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Integral Field Spectroscopy Jeremy Allington-Smith University of Durham.

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Presentation on theme: "Integral Field Spectroscopy Jeremy Allington-Smith University of Durham."— Presentation transcript:

1 Integral Field Spectroscopy Jeremy Allington-Smith University of Durham

2 Contents Advantages of Integral Field Spectroscopy Datacube "theorem" Techniques of IFS Lenslet-array Fibres+lenslets Image-slicing Multiple IFS

3 What is IFS? Integral field spectroscopy produces a spectrum of each part of an image simultaneously This results in a datacube with axes (x, y,  This is sometimes called "3D imaging" or "2D spectroscopy" or even "3D spectroscopy"! 3D techniques which also produce a datacube but not from a single observation (e.g Fabry-Perot or FTS) are not usually called IFS

4 Direct imageRadial velocityClose up SAURON: NGC 4365 (Lyon/Durham/Leiden/ESO) Why use IFS? "Boring" elliptical galaxy with odd kinematics!

5 Where do you put the slit? Slit gives only a 1D slice through object Slit captures only part of the object's light Only a 3D technique reveals the global velocity field

6 IFS – use info from adjacent slices to correct velocity data Slit spectroscopy – velocities in error since blobs not centred in slit dispersion Generic advantage of IFS Spectroscopy over full 2D field with high filling factor No slit losses - all the light is used Point and shoot target acquisition reduces operational overheads Can reconstruct white-light image to aid interpretation (and target acquisition) Almost immune to atmospheric dispersion More accurate radial velocity determination: –Obtain global velocity field - not just a 1-D section –Velocity field can be reconstructed accurately without errors due to position of features within slit

7 Poor seeing? IFS still helps Gives both high spectral resolution and high throughput Not really IFS since the spatial information is not used Extra benefit of IFS system even in poor seeing Slit is narrow to get high spectral resolution but throughput is low Slicing allows more light to be captured without sacrificing spectral resolution Reformatted slit (still narrow) dispersion

8 Applications Galaxy kinematics: stars and gas (em & abs lines) Distribution of ionising radiation (line ratios) Distribution of stellar populations (lines/continuum) Studies of interacting galaxies (kinematic resolution) Unbiassed searches for primaeval line-emitting galaxies (may be invisible in broadband image) Searches for damped Ly  aborbers near line of sight to QSOs (with large impact parameter) Outflows from young stellar objects

9 Dissecting active galaxies NGC4151 observed with SMIRFS-IFU in J-band - Turner et al. MNRAS 331, 284 (2002) Distribution of [FeII] Velocity field (narrow Pa  )

10 Datacube "theorem" To first order… all 3D methods are equally efficient in generating the same datacube volume with the same number of pixels x y Datacube with same equivalent volume Nnm N observations each with n x m pixels Spectral and spatial information encoded on detector in any way you like

11 Imaging spectroscopy E.g. Fabry-Perot interferometry & narrow-band imaging Devote pixels entirely to imaging: Datacube sliced into thin slices in wavelength. Repeat observations with different wavelength range Sensitive to changes in sky background Each slice contains the full field imaged in one passband x y

12 Longslit spectroscopy Longslit spectroscopy: Each longslit pointing produces a x slice Full datacube produced by stepping longslit in y Each slice is one longslit spectrum x y NB: No spatial information in y within each slice

13 Integral field spectroscopy Devote pixels mostly to spectroscopy: datacube sliced into narrow spatial fields - repeat observation with different pointings Each piece contains all the spectra within a narrow field x y

14 ... to second order? Which technique wins depends mostly on: – the dominant noise source detector read noise detector dark current photon noise from sky photon noise from object temporal variability in sky background –how many pixels you can afford –details of the scientifc application, especially: the size of the total field required the length of the total spectrum required A tradeoff between FTS and IFS for NGST/IFMOS indicated that IFS was preferrable

