View
216
Download
1
Category
Tags:
Preview:
Citation preview
3D Spectroscopy3D Spectroscopy
Francisco Müller Sánchez
Instituto de Astrofísica de Canarias
La Laguna, España
Introductory Review and Observational Techniques
Science motivation for 3D Spectroscopy Instrumentation Preparation of Observations and Principles of Data Reduction
Data Analysis
Examples of SINFONI-AO: prototypical merger Examples of SINFONI-AO: prototypical merger NGC6240 NGC6240
velo
city
flux
stars molecular gas ionised gas1”
Analysis of data cubes Analysis of data cubes
Introductory Review and Observational Introductory Review and Observational techniquestechniques
Classical observational techniques Strengths of 3D Data Concepts of Adaptive Optics Instrument techniques used to achieve 3D Spectroscopy
Beckers 1993: ARA&A 31, 13Hardy 1998: Adaptive Optics for Astronomical TelescopesAntichi 2009: ApJ, 695, 1042Kissler-Patig 2005: Science perspectives for 3D spectroscopyalso Sterne & Weltraum articles 1994 (Hippler, Kasper, Davies, Ragazzoni)
Ancient Mayan PhotometryAncient Mayan Photometry
- An eclipse table that predicts times when eclipses may occur.
- A Venus table that predicts the times when Venus appears as morning star and the other apparitions of the planet.
- A Mars table that records the times when Mars goes into retrograde motion. A second Mars table that tracks the planet's motion along the ecliptic has recently been identified.
PhotometryPhotometry
The venerable photographic plate and its more recent version, the CCD, provide objective information in two dimensions concerning the brightness, I(x,y), of an extended object or area of sky.
SpectroscopySpectroscopy
SpectroscopySpectroscopy
Long-slit spectroscopy enables us to split the light that reaches us not just from a point but also from an entire line of points into its constituent colours, thereby providing us with information in two dimensions - position along the slit, x, and colour, lambda:I(x,lambda)
Why not combine them?Why not combine them?
3D spectroscopy attempts to get closer to the fundamental goal of astronomical observing techniques, which is to record the direction, wavelength, polarization state and arrival time for every incoming photon over the largest field of view. In fact using 3D spectroscopy, the wavelength and the incoming direction in a 2D field of view are recorded in a (x,y,λ) data cube, in contrast with standard techniques which either do imaging over a 2D field, or spectroscopy along a 1D slit.
Color
3D spectroscopy yields datacubes3D spectroscopy yields datacubes
Scanning Spectrophotometers (Fabry-Perot Scanning Spectrophotometers (Fabry-Perot interferometer)interferometer)
The FPI can be used to obtain monochromatic images over a full two-dimensional field of view with spectral resolutions comparable to those of grating spectrographs. In a Fabry-Pérot the distance between the plates can be tuned in order to change the wavelengths at which transmission peaks occur in the interferometer.
Imaging Fourier Transform SpectroscopyImaging Fourier Transform Spectroscopy
Imaging SpectroscopyImaging Spectroscopy
Scanning Long-Slit SpectroscopyScanning Long-Slit Spectroscopy
Energy-Resolving DetectorsEnergy-Resolving Detectors
Tantalum superconducting tunnel junctions Peacock et al. 1998
Integral-Field UnitsIntegral-Field Units
Concept of integral-field spectroscopyConcept of integral-field spectroscopy
Don’t confuse IFS with MOS (Multi Object Don’t confuse IFS with MOS (Multi Object Spectroscopy)Spectroscopy)
LUCIFER at the LBT
Three ways of doing IFSThree ways of doing IFS
Lenslets (TIGER Approach)Lenslets (TIGER Approach)
Example of lenslet IFU: SAURON @ WHTExample of lenslet IFU: SAURON @ WHT
Fibers (ARGUS approach)Fibers (ARGUS approach)
Example of fibers IFU: INTEGRAL @ WHTExample of fibers IFU: INTEGRAL @ WHT
SlicersSlicers
SINFONI - made @ MPE
Example of Image slicer IFU: SINFONI @ VLTExample of Image slicer IFU: SINFONI @ VLT
Strengths of 3D data 1Strengths of 3D data 1
- No slit losses: high system efficiency
- Less time consuming
- More accurate radial velocity determination
- Background estimate can be obtained simultaneously
- Kinematics of crowded regions
- It doesn’t suffer from changes of several exposures
Strengths of 3D data 2. Einstein’s cross sectionStrengths of 3D data 2. Einstein’s cross section
Atmospheric TurbulenceAtmospheric Turbulence
2)'()'()( rrfrfrD
for Kolmogorov statistics, the refractive index structure function is
van Karman model includes inner (~1cm) & outer (~30m) scales
3/22)( rCrD Nn
0
23/52
sec2
91.2)( dhCrrD N
for a wavefront propagating through the atmosphere, the phase structure function is
quantified using the structure function
CN2 at Mt Graham
(LBT site)
Atmospheric TurbulenceAtmospheric Turbulence
CN2 is refractive index
structure constant.
