Integral Field Spectroscopy
Jeremy Allington-Smith
University of Durham
Contents
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Advantages of Integral Field Spectroscopy
Datacube "theorem"
Techniques of IFS
Lenslet-array
Fibres+lenslets
Image-slicing
Multiple IFS
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
Why use IFS?
"Boring" elliptical galaxy with odd kinematics!
Direct image
Radial velocity
Close up
SAURON: NGC 4365 (Lyon/Durham/Leiden/ESO)
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
Generic advantage of IFS
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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
Slit spectroscopy – velocities in error
since blobs not centred in slit
IFS – use info from adjacent slices
to correct velocity data
dispersion
Applications
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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
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)
Datacube "theorem"
To first order… all 3D methods are equally efficient
in generating the same datacube volume with the
same number of pixels
N observations
each with
n x m pixels
Datacube
with same
equivalent
volume Nnm
y
x
l
Spectral and spatial information
encoded on detector in any way you like
Imaging spectroscopy
E.g. Fabry-Perot interferometry & narrow-band imaging
Each slice contains the full
field imaged in one passband
Devote pixels entirely to
imaging:
Datacube sliced into thin
slices in wavelength.
y
x
l
Repeat observations with
different wavelength range
Sensitive to changes in sky
background
Longslit spectroscopy
Each slice is one
longslit spectrum
Longslit spectroscopy:
Each longslit pointing
produces a xl slice
y
x
l
Full datacube produced by
stepping longslit in y
NB: No spatial information in y within each slice
Integral field spectroscopy
Each piece contains all the
spectra within a narrow field
Devote pixels mostly to
spectroscopy:
y
x
l
datacube sliced into narrow
spatial fields - repeat
observation with different
pointings
... to second order?
• Which technique wins depends mostly on:
– the dominant noise source
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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
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
Techniques of IFS
Telescope
focus
Spectrograph
input
Spectrograph
output
Like SAURON and OASIS.
Overlaps must be avoided
 low information density
in datacube
Pupil
imagery
Lenslets
Datacube
Fibres+
lenslets
slit
Fibres
y
x
Image
slicer
Mirrors
1
1
2
3
4
l
slit
2 3 4
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
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)
Lenslet+fibres: optical principle
Slit (out
of page)
Pickoff
mirror
Enlarger
Microlens
array
Telescope
focus
GMOS-IFU
Allington-Smith et al
PASP 114, 892 (2002)
sky
Fibre bundle
image
pupil
image
fibre
fibre
grating
slit
Spectrograph
Fibre+lenslet detection process
Input
image
Original
x
y
x
Pseudo-slit
y
x
Overlaps here
don't matter
y
Computer
Detector
x
y
monochromatic image of pseudo-slit
x
y’
reconstructed
monochromatic
image of sky
y
x
Allington-Smith & Content, PASP 110,1216 (1999)
Ensure critical
sampling here!
GMOS-IFU
GMOS
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0.07 arcsec/pixel image scale
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5.5 x 5.5 arcmin field
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0.4 - 1.1mm wavelength coverage
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R = 10,000 with 0.25” slits
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Multiobject mode using slit masks
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Integral field spectroscopy mode
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Active control of flexure
collimator
Integral Field Unit
Gemini instrument
support
structure
fore optic
support
structure
on-instrument
wavefront
sensor
filter wheels
IFU/mask
cassettes
GMOS
without enclosure and
electronics cabinets
grating turret
& indexer unit
camera
main optical
shutter
support structure
CCD unit Dewar
The IFU
Location of slits
(covered)
Slit mask
(containing two
pseudoslits)
interfaces with
GMOS mask
changer
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
Field to slit mapping
1 slit block containing 2 rows
6144
pixels
1 arcmin
Optionally block
off this slit to
double spectrum
length but halve
field
4608 pixels
Field to slit mapping
One slit blocked to give
• Longer spectra
• Half the field (can still beam-switch)
6144
pixels
4608 pixels
Background subtraction
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Various subtraction strategies
Beam switching supported
Optimised for AO (Altair in I)
5.5'
Typical/generous
isoplanatic patch
Position of reference
star during beam-switch
Object
field
Background
field
1 arcmin
Field for Adaptive Optics
GMOS integral field unit observes NGC1068
Image taken
by
GMOS
without
using the IFU
One
image
at each
velocity
form the
datacube
(only 4%
shown)
The IFU
records a
spectrum
for each
element
One spectrum
for each
element
(only 4%
shown)
NGC1068 - raw data
Red
Individual
fibre
spectra
Blue
[OIII]
NGC1068 - spectra
• Composite plot of
representative
[OIII]4959+5007
spectra over the field
• The velocity structure
is very complex.
NGC1068 - datacube
NE
• 8 x 10" field
(mosaiced from
5 pointings)
• Scan through
[OIII]5007 line
Bowshock
NE
Jet
Observer
SW
Galaxy
disk
Nucleus
SW
Miller, Allington-Smith, Turner, Jorgensen
Advanced Image
Slicer (AIS)
To spectrograph
Field optics
(slit mirrors S3)
• Developed from MPE's 3D by the
University of Durham for highlyefficient spectroscopy over a twodimensional field
Pseudo-slit
Focal
plane
• Optimum use of detector pixels since
complete slices of sky are imaged (no
dead space between spatial samples)
Spectrogram
Slicing mirror (S1)
• 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
Pupil mirrors
(S2)
Field before
slicing
From telescope
and fore-optics
Gemini Near-IR Spectrograph
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Cryogenic 1-5mm spectrograph for GEMINI with IFU deployable via slit slide
GNIRS - NOAO, GNIRS-IFU - University of Durham
(0.2 x 0.1 x 0.1)m3
and 1Kg
GNIRS-IFU summary
4.4" = 29 px of 0.15"
• Wavelength range:
– Optimal: 1.0-2.5 mm
– Total: 1.0-5.0 mm
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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
Field
3.2 "= 21 slices of 0.15"
29 pixels
2 pixels
Slit
Detector
Detector: 1024 x 1024 pixels
Slit length (short camera)
= 100" = 667 pixels
Optical layout
From GNIRS fore-optics
F2, 1st reimaging
mirror
F1, pickoff
mirror
S3, slit mirrors
S2, pupil
mirrors
S1, Slicing mirror
Slice 1
Slice 2
F3, 2nd
reimaging
mirror
To GNIRS collimator
Optical layout
Monolithic S3
Monolithic S2
F2
S1
Bi-lithic S1
showing split
MOS with IFS? - NGST/IFMOS
HR
Fore-optics
Field 46x40"
Sampling 0.19x0.19"
LR
Field 3.8x2.6"
Sampling 0.05x0.05"
Fore-optics
Fore-optics
Fore-optics
Slicing unit
Slicing unit
4k x 4k
detector
1 slit
Blue+Red spectrograph
(9 slits)
2kx2k
detector
9 slits
Fore-optics
Fore-optics
Slicing unit
Slicing unit
Spectrograph
(1 slit)
Work by NGSTIFMOS consortium
sponsored by ESA
Fore-optics
Slicing unit
Slicing unit
Blue+Red spectrograph
(9 slits)
Slicing unit
Blue+Red spectrograph
(9 slits)
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
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
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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)
Large-field
multi-IFU
prototype
• Complete deployable IFU module
of 225 elements (Subaru F/2)
• Fishing rod deployment
Output
(slit for test only)
Input
30' prime
focus field
Probe arm + optics
Individual field
15 x 15 (4.5" x 4.5")
Deqing Ren, PhD thesis, 2001. University of Durham
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
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
UK-ATC
To fixed image slicer IFU
From fore-optics