Integral Field Spectroscopy. Integral Field Spectroscopy. Integral Field Spectroscopy. Integral Field Spectroscopy.

David Lee, David Lee,

Anglo-Australian Observatory.Anglo-Australian Observatory.

dl@aaoepp.aao.gov.audl@aaoepp.aao.gov.au

http://www.aao.gov.au/local/www/dlhttp://www.aao.gov.au/local/www/dl

Why integral field spectroscopy?Why integral field spectroscopy?Why integral field spectroscopy?Why integral field spectroscopy?

• Traditional spectroscopic techniques include: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 For observations of many types of object it would be useful to obtain information about the two-dimensional spatial structure obtain information about the two-dimensional spatial structure as well as spectral information.as well as spectral information.

• Integral field spectroscopy is a relatively new technique Integral field spectroscopy is a relatively new technique developed to achieve this.developed to achieve this.

Integral field spectroscopy.Integral field spectroscopy.Integral field spectroscopy.Integral field spectroscopy.

Integral field spectroscopy is a technique to produce a Integral field spectroscopy is a technique to produce a spectrum for each spatial element in an extended two-spectrum for each spatial element in an extended two-dimensional field. The observation produces a data-dimensional field. The observation produces a data-cube containing both spatial and spectral information.cube containing both spatial and spectral information.

Various methods of IFS:Various methods of IFS:Various methods of IFS:Various methods of IFS:

http://aig-www.dur.ac.uk/

Advantages of IFSAdvantages of IFSAdvantages of IFSAdvantages of IFS• Obtain both imaging and spectroscopic information Obtain both imaging and spectroscopic information

simultaneously - maximises the information available on the simultaneously - maximises the information available on the detector.detector.

• In bad seeing the large field of view of an IFS will help to In bad seeing the large field of view of an IFS will help to prevent slit losses.prevent slit losses.

• Resolution is fixed by the fibre / mirror size not by the slit Resolution is fixed by the fibre / mirror size not by the slit width.width.

• Use of optical fibres allows the instrument to be removed Use of optical fibres allows the instrument to be removed from the telescope and located in a more stable environment.from the telescope and located in a more stable environment.

• Target acquisition is straightforward.Target acquisition is straightforward.

Disadvantages of IFSDisadvantages of IFSDisadvantages of IFSDisadvantages of IFS• The integral field unit optics can decrease the transmission of The integral field unit optics can decrease the transmission of

the instrument.the instrument.

• Accurate sky subtraction becomes more difficult than with a Accurate sky subtraction becomes more difficult than with a longslit or multiple-slit spectrograph.longslit or multiple-slit spectrograph.

• Data analysis can be difficult.Data analysis can be difficult.

• The “slit length” is much less than with a longslit The “slit length” is much less than with a longslit spectrograph. spectrograph.

Science with IFSScience with IFSScience with IFSScience with IFS• Spectroscopy of extended objectsSpectroscopy of extended objects

• Spatially resolved spectroscopy (aperture effect)Spatially resolved spectroscopy (aperture effect)

• Dynamics, kinematics, velocity maps - rotation curvesDynamics, kinematics, velocity maps - rotation curves

• Velocity dispersion informationVelocity dispersion information

• Variation of spectrum within object (starburst / AGN etc)Variation of spectrum within object (starburst / AGN etc)

• Line strength distributionsLine strength distributions

• Maps of emission / absorption linesMaps of emission / absorption lines

• ““Large aperture spectroscopy” without loss of resolution (Low Large aperture spectroscopy” without loss of resolution (Low Surface Brightness galaxies)Surface Brightness galaxies)

Schematic of SPIRAL on the AAT.Schematic of SPIRAL on the AAT.Schematic of SPIRAL on the AAT.Schematic of SPIRAL on the AAT.

