Keck NGAO Science Requirements
Claire Max
UC Santa Cruz
Caltech NGAO Meeting
November 14, 2006
Outline
• Background
• JWST and ALMA
• Science requirements for selected key areas
– Multiplicity and size of minor planets
– Imaging extrasolar planets around brown dwarfs and low mass
stars
– General relativistic effects in the Galactic Center
– Galaxy assembly and star formation history
• Other science cases are in progress
• Roll-up of requirements to date
• Some key issues that have emerged
Background
• Science Requirements Document (SRD) is a “living document” and
will be updated as the science case is developed with increasing
fidelity.
• Initially, SRD will heavily reference the science cases developed
for Proposal to Keck SSC in June 2006.
• Key issues:
– Importance of science enabled by NGAO system and accompanying
instruments
– Advances offered by NGAO relative to existing systems and new AO
systems being developed on other telescopes
– Complementarity to JWST and ALMA, which will be commissioned on
the same timescale as Keck NGAO will be commissioned.
JWST Capabilities
• Cryogenic 6.5m space telescope to be launched in 2013
• Higher faint-source sensitivity than Keck NGAO (very low
backgrounds)
• NIRCAM will image from 0.6-5 µm
– 2.2 x 2.2 arc-minute field of view, pixel scale of 0.035 arc sec for 0.62.3 µm, and coronagraphic capability
• NIRSpec multi-object spectrograph with an IFU
– In R~100 and R~1000 modes will obtain simultaneous spectra of >100
objects in 3.4 x 3.4 arcmin field of view
– Has an IFU with field of view 3” x 3” (R=1000 or 3000; not clear which)
– Spatial pixel size will be 0.1 arc sec in all cases
• Conclusions for NGAO? We can compete at higher spatial resolution
(<0.1 arc sec) and shorter wavelengths (<2 µm) where JWST will not
be diffraction limited or Nyquist sampled.
ALMA Capabilities
• Very powerful new facility for mm and sub-mm astrophysics
• Currently scheduled to begin science operations in 2012
• Consists of 54 12-m and 12 7-m antennas located at 5000m
(16,500 feet) in the Atacama desert
• Typical spatial resolution 0.1 arc-second (down to 0.01 arcseconds at high frequencies)
• Chemical evolution in star-forming regions at z~3, dust-gas
interactions, molecules surrounding stars, molecular clouds, dust
emission out to z=20, kinematics of obscured galactic nuclei and
quasi-stellar objects on spatial scales smaller than 100 pc
• Conclusions for NGAO? A renaissance in star formation studies
near and far; new insights into highly obscured distant galaxies
Science case: Size and shape of
minor planets
• Shape and size
Ceres, K band, Keck NGS AO
– Some are round, many are not
– IAU planet definition debate!
