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3D 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 NGC6240 velocity flux 1” stars molecular gas ionised gas Analysis of data cubes Introductory Review and Observational techniques Beckers 1993: ARA&A 31, 13 Hardy 1998: Adaptive Optics for Astronomical Telescopes Antichi 2009: ApJ, 695, 1042 Kissler-Patig 2005: Science perspectives for 3D spectroscopy also Sterne & Weltraum articles 1994 (Hippler, Kasper, Davies, Ragazzoni) Classical observational techniques Strengths of 3D Data Concepts of Adaptive Optics Instrument techniques used to achieve 3D Spectroscopy Ancient 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. Photometry 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. Spectroscopy Spectroscopy 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? 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. 3D spectroscopy yields datacubes Color Scanning Spectrophotometers (Fabry-Perot interferometer) The FPI can be used to obtain monochromatic images over a full twodimensional 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 Spectroscopy Imaging Spectroscopy Scanning Long-Slit Spectroscopy Energy-Resolving Detectors Tantalum superconducting tunnel junctions Peacock et al. 1998 Integral-Field Units Concept of integral-field spectroscopy Don’t confuse IFS with MOS (Multi Object Spectroscopy) LUCIFER at the LBT Three ways of doing IFS Lenslets (TIGER Approach) Example of lenslet IFU: SAURON @ WHT Fibers (ARGUS approach) Example of fibers IFU: INTEGRAL @ WHT Slicers Example of Image slicer IFU: SINFONI @ VLT SINFONI - made @ MPE Strengths 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 section Atmospheric Turbulence quantified using the structure function D( r ) f (r' ) f (r'r )2 for Kolmogorov statistics, the refractive index structure function is Dn ( r ) CN2 r 2 / 3 for a wavefront propagating through the atmosphere, the phase structure function is 2 5/ 3 D ( r ) 2.91 sec r CN2 dh 0 2 van Karman model includes inner (~1cm) & outer (~30m) scales Atmospheric Turbulence CN2 is refractive index structure constant. The integral of CN2 is Fried’s parameter r0 C dh 0 3 / 5 2 N and variance of wavefront aberrations is just D 1.030 r0 5/ 3 2 Turbulence limits the resolution of a telescope to λ/r0 instead of λ/D. CN2 at Mt Graham (LBT site) Everything depends on CN2 coherence length r0 where coherence timescale r0 0.1856 / 5 sec CN2 dh 0 3/ 5 3 / 5 0 0.314r0 Vwind Vwind CN2 v 5 / 3dh 0 where 0 CN2 dh 3/ 5 assumes Taylor’s frozen flow hypothesis isoplanatic angle 0 0.314r0 / H H sec C h dh 0 where 2 N 5/ 3 0 C dh 2 N 3/ 5 Impact of a Perturbed Wavefront parallel light rays can be focussed Point focus how well spatial frequencies are transferred through the optical system light rays affected by turbulence blur resulting shape of a point source Modal Decomposition Most common & simplest for a circular aperture are Zernike modes. For an annular aperture, Karhunen-Loève modes are better. coma & trefoil A simple adaptive optics system open & closed loop images Neptune (Keck, NGS) star (Calar Alto, LGS) Shack Hartmann Sensor (developed in 1900 by J.Hartmann) 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 Shack Hartmann Sensor Piezo Actuator Mirrors incoming wavefront will be flat when it reflects off the mirror 349 actuator DM thin flexible (glass) mirror reference block piezo actuators which contract & lengthen when voltages are applied wiring on back side Curvature Sensor (developed in 1994 by F.Roddier) A few things to bear in mind amplitude of aberration - 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 Residual Wavefront Variance & Strehl Ratio coherence length 2fitting ~ 0.2944 j 3 2 D r0 5 / 3 for large j (number of Zernike modes) coherence timescale 5/ 3 2 timedelay 0 isoplanatic angle total wavefront variance 2 2 2 2 total 2fitting angle timedelay noise ... Strehl ratio SR ~ exp 2 2 angle 0 5/ 3 ratio of peak intensity to that for a perfect optical system Sodium & Rayleigh Laser Guide Stars sky coverage few % with NGS but ~50% with LGS (most coverage in galactic plane; almost none at galactic pole) Keck MMT VLT 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 Stars 1. laser technology 2. elongation of spot due to finite thickness of layer 3. variations in height of sodium layer 4. need for tip-tilt star on-axis LGS spot off-axis LGS spot height (km) sodium density time (min) MultiConjugate Adaptive Optics MAD strehl maps reference stars 1 star & 1 DM high turbulence layer low turbulence layer telescope 3 stars & 2 DMs DM2 DM1 WFSs one wavefront sensor per star MultiConjugate Adaptive Optics Classical MCAO needs multiple guide stars (e.g. Gemini South MCAO needs 5 LGS & 3 NGS). reference stars high turbulence layer This is computationally complex Instead, one can use the layer oriented approach, with LGS or NGS. 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 low turbulence layer telescope DM2 DM1 WFS1 WFS1 one wavefront sensor per deformable mirror Examples of LGS-AO: interacting galaxies IRAS 09061-1248 K-band image of these interacting galaxies shows the vast amount more detail that LGS-AO can reveal UKIRT (archive) NACO-LGS/VLT Examples of LGS-AO: prototypical merger NGC6240 2µm continuum 1” Komossa et al. 2003 Tecza et al. 2000 Examples of LGS-AO: prototypical merger NGC6240 velocity flux 1” stars molecular gas ionised gas Examples of LGS-AO: high redshift galaxies Future perspectives: FRIDA @ GTC Future perspectives: SERPIL @ LBT Multiple IFS: KMOS @ VLT Multiple IFS: KMOS @ VLT IFS @ ELT Outlook for tomorrow’s lecture Science perspectives for IFS Galactic astronomy The Galactic Center Nearby AGN Quasars and high-z galaxies Bimorph Mirrors 2 layer piezo ceramic which bends when a voltage is applied incoming wavefront will be flat when it reflects off the mirror thin glass mirror continuous electrode control electrodes bimorph mirror for Gemini, showing the zones Realistic Expectations Extreme 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 600nm 2.2µm Circinus Galaxy 5” no bright point source for AO reference; and bright background. 5” with an IR-WFS (i.e. NACO) Multiple Layers of Turbulence with 2 turbulent layers, on- and off-axis wavefronts are different Turbulence Layers adapted from Rigaut 2000 Multiple Layers of Turbulence with 2 turbulent layers, on- and off-axis wavefronts are different and cannot be corrected with a single DM Turbulence Layers Deformable mirror adapted from Rigaut 2000 Multiple Layers of Turbulence with 2 turbulent layers, on- and off-axis wavefronts are different and cannot be corrected with a single DM Turbulence Layers Deformable mirrors but they can be corrected with multi-conjugate DMs adapted from Rigaut 2000