Calibration by Field Stars
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Modeling assumptions
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Optical surface errors Random, with shift-invariant statistics Defined by PSDs
Induced wavefront errors Linear sum of contributions from “phase screens” at each surface
Resulting image distortion RMS best-fit tilt to exit pupil wavefront
Random boresight errors Normally distributed in 2 dimensions
Intentional field-of-view offset or rotation
Linear displacement of optical path through some or all optical surfaces
Distortion calibration by reference sources/stars
Measures image distortion map up to the Nyquist rate defined by reference source spacing
Distortion calibration by field stars
Removes low-order (polynomial) modes of image distortion map
Other simplifying assumptions
Circular, unobscured apertureCircular field-of-viewCircularly symmetric error PSDs
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Formulation of Results
Mean-square error a sum of contributions from each surface:
For quasi-static errors
For dither/rotation errors (random boresight errors similar)
For calibration errors
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i
i22
M
m
mii b
bJmDDJd
D 0
21
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222 )2()1(1)(4)( 2
M
m
miii b
bJmhJDDJd
D 0
21
0
22
222 )2()1(1222)(4)( 2
1)2(
22
222 )(4)( 22
dh iii D
DJdD
Domain of aliasing
Translation filter
Error PSD
Tip/Tilt filter
Low-order Mode removal filter
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Optical Train Schematic for NFIRAOS (Facility AO System) + IRIS (Near IR Imager/Spectrograph)
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TMT
NFIRAOS Windows
(Reference Source Grid)
NFIRAOS Optics
IRIS Window
IRIS Optics
Focal Plane
(Rotation Bearing)
Rotates in IRIS Coordinate System
Distortion Calibration via Stars
Distortion Calibration via
Reference Sources
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TMT+NFIRAOS Optical Layout
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2-5: Input windows1,16: Input/output focus7,9,10,14: OAPs8,11: DMs12: Science beamsplitter15: Instrument selection fold TMT.AOS.PRE.13.087.REL01
AO4ELT3, Florence, 05/28/13
(Initial) NFIRAOS Optical Surface Specifications
Developed based upon overall NFIRAOS wavefront error budgets– Science field– Off-axis guidestars– Compensation by NFIRAOS
deformable mirrors permittedValues specify transmitted wavefront errors over surface clear aperture with tip/tilt/focus removedPower law error PSD specified
– p=-2.5
Total 76.5 nm RMSWin 1 (-191.1 km) 30.9
Win 2 (-198.2 km) 31.0
OAP1 (49.2 km) 25.1
OAP2 (-36.6 km) 24.6
OAP3 (35.2 km) 24.9
Sci B/S (-9.7 km) 27.5
OAP4 (-43.6 km) 24.3
Inst. Fold (-85.2 k m) 27.1
TMT.AOS.PRE.11.123.REL01AO4ELT, Victoria, September 26 2011
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Uniform tolerance of 25 nm RMS assumed for initial astrometry budgeting
9.05.5
Input window specifications tightened as needed to achieve astrometry budget
NFIRAOS focal plane
Entrance window
Collimator lens
Fold mirror
Fold mirror Collimator lenses
Pupil
ADC
Filter
Camera TMADetector
IRIS Imaging Channel(ApT Collimator + TMA Camera)
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IRIS Optical Surface Specifications
# Surf.Name
DA, m
h, km
PSD pwr
, nm
# Surf. Name
DA, m
h, km
PSD pwr
, nm
1 Win f 0.10 2890 2.30 2.8 13 ADC 1f 0.10 -19.8 2.12 3.2
2 Win b 0.10 2520 2.29 2.7 14 ADC 1c 0.10 -21.5 2.12 0.7
3 Col 1f 0.12 1290 2.23 2.6 15 ADC 1b 0.10 -22.8 2.12 4
4 Col 1b 0.12 1190 2.22 2.6 16 ADC 2f 0.10 -25.7 2.12 4
5 Fold 1 0.12 895 2.21 10.6 17 ADC 2c 0.10 -27 2.12 0.7
6 Fold 2 0.12 156 2.15 9.6 18 ADC 2b 0.10 -28.7 2.12 3.2
7 Col 2f 0.12 63.5 2.14 6.9 19 Filt. F 0.08 -61.5 2.12 2.0