Turbulence limits the resolution of a telescope to λ/r0 instead of λ/D.
5/3
0
20
dhCr N
The integral of CN2 is
Fried’s parameter
and variance of wavefront aberrations is just
3/5
0
2 030.1
r
D
Everything depends on CEverything depends on CNN22
5/3
0
25/35/60 sec185.0
dhCr N
coherence length 0r
where
windVr00 314.0
5/3
0
2
0
3/52
dhCdhvCV NNwind
coherence timescale
where
assumes Taylor’s frozen flow hypothesis
Hr /314.0 00 5/3
0
2
0
3/52sec
dhCdhhCH NNwhere
isoplanatic angle
Impact of a Perturbed WavefrontImpact of a Perturbed Wavefront
blurPoint focus
parallel light rays can be focussed
light rays affected by turbulence
how well spatial frequencies are transferred through the optical system
resulting shape of a point source
coma & trefoil
Modal DecompositionModal Decomposition
Most common & simplest for a circular aperture are Zernike modes.
For an annular aperture, Karhunen-Loève modes are better.
A simple adaptive optics systemA simple adaptive optics system
open & closed loop images
Neptune (Keck, NGS)
star (Calar Alto, LGS)
Shack Hartmann SensorShack Hartmann Sensor
Measures first derivative of wavefront (gradients)
Displacement of spots is proportional to the wavefront tilt
Many algorithms possible for centroiding
Easy to extend to very high order systems
Divides pupil into subapertures
(developed in 1900 by J.Hartmann)
Shack Hartmann SensorShack Hartmann Sensor
Piezo Actuator MirrorsPiezo Actuator Mirrors
349 actuator DM
wiring on back sidereference block
thin flexible (glass) mirror
piezo actuators which contract & lengthen when voltages are applied
incoming wavefront will be flat when it reflects off the mirror
Curvature SensorCurvature Sensor (developed in 1994 by F.Roddier)
A few things to bear in mindA few things to bear in mind
- AO works better at longer wavelengths (dependence of r0 on λ6/5)e.g. consider a phase change of 250nm with respect to 500nm optical light and 2.2μm near infrared light. So at longer wavelengths, coherence length is greater & timescales are longer
- One can measure in optical & correct in infrared (absolute phase change is same)- AO systems have to run fast (bandwidth ~1/10 of the frame rate)
prediction would be great…
time
ampl
itude
of a
berr
atio
n
Residual Wavefront Variance & Strehl RatioResidual Wavefront Variance & Strehl Ratio
coherence length
isoplanatic angle
3/50
232 2944.0~ rDjfitting
3/50
2 angle
total wavefront variance ...22222 noisetimedelayanglefittingtotal
for large j (number of Zernike modes)
Strehl ratio 2exp~ SR ratio of peak intensity to that for a perfect optical system
coherence timescale 3/50
2 timedelay
Sodium & Rayleigh Laser Guide StarsSodium & Rayleigh Laser Guide Stars
MMT
VLT
Keck
sky coverage few % with NGS but ~50% with LGS(most coverage in galactic plane; almost none at galactic pole)
Starfire Optical Range, Calar Alto, Lick, MMT, Keck, VLT, Subaru, Gemini North, WHT, Palomar 200”, Mt Wilson 100”, (LBT, Gemini South)
A few issues with Laser Guide StarsA few issues with Laser Guide Stars
sodium density
heig
ht (
km)
time (min)
on-axis LGS spot
off-axis LGS spot
1. laser technology2. elongation of spot due to finite thickness of layer 3. variations in height of sodium layer 4. need for tip-tilt star
MultiConjugate Adaptive OpticsMultiConjugate Adaptive Optics
1 star & 1 DM
3 stars & 2 DMs
MAD strehl maps reference stars
high turbulence layer
low turbulence layer
telescope
one wavefront sensor per star
DM1
DM2
WFSs
MultiConjugate Adaptive OpticsMultiConjugate Adaptive Optics
This is computationally complex
Classical MCAO needs multiple guide stars (e.g. Gemini South MCAO needs 5 LGS & 3 NGS).