15 IFS "efficiency" Aim is to maximise a figure of merit that is a function of: # spatial samples, # spectral samples, throughput # spatial samples: pack spectra together tightly along slit. Overlaps will result between samples at the slit but this is okay if: –there is Nyquist sampling of the field at the IFU input –adjacent spectra come from adjacent elements on the sky –there is no wavelength offset between adjacent spectra # spectral samples: maximise length of spectrum to fill complete detector length but, for a given detector, (#spatial  #spectral)  constant so can have multiple slits to increase #spatial by reducing #spectral throughput: efficient design  Make the best possible use of the available detector pixels by minimising the dead space between spectra

16 Techniques of IFS Lenslets Fibres+ lenslets Image slicer Telescope focus Spectrograph input Spectrograph output Pupil imagery Fibres Mirrors slit x y Datacube Both designs maximise the spectrum length and allows more efficient utilisation of detector surface. Only the image slicer retains spatial information within each slice/sample  high information density in datacube Like SAURON and OASIS. Overlaps must be avoided  low information density in datacube

17 Lenslet IFU Example: SAURON* designed for wide-field galaxy kinematics Short wavelength range for low-redshift MgB (517.4nm) Spectra must not overlap otherwise information lost Sauron built by CRAL (Lyon) *Bacon et al. MNRAS 326, (2001)

18 Lenslet+fibres: optical principle Microlens array Pickoff mirror Enlarger fibre slit Spectrograph grating Fibre bundle Slit (out of page) Telescope focus sky image pupil image fibre GMOS-IFU Allington-Smith et al PASP 114, 892 (2002)

19 Input x y x y Original image Allington-Smith & Content, PASP 110,1216 (1999) Pseudo-slit x y Ensure critical sampling here! Fibre+lenslet detection process x Detector y x monochromatic image of pseudo-slit x y reconstructed monochromatic image of sky Computer y’ Overlaps here don't matter

20 GMOS 0.07 arcsec/pixel image scale 5.5 x 5.5 arcmin field  m wavelength coverage R = 10,000 with 0.25” slits Multiobject mode using slit masks Integral field spectroscopy mode Active control of flexure GMOS without enclosure and electronics cabinets fore optic support structure IFU/mask cassettes Gemini instrument support structure Dewar CCD unit shutter main optical support structure camera grating turret & indexer unit filter wheels collimator on-instrument wavefront sensor Integral Field Unit GMOS-IFU

21 Slit mask (containing two pseudoslits) interfaces with GMOS mask changer Location of slits (covered) The IFU

22 Requirements & solutions Exploit good images from GEMINI  0.2" sampling Unit filling factor  Fibres coupled to close-packed lenslet array at input Largest possible object field  7" x 5" (1000 fibres) Provision to optimise accuracy of background subtraction  extra 5" x 3.5" field offset by 60" from object field for background estimation (500 fibres) Transparent change between modes  IFU deployed by mask exchanger, input & output focus coplanar with masks High efficiency  lenslet-coupled at output and input to convert F/16 beam to ~F/5 for efficient use with fibres Use of low risk construction technique (GEMINI request to reduce risk to schedule)  fibre+lenslet not image slicer

23 4608 pixels 6144 pixels Optionally block off this slit to double spectrum length but halve field 1 arcmin 1 slit block containing 2 rows Field to slit mapping

24 4608 pixels 6144 pixels Field to slit mapping One slit blocked to give Longer spectra Half the field (can still beam-switch)

25 5.5' Background subtraction Field for Adaptive Optics Object field Background field 1 arcmin Various subtraction strategies Beam switching supported Optimised for AO (Altair in I) Position of reference star during beam-switch Typical/generous isoplanatic patch Position of reference star during beam-switch Typical/generous isoplanatic patch

26 GMOS-IFU performance Direct image of one block with uniform illumination Filter IFU throughput g 62% r 65% i 62% z 58% Lab (633nm) 68% Theory 59% Reconstructed white-light image of star (~0.6"  same as in direct image) Object field Background field

27 One image at each velocity form the datacube (only 4% shown) One spectrum for each element (only 4% shown) The IFU records a spectrum for each element Image taken by GMOS without using the IFU GMOS integral field unit observes NGC1068

