Presentation is loading. Please wait.

Presentation is loading. Please wait.

Integral Field Spectroscopy

Similar presentations

Presentation on theme: "Integral Field Spectroscopy"— 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,l) 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 "Boring" elliptical galaxy with odd kinematics!
Why use IFS? "Boring" elliptical galaxy with odd kinematics! Direct image Radial velocity Close up SAURON: NGC 4365 (Lyon/Durham/Leiden/ESO)

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 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 IFS – use info from adjacent slices to correct velocity data Slit spectroscopy – velocities in error since blobs not centred in slit dispersion

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 dispersion Reformatted slit (still narrow) Slicing allows more light to be captured without sacrificing spectral resolution

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 Lya aborbers near line of sight to QSOs (with large impact parameter) Outflows from young stellar objects

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

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

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

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

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 y  x Lenslets slit Fibres+ lenslets Image slicer
Like SAURON and OASIS. Overlaps must be avoided  low information density in datacube Lenslets Fibres+ lenslets Image slicer Telescope focus Spectrograph input output Pupil imagery Fibres Mirrors slit 1 2 3 4 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

17 Sauron built by CRAL (Lyon)
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 bundle Slit (out of page) Telescope focus Spectrograph grating GMOS-IFU Allington-Smith et al PASP 114, 892 (2002) fibre slit sky image pupil fibre

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

20 GMOS-IFU Integral Field Unit GMOS 0.07 arcsec/pixel image scale
5.5 x 5.5 arcmin field mm 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 Dewar CCD unit shutter main optical support structure camera grating turret & indexer unit filter wheels collimator on-instrument wavefront sensor

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

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

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

25 Background subtraction
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 Field for Adaptive Optics Object field Background 1 arcmin 5.5'

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

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

28 NGC raw data [OIII] Red Blue Individual fibre spectra

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

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

31 Advanced Image Slicer (AIS)
Pseudo-slit Slicing mirror (S1) Spectrogram Pupil mirrors (S2) To spectrograph Field optics (slit mirrors S3) From telescope and fore-optics 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 Focal plane Field before slicing

32 Gemini Near-IR Spectrograph

33 GNIRS-IFU summary Wavelength range: Field: 3.2”x 4.4” Sampling: 0.15”
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 The main instrument capabilities are summarized in the table below. Image Scale: arcsec/pixel (long camera 0.15 arcsec/pixel (short camera) Slit length arcsec (long camera) 100 arcsec (short camera) Slit widths , 0.15, 0.20, 0.30, 0.45, 0.6, 1, 3 arcsec + "open" Wavelength range m m Resolutions: ,6000,18000 (long camera) 667, 2000, 6000 (short camera) Other options present: cross-dispersion ( m m) Wollaston prism for polarization analysis Detector K x 1K InSb 27 m m pixels (ALADDIN) In addition, a rear slit viewing configuration will be used to facilitate acquisition of objects, and the internal (IR) wavefront sensor will provide tip-tilt correction and fine focus correction. Overall efficiency of the instrument depends not only on instrumental throughput, but on ease and speed of set-up, and on configuration stability and repeatability as well. It is important to minimize the amount of observing time that mustbe used for real-time calibration sequences.

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

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

HR LR Field 46x40" Sampling 0.19x0.19" Field 3.8x2.6" Sampling 0.05x0.05" Work by NGST-IFMOS consortium sponsored by ESA 2kx2k detector 9 slits 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

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

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

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

41 To fixed image slicer IFU
GIRMOS pickoff arm 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 light path To fixed image slicer IFU From fore-optics UK-ATC

Download ppt "Integral Field Spectroscopy"

Similar presentations

Ads by Google