Integral Field Spectroscopy'

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Integral Field Spectroscopy.

• David Lee,
• Anglo-Australian Observatory.
• dl_at_aaoepp.aao.gov.au
• http//www.aao.gov.au/local/www/dl

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Why integral field spectroscopy?

• Traditional spectroscopic techniques include
• Longslit spectroscopy. This provides a spectrum
  with one dimensional spatial information along
  the slit.
• Multiple-object spectroscopy either with multiple
  slits or multiple fibres. These techniques
  simultaneously provide spectral information from
  many objects (100) but with limited spatial
  information along the slit and no spatial
  information from fibres.
• For observations of many types of object it would
  be useful to obtain information about the
  two-dimensional spatial structure as well as
  spectral information.
• Integral field spectroscopy is a relatively new
  technique developed to achieve this.

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Integral field spectroscopy.
Integral field spectroscopy is a technique to
produce a spectrum for each spatial element in an
extended two-dimensional field. The observation
produces a data-cube containing both spatial and
spectral information.
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Various methods of IFS
http//aig-www.dur.ac.uk/
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Advantages of IFS

• Obtain both imaging and spectroscopic information
  simultaneously - maximises the information
  available on the detector.
• In bad seeing the large field of view of an IFS
  will help to prevent slit losses.
• Resolution is fixed by the fibre / mirror size
  not by the slit width.
• Use of optical fibres allows the instrument to be
  removed from the telescope and located in a more
  stable environment.
• Target acquisition is straightforward.

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Disadvantages of IFS

• The integral field unit optics can decrease the
  transmission of the instrument.
• Accurate sky subtraction becomes more difficult
  than with a longslit or multiple-slit
  spectrograph.
• Data analysis can be difficult.
• The slit length is much less than with a
  longslit spectrograph.

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Science with IFS

• Spectroscopy of extended objects
• Spatially resolved spectroscopy (aperture effect)
• Dynamics, kinematics, velocity maps - rotation
  curves
• Velocity dispersion information
• Variation of spectrum within object (starburst /
  AGN etc)
• Line strength distributions
• Maps of emission / absorption lines
• Large aperture spectroscopy without loss of
  resolution (Low Surface Brightness galaxies)

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Schematic of SPIRAL on the AAT.
Figure courtesy of Matthew Kenworthy
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How the IFU works

• Fore-optics re-image the telescope focal plane.
• A micro-lens array is used to sample the
  magnified image
• Optical fibres are used to re-format the
  two-dimensional image into a one-dimensional
  slit.
• The fibres feed a dedicated bench mounted optical
  spectrograph.
• The light is dispersed to form a spectrum on the
  detector

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Photograph of micro-lens array.
The LIMO micro-lens array contains 512 - 1 mm
Square lenses, all Silica construction, with
anti-reflection coatings.
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SPIRALs two-dimensional fibre array
The two-dimensional fibre array containing 512
optical fibres. The fibres are positioned within
a machined brass plate to an accuracy of 5
microns RMS.
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SPIRALs output slit.
At the output slit the fibres are reformatted
into a linear array which forms the entrance slit
to the spectrograph. SPIRALs output slit is 60
mm in length.
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The SPIRAL spectrograph

• Littrow design
• Mounted on a stable optical bench
• Operates at F/4.8
• Spectral resolution from 1000 - 8000
• Wavelength range 480 - 900 nm

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IFS observing sequence

• Due to the large amount of data obtained with a
  single IFS observation some care has to be taken
  to ensure that appropriate calibration exposures
  and sky observations are taken.
• Arc lamp exposures for wavelength calibration -
  each fibre is individually calibrated.
• Dome - flat exposures (white light source) to
  allow identification of spectra and to remove
  pixel to pixel variations on the detector.
• Twilight sky exposures to accurately determine
  the transmission of each fibre / microlens - this
  allows flat-fielding of reconstructed images.
• Object / Sky exposures - separate object and sky
  observations may be required. Care has to be
  taken to obtain accurate sky subtraction.
• Spectrophotometric standard stars for flux
  calibration or velocity templates.

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Example CCD data - MITLL2
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E-IFU standard star observation.
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Example data PKS 1733-565

• V17 Compact radio galaxy

Continuum Emission IFU image at wavelength 5550Å
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PKS 1733-565
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PKS 1733-565
Emission line map in OIII 5007Å (5500Å at
z0.0985)
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Sky subtraction mean sky method

• Allocate sky fibres within field of view
• Simultaneous observation of both object and sky
• object must not fill field of view
• sky spectrum obtained from mean of all sky fibres
• Problems can arise due to systematic errors such
  as
• Wavelength calibration errors
• Errors in determination of fibre throughput
• contamination of sky spectrum

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Sky subtraction mean sky method
Before
After sky subtraction (note sky residuals)
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Low Surface Brightness galaxy F362-030

• Integrated photographic B magnitude 18.6 mag
• Surface brightness 23.9 mag/square arc-second
• Object size 10 arc-seconds

DSS image
Spectrum from 45 minute exposure (3 x 900 s
object, 3 x 900 s sky)
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Spectrum of LSB galaxy
Reconstructed IFU image

• Note the excellent subtraction of the night sky
  emission lines
• Measured redshift z 0.029

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Planetary nebula NGC6302
Reconstructed IFU image in NII. This image
consists of a mosaic of 8 IFU images.
DSS image
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The jet of R-mon (NGC 2261)
SPIRAL observations of the reflection nebula
around star R-mon. Green is H-alpha, blue is OI
6300 Å, red is SII 6716 Å. Image size is 35 x
20.
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Example spectra from R-Mon
Spectra, SII 6716 and 6731 Å, from two of the
SPIRAL slit blocks with the continuum subtracted.
Weak emission from the nebula can be seen, with
stronger blue shifted emission from the jet.
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Supernova 1987A
Composite colour IFU image with OI 6300 Å (red)
and continuum (blue and green).
HST image
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SAURON data NGC 2549
Reconstructed image of NGC 2549
http//www-obs.univ-lyon1.fr/ycopin/sauron.html
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IFS further information

• Lenslet - fibre type systems
• SPIRAL - A Kenworthy et al., 2001, PASP, Vol.
  113, p 215
• INTEGRAL (WHT) http//www.ing.iac.es/bgarcia/int
  egral/html/integral_home.html
• Lenslet only type systems
• TIGER Bacon et al., 1995, Astronomy and
  Astrophysics Supplement, v.113, p.347
• SAURON Bacon et al., 2001, MNRAS, in press
  (astro-ph/0103451)
• Image slicer spectrographs
• NIFS (Gemini) http//www.mso.anu.edu.au/nifs/
• 3D Weitzel et al., 1996, Astronomy and
  Astrophysics Supplement, v.119, p.531

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Summary of IFU characteristics.

• Field of view 22 x 11 (32 x 10)
• Spatial sampling 0.7 (1.0)
• Wavelength range 480 nm - 900 nm
• Resolution 1000 - 8000 (R4000 with 600 l/mm)
• IFU data reduction software available on-line
  during observations at AAT
• SPIRAL - Nod Shuffle observing mode for
  improved sky subtraction accuracy

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Diagram of fore-optics.
1
2
4
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1 - Corrector lens 2 - Magnification lens 3 -
Field lens 4 - Microlens array
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Fibre Transmission.
SPIRAL uses blue fibres for better UV
performance but with absorption in the red.