8 Col 2b 0.12 61 2.14 6.9 20 Filt. B 0.08 -63.5 2.12 2.0
9 Col 3f 0.12 50.8 2.13 2.1 21 Cam 1 0.16 -171 2.14 9.7
10 Col 3b 0.12 47.3 2.13 2.1 22 Cam 2 0.16 -370 2.16 10.0
11 Col 4f 0.12 45.3 2.13 2.2 23 Cam 3 0.24 -620 2.17 10.5
12 Col 4b 0.12 40.2 2.13 2.213
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Distortion Calibration Errors vs. Reference Source Spacing
All optics except NFIRAOS Windows
Spacing of 0.7 arc sec yields 5 m arc sec error
NFIRAOS windows, original specs
Errors too large!NFIRAOS windows, revised specs
Spacing of 5 arc sec yields 6 m arc sec error
Source Spacing arc sec0.1 10.01.0
Cal
ibra
tion
Err
or, m
arc
sec
0.1
1.0
10.0
100.0
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Distortion Calibration Error Contribution by Surface
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Surface number in optical train
Cal
ibra
tion
Err
or, m
arc
sec Camera
mirrors 2 and 3
NFIRAOS windows, original specs
NFIRAOS windows, revised specs
IRIS input window Collimator
lens 1
Fold mirror 1
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Differential Image Distortion Due to Iris/NFIRAOS Rotation
Fourier model approximates field rotation by a global translation:
16Line-of-sight shift in NFIRAOS, arc sec0.1 10.01.0Fiel
d-av
erag
ed d
iffer
entia
l dis
torti
on, m
arc
sec
1.0
100
10
1000Red: Original window tolerancesBlue: Revised tolerancesSolid: Global tip/tilt calibration using reference starsDashed: Plate scale calibration
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Sensitivities for Quasi-Static DM Figure Errors on DM11.2 in NFIRAOS
D=30m30” FoV11.2 km DM conjugate range0.5m actuator pitchMax. sensitivity of ~2.5 mas/nm with global tip/tilt calibration~0.15 with plate scale calibration
17Spatial frequency, cycles/m1.00.10.010.001
1.0
10.0
0.1
0.01
Sen
sitiv
ity, m
as/n
m
0.001
0.0001
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Summary
A Fourier model for evaluating the magnitude of image distortions due to optical surface errors has been applied to develop astrometry error budget terms for observations with IRIS+NFIRAOS on TMTResults are preliminary, but confirm intuition and are encouraging:– Calibration of static distortions to 5-7 m arc sec is feasible, but…
Tolerances on surfaces near focal planes are tightEven so, many references sources are neededIRIS optical design may be iterated to adjust surface conjugates
– As with K-mirrors, a consistent image rotator angle must be used for repeated observations of the same field
– Image distortion due to quasi-static errors on DM11.2 are small, provided that overall plate scale changes can be calibrated using reference stars
Further modeling/simulation is planned to using more realistic models of static errors, calibration procedures, quasistatic errors, etc.
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Acknowledgements
The TMT Project gratefully acknowledges the support of the TMT partner institutions.They are
– the Association of Canadian Universities for Research in Astronomy (ACURA),– the California Institute of Technology– the University of California– the National Astronomical Observatory of Japan– the National Astronomical Observatories and their consortium partners– And the Department of Science and Technology of India and their supported institutes.
This work was supported as well by– the Gordon and Betty Moore Foundation– the Canada Foundation for Innovation– the Ontario Ministry of Research and Innovation– the National Research Council of Canada– the Natural Sciences and Engineering Research Council of Canada– the British Columbia Knowledge Development Fund– the Association of Universities for Research in Astronomy (AURA)– and the U.S. National Science Foundation.
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