• Surface features
– Ceres is one example: low contrast
will be helped by high NGAO Strehl
ratio
• Observations of the 15 - 20 largest
asteroids will provide strong
constraints on frequency of large
collisions
• NGAO should be able to resolve
~800 main-belt asteroids
Eros
Science case: Multiplicity of minor
planets
• Recent data suggest that primary
asteroid of most binary asteroid
systems has rubble-pile
structure, weak shear strength
• Hence shape is directly related
to angular momentum at
formation
• Moonlet orbit plus shape of
primary gives mass of primary
• NGAO, particularly at R band,
increases detection rate of
moonlets dramatically
Simulation of fake moonlet
around 87 Sylvia
Minor planets: science requirements
• Driver for visible wavelengths: 0.7 <  < 2.4 microns
– Reflected sunlight, important spectral bands
• Preferred instrument: visible imager
• Other instruments: visible IFU
• Instantaneous FOV: 2 arc sec, Nyquist-sampled
• Image quality: 170 nm OK, still doing simulations
• Photometric accuracy: 5% for satellite relative to primary
• Astrometric accuracy: Nyquist/4
• Contrast ratio: m > 5.5 at 5 arc sec from primary
• Other important considerations:
– Need non-sidereal tracking; need rapid retargeting in LGS mode (≤10
min compared with 25 min today); request service observing
Science case: Extrasolar planets
around nearby stars
• Gemini + ESO “extreme AO”
systems very powerful, but
can’t look around low-mass
stars or brown dwarfs
– Too faint for wavefront sensing
• Low-mass stars are much more
abundant than higher mass
stars; they might be most
common hosts of planetary
systems
• Survey of young T Tauri stars
will constrain planet formation
timescales
Extrasolar planets: Science Requirements
• Wavelength range: 0.9 <  < 2.4 microns
• Preferred instrument: NIR imager
• Other instruments: Low-resolution (R~100) near-IR spectroscopy
(could this be done with narrow-band filters?), L-band imager
• Instantaneous FOV: 5 - 10 arc sec, 5 - 10 mas sampling
• Image quality: 140 nm OK, still doing simulations of ≥170 nm
• Photometric accuracy: 5% for planet relative to primary
• Astrometric accuracy: < 5 mas
• Contrast ratio: H=10 at 0.5” separation
• Other important considerations:
– Need coronagraph; Need low residual static WFE (how low?); Need
rapid retargeting in LGS mode (≤10 min compared with 25 min
today); Need IR tip-tilt (both on and off axis)
Science Case: General Relativistic Effects
at Galactic Center
• Detect deviations from Keplerian
orbits around black hole
• Highest priority: strong-field GR
precession
• Can be measured even for single
orbits of known stars (S0-2) if
astrometric precision is ~100 μas
coupled with radial velocity
precision of ~10 km/s
• If NGAO allows discovery of other
(fainter) close-in stars, may be
able to measure other effects too
Galactic Center: science requirements
• Wavelength range: K band
• Preferred instruments: NIR imager and NIR IFU
• Imager instantaneous FOV: 10 arc sec (now 20 km/s), Nyquist samp
• IFU instantaneous FOV: 1 arc sec, 20 or 35 mas sampling
• Other instruments: R=15,000 IR spectrograph would be good
• Spectral resolution: 3000 - 4000
• Image quality: 170 nm OK, doing simulations of other WFEs
• Astrometric accuracy: 0.1 mas
• Radial velocity accuracy: 10 km/s
• Contrast ratio: K=4 at 0.05” separation
• Other important considerations:
– Need IR tip-tilt (consider H or K band, because of very high extinction
at J band)
We need to understand what is
limiting astrometric accuracy today
• Uncertainty decreases as expected for brighter stars, then
hits a floor.
• Why the floor? Tip-tilt anisoplanatism? Work is underway.
Comment on astrometric accuracy
and AO design
• MCAO systems are known to suffer from focal plane
distortions.
• In addition to tip and tilt, differential astigmatism and
defocus between the DMs is unconstrained. These
three unconstrained modes do not influence on-axis
image quality, but produce differential tilt between
the different parts of the field of view.
• Our Point Design has a large DM for high stroke
correction, and a smaller DM (MEMS or other) for highorder correction. Need to analyze interaction of the
two DMs to avoid or minimize focal plane distortions.
Science Case: Galaxy assembly and
star formation history
• Overview
–
–
–
–
Study galaxies at z > 1 via their emission lines
Star formation: H
Metallicity: NII / H
Excitation: OII, OIII (star formation, AGN activity)
Space densities of types of galaxies
Approx density
Type of Object
SCUBA sub -mm galaxies
to 8 mJy
Old and red galaxies wit h
0.85 < z < 2.5 and R < 24.5
Field galaxies w/ em issio n
lines in JHK windows
per square arc min ute
0.1
2
> 25
0.8 < z < 2.6 & R < 25
Center of distant rich cluste r
of galaxies at z > 0.8
> 20
All galaxies K < 23
> 40
• Tens of galaxies per
square arc min
• Clear benefit to
deployable IFUs
• How many? Decide
based on total cost
and design issues
(e.g. all fit into one
dewar)
• Reasonable number?
