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
Cryogenic 1-5m 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
LIGHT PICKED-OFF FROM TELESCOPE FOCAL PLANE TAKE ITS WAY THROUGH OFNER RELAY AND SLICER IFU DECKER SLIDE – A BOX OF 20X10X10CM AND 1 KG – TO BE SLICED AND SLITED BEFORE GO TO COLIMATION AND FOCOLIZATION BY CAMERA.

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

36. MOS with IFS? - NGST/IFMOSHR 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