Figure courtesy of Matthew Kenworthy

How the IFU works:How the IFU works:How the IFU works:How the IFU works:

• Fore-optics re-image the telescope focal plane.Fore-optics re-image the telescope focal plane.• A micro-lens array is used to sample the magnified A micro-lens array is used to sample the magnified

imageimage• Optical fibres are used to re-format the two-Optical fibres are used to re-format the two-

dimensional image into a one-dimensional slit.dimensional image into a one-dimensional slit.• The fibres feed a dedicated bench mounted optical The fibres feed a dedicated bench mounted optical

spectrograph.spectrograph.• The light is dispersed to form a spectrum on the The light is dispersed to form a spectrum on the

detectordetector

Photograph of micro-lens array.Photograph of micro-lens array.Photograph of micro-lens array.Photograph of micro-lens array.

The LIMO micro-lens array contains 512 - 1 mm Square lenses, The LIMO micro-lens array contains 512 - 1 mm Square lenses, all Silica construction, with anti-reflection coatings.all Silica construction, with anti-reflection coatings.

SPIRAL’s two-dimensional fibre arraySPIRAL’s two-dimensional fibre arraySPIRAL’s two-dimensional fibre arraySPIRAL’s two-dimensional fibre array

The two-dimensional fibre array containing 512 optical The two-dimensional fibre array containing 512 optical fibres. The fibres are positioned within a machined brass fibres. The fibres are positioned within a machined brass plate to an accuracy of ~5 microns RMS.plate to an accuracy of ~5 microns RMS.

SPIRAL’s output slit.SPIRAL’s output slit.SPIRAL’s output slit.SPIRAL’s output slit.

At the output slit the fibres are reformatted into a linear array At the output slit the fibres are reformatted into a linear array which forms the entrance slit to the spectrograph. SPIRAL’s which forms the entrance slit to the spectrograph. SPIRAL’s output slit is 60 mm in length.output slit is 60 mm in length.

The SPIRAL spectrographThe SPIRAL spectrographThe SPIRAL spectrographThe SPIRAL spectrograph

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

IFS: observing sequenceIFS: observing sequenceIFS: observing sequenceIFS: observing sequenceDue to the large amount of data obtained with a single IFS observation some 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 care has to be taken to ensure that appropriate calibration exposures and sky observations are taken.sky observations are taken.

• Arc lamp exposures for wavelength calibration - each fibre is individually Arc lamp exposures for wavelength calibration - each fibre is individually calibrated.calibrated.

• Dome - flat exposures (white light source) to allow identification of Dome - flat exposures (white light source) to allow identification of spectra and to remove pixel to pixel variations on the detector.spectra and to remove pixel to pixel variations on the detector.

• Twilight sky exposures to accurately determine the transmission of each Twilight sky exposures to accurately determine the transmission of each fibre / microlens - this allows flat-fielding of reconstructed images.fibre / microlens - this allows flat-fielding of reconstructed images.

• Object / Sky exposures - separate object and sky observations may be Object / Sky exposures - separate object and sky observations may be required. Care has to be taken to obtain accurate sky subtraction.required. Care has to be taken to obtain accurate sky subtraction.

• Spectrophotometric standard stars for flux calibration or velocity Spectrophotometric standard stars for flux calibration or velocity templates.templates.

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E-IFU standard star observation.E-IFU standard star observation.E-IFU standard star observation.E-IFU standard star observation.

Example data: PKS 1733-565Example data: PKS 1733-565Example data: PKS 1733-565Example data: PKS 1733-565

V=17 Compact radio galaxyV=17 Compact radio galaxy

ContinuumEmission IFU image atwavelength 5550Å

PKS 1733-565PKS 1733-565PKS 1733-565PKS 1733-565

PKS 1733-565PKS 1733-565PKS 1733-565PKS 1733-565

Emission line map in O[III] 5007Å (~5500Å at z=0.0985)

Sky subtraction: mean sky methodSky subtraction: mean sky methodSky subtraction: mean sky methodSky subtraction: mean sky method

Allocate sky fibres within field of viewAllocate sky fibres within field of view• Simultaneous observation of both object and skySimultaneous observation of both object and sky• object must not fill field of view object must not fill field of view • sky spectrum obtained from mean of all sky fibressky spectrum obtained from mean of all sky fibres