6 - 12 IFU heads
Low backgrounds are key
• Backgrounds are current
limit for OSIRIS science
in this field
• Requirement:
background AO system
less than 10 to 20% of
that from sky and
telescope
• We need to address
cooling issues vigorously
– What is practical, what
are costs?
High z Galaxies: science requirements
• Wavelength range: JHK bands
• Preferred instruments: deployable NIR IFUs (6 - 12)
• IFU instantaneous FOV: 3 x1 arc sec requirement, 3 x 3 arc sec
goal
• Spectral resolution: 3000 - 4000
• Spatial sampling: 50 mas
• Image quality: 50 mas enclosed energy (what fraction?) for
optimal tip-tilt star configuration
• Sky coverage fraction: > 30% on average, if consistent with above
image quality spec. If not, iterate.
• Sky background: less than 10-20% above sky + telescope
• Other important considerations:
– No. of IFUs should be determined by total cost, and by design issues
Spreadsheet summary
Next Generation Adaptive Optics - Science Requirements November 14, 2006
Science Case
Solar System
Multiple Asteroidal Systems
Main Belt Multiples (ex: 87 Sylvia)
Galactic
Galactic Center Dynamics
Astrometry
Radial Velocities
Direct Imaging of Planets
Search & detection (BDs, young *s)
Extragalactic
Field Galaxies
2<z<3
Imag Spec
PhotoSampli Sampli Spectra metric
ng
ng
l Reso- accur.
(mas) (mas)
lution
(mag)
Wavelength
(microns)
Instan-taneous FOV
(")
Image
quality
0.7-2.5
2
170nm OK
Nyq
K band
K band
10
1
170 nm OK
170 nm OK
Nyq
na
0.9-2.5
5 to 10
140nm OK
5-10
JHK
3x1 to 3x3
50 mas for
optimal TT *s
na
na
na
na
na
20 or 35 3K - 4K
na
50
na
3000 to
4000
Astrometric
IFU
accur. Multipli
(mas)
city
Bkgnd
level
Contrast ( 
mag)
Science Inst 1st
priority
> 5.5 at 0.5"
Vis camera
0.05 (rel)
Nyq/4
1
na
na
na
0.1
na
na
1
?
?
0.05 (rel)
<5
na
na
6 to 12
10 to
20% >
atm+tel
na
na
Additional Requirements
Differential tracking (<70"/hr)
Service observing; <10 min overhead to acquire new LGS on-axis target
confusion limited
IR Tip-Tilt needed (consider H or K band)
10km/s accuracy needed; an R~15,000 spectrometer would also be nice
Static wfe~XX nm needed (must quantify); coronagraph needed; L-band imaging would be useful
H=10 @ 0.5"NIR imager/coronagraph IR tip-tilt (both on and off axis); <10 min overhead to acquire new LGS on-axis target
K=4 @ 0.05"
K=4 @ 0.05"
NIR imager
NIR IFU
na
deployable NIR IFUs
low backgrounds are crucial; no. of IFUs should be determined by cost
Key issues that have emerged
• Keep asking “how does this science complement JWST
capabilities?” or “where is NGAO’s sweet spot relative to JWST?”
• Need non-sidereal tracking (asteroids)
• Need rapid retargeting in LGS mode (≤10 min compared with 25
min today)
• Need coronagraph and low residual static WFE (how low?) (planet
detection)
• Need IR tip-tilt (think about H or K for Galactic Ctr)
• Need to understand what is limiting astrometric accuracy for
Galactic Center today (need 0.1 mas)
• Need to understand astrometric implications of having > 1 DM
• Need sky background less than 10-20% above sky + telescope
• Determine # of IFUs from total cost and from design issues (below
what # is it possible to fit all into one dewar?)