28 [OIII] RedBlue Individual fibre spectra NGC raw data

29 Composite plot of representative [OIII] spectra over the field The velocity structure is very complex. NGC spectra

30 NGC datacube 8 x 10" field (mosaiced from 5 pointings) Scan through [OIII]5007 line Miller, Allington-Smith, Turner, Jorgensen Jet Galaxy disk Nucleus NE SW Observer Bowshock NE SW

31 Advanced Image Slicer (AIS) Developed from MPE's 3D by the University of Durham for highly- efficient spectroscopy over a two- dimensional field Optimum use of detector pixels since complete slices of sky are imaged (no dead space between spatial samples) Correct spectral sampling is obtained without degrading spatial resolution in dispersion direction Diffraction is only a 1-D issue  reduction in optics size/mass Optics may be diamond-turned from the same material as the mount to reduce thermal mismatch  good for space/cryo applications Adopted by GEMINI 8m Telescopes Project (GNIRS-IFU) and proposed by ESA for NGST Field before slicing Pseudo-slit Slicing mirror (S1) Spectrogram Pupil mirrors (S2) To spectrograph Field optics (slit mirrors S3) From telescope and fore-optics Focal plane

32 Gemini Near-IR Spectrograph (0.2 x 0.1 x 0.1)m 3 and 1Kg Cryogenic 1-5  m spectrograph for GEMINI with IFU deployable via slit slide GNIRS - NOAO, GNIRS-IFU - University of Durham

33 GNIRS-IFU summary Wavelength range: –Optimal:  m –Total:  m Field: 3.2”x 4.4” Sampling: 0.15” Spatial elements: 625 Spectrum length: 1024 px Cryogenic environment IFU fits in module in GNIRS slit slide

34 Optical layout S1, Slicing mirror S3, slit mirrors S2, pupil mirrors F2, 1st reimaging mirror F1, pickoff mirror F3, 2nd reimaging mirror From GNIRS fore-optics To GNIRS collimator Slice 1 Slice 2

35 Optical layout S1 Monolithic S3 Monolithic S2 Bi-lithic S1 showing split F2

36 Field 46x40" Sampling 0.19x0.19" Fore-optics Slicing unit Blue+Red spectrograph (9 slits) Fore-optics Slicing unit Blue+Red spectrograph (9 slits) Fore-optics Slicing unit Spectrograph (1 slit) 4k x 4k detector 1 slit Field 3.8x2.6" Sampling 0.05x0.05" MOS with IFS? - NGST/IFMOS HR LR 2kx2k detector 9 slits Fore-optics Slicing unit Blue+Red spectrograph (9 slits) Work by NGST- IFMOS consortium sponsored by ESA

37 Did IFMOS get on NGST? Work by NGST-IFMOS consortium sponsored by ESA. Picture from Astrium No, but small- field IFU may be included in NIRSPEC alongside MOS mode

38 Multiple IFS IFS of multiple targets over wide field via deployable IFUs  MOS with mapping to e.g. measure mass of many galaxies Total number of elements set by number of detector pixels: –This must be divided amongst the different IFUs –For example, 20 modules with 200 elements each could be accommodated on a 4k x 4k detector  small field/module Main focus is on near-infrared Exploit "wide-field" AO on GEMINI and VLT Existing small-field IFU system: VLT/Flames (NB: Falcon) Prototyping underway for image-slicing (e.g. VLT/KMOS)

39 Large-field multi-IFU prototype Complete deployable IFU module of 225 elements (Subaru F/2) Fishing rod deployment Individual field 15 x 15 (4.5" x 4.5") Input Output (slit for test only) Probe arm + optics 30' prime focus field Deqing Ren, PhD thesis, University of Durham

40 The enclosing circle is 530mm diameter for a 93mm diameter field-of-view UK-ATC GIRMOS: gnomes around a pond Feeds fixed image-slicing IFUs

41 stepping motor drive via worm gears for both ‘shoulder’ and ‘elbow’ actions two tubular arms in CFRP the arms are not co-planar four folds in each optical path light re-imaged at x1.5 magnification UK-ATC light path To fixed image slicer IFU From fore-optics GIRMOS pickoff arm


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