Problems can arise due to systematic errors such as:Problems can arise due to systematic errors such as:• Wavelength calibration errorsWavelength calibration errors• Errors in determination of fibre throughputErrors in determination of fibre throughput• contamination of sky spectrumcontamination of sky spectrum

Sky subtraction: mean sky methodSky subtraction: mean sky methodSky subtraction: mean sky methodSky subtraction: mean sky method

Before

After sky subtraction (note sky

residuals)

Low Surface Brightness galaxy F362-030Low Surface Brightness galaxy F362-030Low Surface Brightness galaxy F362-030Low Surface Brightness galaxy F362-030

• Integrated photographic B magnitude = 18.6 magIntegrated photographic B magnitude = 18.6 mag

• Surface brightness 23.9 mag/square arc-secondSurface brightness 23.9 mag/square arc-second

• Object size ~10 arc-secondsObject size ~10 arc-seconds

DSS imageDSS image

Spectrum from 45 minute exposure Spectrum from 45 minute exposure (3 x 900 s object, 3 x 900 s sky)(3 x 900 s object, 3 x 900 s sky)

Spectrum of LSB galaxySpectrum of LSB galaxySpectrum of LSB galaxySpectrum of LSB galaxy

Reconstructed IFU imageReconstructed IFU image

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

Planetary nebula NGC6302Planetary nebula NGC6302Planetary nebula NGC6302Planetary nebula NGC6302Reconstructed IFU Reconstructed IFU image in N[II]. This image in N[II]. This image consists of a image consists of a mosaic of 8 IFU mosaic of 8 IFU images.images.

DSS imageDSS image

The jet of R-mon (NGC 2261)The jet of R-mon (NGC 2261)The jet of R-mon (NGC 2261)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”.

Example spectra from R-MonExample spectra from R-MonExample spectra from R-MonExample spectra from R-MonSpectra, S[II] 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.

Supernova 1987ASupernova 1987ASupernova 1987ASupernova 1987A

Composite colour IFU image with [OI] 6300 Å (red) and continuum (blue and green).

HST image

SAURON data: NGC 2549SAURON data: NGC 2549SAURON data: NGC 2549SAURON data: NGC 2549

http://www-obs.univ-lyon1.fr/~ycopin/sauron.html

Reconstructed image of NGC

2549

Velocity mapVelocity dispersion map

IFS: further informationIFS: further informationIFS: further informationIFS: further information• Lenslet - fibre type systems:Lenslet - fibre type systems:

– SPIRAL - A: Kenworthy et al., 2001, PASP, Vol. 113, p 215

– INTEGRAL (WHT): http://www.ing.iac.es/~bgarcia/integral/html/integral_home.html

• Lenslet only type systems: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: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

Summary of IFU characteristics.Summary of IFU characteristics.Summary of IFU characteristics.Summary of IFU characteristics.

• Field of view 22” x 11” Field of view 22” x 11” (32” x 10”)(32” x 10”)• Spatial sampling 0.7” Spatial sampling 0.7” (1.0”)(1.0”)• Wavelength range 480 nm - 900 nm Wavelength range 480 nm - 900 nm • Resolution 1000 - 8000 Resolution 1000 - 8000 (R=4000 with 600 l/mm)(R=4000 with 600 l/mm)• IFU data reduction software available “on-line” IFU data reduction software available “on-line”

during observations at AATduring observations at AAT• SPIRAL - Nod & Shuffle observing mode for SPIRAL - Nod & Shuffle observing mode for

improved sky subtraction accuracyimproved sky subtraction accuracy

Diagram of fore-optics.Diagram of fore-optics.Diagram of fore-optics.Diagram of fore-optics.

1 2 3 4

1 - Corrector lens1 - Corrector lens2 - Magnification lens2 - Magnification lens3 - Field lens3 - Field lens4 - Microlens array4 - Microlens array

Fibre Transmission.Fibre Transmission.Fibre Transmission.Fibre Transmission.SPIRAL uses “blue” fibres for better UV performance but with absorption in the red.