Instead, one can use the layer oriented approach, with LGS or NGS.
reference stars
high turbulence layer
low turbulence layer
telescope
one wavefront sensor per deformable mirror
DM1
DM2
WFS1
WFS1
LINC-NIRVANA on the LBT uses pyramid sensors to co-add the light from many faint stars on the detector;but note that the strehl ratio is expected to be limited & vary a bit over the field
Examples of LGS-AO: interacting galaxies IRAS 09061-Examples of LGS-AO: interacting galaxies IRAS 09061-12481248
NACO-LGS/VLTUKIRT (archive)
K-band image of these interacting galaxies shows the vast amount more detail that LGS-AO can reveal
Examples of LGS-AO: prototypical merger NGC6240 Examples of LGS-AO: prototypical merger NGC6240
Komossa et al. 2003 Tecza et al. 2000
2µm continuum
1”
Examples of LGS-AO: prototypical merger NGC6240 Examples of LGS-AO: prototypical merger NGC6240 ve
loci
tyflu
x
stars molecular gas ionised gas1”
Examples of LGS-AO: high redshift galaxiesExamples of LGS-AO: high redshift galaxies
Future perspectives: FRIDA @ GTCFuture perspectives: FRIDA @ GTC
Future perspectives: SERPIL @ LBTFuture perspectives: SERPIL @ LBT
Multiple IFS: KMOS @ VLTMultiple IFS: KMOS @ VLT
Multiple IFS: KMOS @ VLTMultiple IFS: KMOS @ VLT
IFS @ ELTIFS @ ELT
Outlook for tomorrow’s lectureOutlook for tomorrow’s lecture
Science perspectives for IFS
Galactic astronomy
The Galactic Center
Nearby AGN Quasars and high-z galaxies
Bimorph MirrorsBimorph Mirrors
bimorph mirror for Gemini, showing the zones
2 layer piezo ceramic which bends when a voltage is applied
continuous electrode
control electrodes
thin glass mirror
incoming wavefront will be flat when it reflects off the mirror
Realistic ExpectationsRealistic ExpectationsExtreme AO (e.g. “planet finders”) aims for >90% strehl at K… but with bright stars
AGN are not particularly bright (fainter than typical limit of R~15mag), and tend to be fuzzy with a relatively bright background.Off-axis correction is usually not an option.LGS performance can vary from 0.1” resolution to ~20% Strehl at K.
One can do much better than the seeing limit, but don’t expect perfect performance every time; and beware of spatial & temporal
variations
5”
600nm2.2µm
5”
Circinus Galaxy
no bright point source for AO reference; and bright background.
with an IR-WFS (i.e. NACO)
adapted from Rigaut 2000
Multiple Layers of TurbulenceMultiple Layers of Turbulence
Turbulence Layers
with 2 turbulent layers, on- and off-axis wavefronts are different
adapted from Rigaut 2000
Deformable mirror
Turbulence Layers
Multiple Layers of TurbulenceMultiple Layers of Turbulence
with 2 turbulent layers, on- and off-axis wavefronts are different
and cannot be corrected with a single DM
Multiple Layers of TurbulenceMultiple Layers of Turbulence
adapted from Rigaut 2000
with 2 turbulent layers, on- and off-axis wavefronts are different
Deformable mirrors
Turbulence Layers
but they can be corrected with multi-conjugate DMs
and cannot be corrected with a single DM
Recommended