Integral Field Spectroscopy
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Transcript of Integral Field Spectroscopy
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Integral Field Spectroscopy
Jeremy Allington-SmithUniversity of Durham
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Contents
• Advantages of Integral Field Spectroscopy• Datacube "theorem"• Techniques of IFS• Lenslet-array• Fibres+lenslets• Image-slicing• Multiple IFS
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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
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Direct image Radial velocity Close up
SAURON: NGC 4365 (Lyon/Durham/Leiden/ESO)
Why use IFS?
"Boring" elliptical galaxy with odd kinematics!
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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
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IFS – use info from adjacent slicesto 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
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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
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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)
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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 observationseach withn x m pixels
Spectral and spatial informationencoded on detector in any way you like
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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 fullfield imaged in one passband
x
y
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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
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Integral field spectroscopy
Devote pixels mostly to spectroscopy:
datacube sliced into narrow spatial fields - repeat observation with different pointings
Each piece contains all thespectra within a narrow field
x
y
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... 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
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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
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Techniques of IFS
Lenslets
Fibres+lenslets
Imageslicer
Telescopefocus
Spectrographinput
Spectrographoutput
Pupilimagery
Fibres
Mirrors
slit
slit
1 2 3 4
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
Like SAURON and OASIS. Overlaps must be avoided low information density
in datacube
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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, 23-35 (2001)
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Lenslet+fibres: optical principle
Microlens array
Pickoff
mirror
Enlarger
fibre
slit
Spectrograph
grating
Fibre bundle
Slit (outof page)
Telescope focus
skyimage
pupil
image
fibre
GMOS-IFUAllington-Smith et alPASP 114, 892 (2002)
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Input
x y
x
y
Original image
Alli
ngto
n-S
mit
h &
Conte
nt,
PA
SP 1
10
,12
16
(1
99
9)
Pseudo-slit
x y
Ensure critical sampling
here!
Fibre+lenslet detection process
x
Detector
y
x
monochromatic image of pseudo-slit
x
yreconstructedmonochromaticimage of sky
Computer
y’
Overlaps here don't
matter
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GMOS• 0.07 arcsec/pixel image scale
• 5.5 x 5.5 arcmin field
• 0.4 - 1.1m wavelength coverage
• R = 10,000 with 0.25” slits
• Multiobject mode using slit masks
• Integral field spectroscopy mode
• Active control of flexure
GMOSwithout enclosure and electronics cabinets
fore opticsupportstructure
IFU/maskcassettes
Gemini instrumentsupportstructure
DewarCCD unitshuttermain optical
support structurecamera
grating turret& indexer unit
filter wheels
collimator
on-instrumentwavefrontsensor
Integral Field Unit
GMOS-IFU
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Slit mask (containing two pseudoslits) interfaces with GMOS mask changer
Location of slits(covered)
The IFU
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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
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4608 pixels
6144pixels
Optionally block off this slit to double spectrumlength but halvefield
1 arcmin
1 slit block containing 2 rows
Field to slit mapping
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4608 pixels
6144pixels
Field to slit mappingOne slit blocked to give• Longer spectra• Half the field (can still beam-switch)
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5.5'
Background subtraction
Field for Adaptive Optics
Objectfield
Backgroundfield
1 arcmin
• Various subtraction strategies • Beam switching supported• Optimised for AO (Altair in I)
Position of reference star during beam-switch
Typical/generousisoplanatic patch
Position of reference star during beam-switch
Typical/generousisoplanatic patch
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One image at each velocity form the datacube (only 4% shown)
One spectrum for each element(only 4% shown)
The IFUrecords aspectrum for each element
Image taken by GMOS withoutusing the IFU
GMOS integral field unit observes NGC1068
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[OIII]
Red Blue
Individualfibre
spectra
NGC1068 - raw data
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• Composite plot of representative [OIII]4959+5007 spectra over the field
• The velocity structure is very complex.
NGC1068 - spectra
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NGC1068 - datacube
•8 x 10" field (mosaiced from 5 pointings)
•Scan through [OIII]5007 line
Miller, Allington-Smith, Turner, Jorgensen
Jet
Galaxydisk
Nucleus
NE
SW
Observer
Bowshock
NE
SW
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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 beforeslicing
Pseudo-slit
Slicing mirror (S1)
Spectrogram
Pupil mirrors(S2)
To spectrograph
Field optics (slit mirrors S3)
From telescopeand fore-optics
Focalplane
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Gemini Near-IR Spectrograph
(0.2 x 0.1 x 0.1)m3
and 1Kg
• Cryogenic 1-5m spectrograph for GEMINI with IFU deployable via slit slide• GNIRS - NOAO, GNIRS-IFU - University of Durham
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Field
Slit
Detector
3.2 "= 21 slices of 0.15"
4.4" = 29 px of 0.15"
2 pixels29 pixels
Detector: 1024 x 1024 pixelsSlit length (short camera)= 100" = 667 pixels
GNIRS-IFU summary • Wavelength range:
– Optimal: 1.0-2.5 m– Total: 1.0-5.0 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
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Optical layout
S1, Slicing mirror
S3, slit mirrors
S2, pupil mirrors
F2, 1st reimagingmirror F1, pickoff
mirror
F3, 2nd reimagingmirror
From GNIRS fore-optics
To GNIRS collimatorSlice 1
Slice 2
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Optical layout
S1
Monolithic S3Monolithic S2
Bi-lithic S1showing split
F2
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Field 46x40" Sampling 0.19x0.19"
Fore-opticsFore-optics
Slicing unit Slicing unit
Blue+Red spectrograph(9 slits)
Fore-opticsFore-optics
Slicing unit Slicing unit
Blue+Red spectrograph(9 slits)
Fore-optics
Slicing unit
Spectrograph(1 slit)
4k x 4kdetector
1 slit
Field 3.8x2.6"Sampling 0.05x0.05"
MOS with IFS? - NGST/IFMOS HR LR
2kx2kdetector
9 slits
Fore-opticsFore-optics
Slicing unit Slicing unit
Blue+Red spectrograph(9 slits)
Work by NGST-IFMOS consortium sponsored by ESA
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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
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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)
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Large-field multi-IFU prototype
•Complete deployable IFU module of 225 elements (Subaru F/2)
•Fishing rod deployment
Individual field15 x 15 (4.5" x 4.5")
Input
Output(slit for test only)
Probe arm + optics
30' primefocus field
Deqing Ren, PhD thesis, 2001. University of Durham
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The enclosing circle is530mm diameterfor a 93mm diameterfield-of-view
UK-ATC
GIRMOS: gnomes around a pond
Feeds fixedimage-slicing IFUs
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• 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