Instrumentation Concepts Ground-based Optical Telescopes

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Instrumentation ConceptsGround-based Optical
Telescopes

• Keith Taylor
• (IAG/USP)
• Aug-Nov, 2008

Aug-Sep, 2008
IAG-USP (Keith Taylor)
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Integral Field Units

• Three principal types of IFUs at UV, optical and
  near IR wavelengths
• Reflective
• Refractive (microlenses)
• Optical fibre
• Also combinations of microlenses and fibres.

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Why do we want to use an image slicer?

• To get spatial information on resolved sources.
  Usually these image slicers are called Integral
  Field Spectrographs.
• To preserve light from extended sources and
  sources whose image profile is broadened by the
  atmosphere.

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Image Slicers

• Slit spectrographs are inherently restricted
  because light from outside of a narrow slice of
  the sky does not enter the instrument.
• This entrance slit can be long and in some
  circumstances it can even be curved. However in
  one direction it is narrow.
• Many images, including in many cases the images
  of point sources (broadened by seeing) are wider
  than this.
• Image slicers reformat the image, allowing more
  of it to pass through the slit.

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Lenslet array (example)
LIMO (glass) Pitch 1mm Some manufacturers use
plastic lenses. Pitches down to 50?m
Used in SPIRAL (AAT) VIMOS (VLT) Eucalyptus (OPD)
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Integral Field Spectroscopy

• Extended (diffuse) object with lots of spectra
• Use contiguous 2D array of fibres or mirror
  slicer to obtain a spectrum at each point in an
  image

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Mirror Image Slicers
Pioneered by MPI (3D) (Gensel)
Compact Efficient Slicer of choice but Cannot
be retrofitted to existing spectrographs
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Image Slicers
Principle of a simple image slicer, arranging
several slices of the sky in a line along the
entrance slit of the spectrograph.
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Reflective Image Slicer
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Reflective Image Slicer

• Consists of a stack of reflectors of rectangular
  aspect, tilted at different angles.
• Relay mirrors reimage the light reflected off
  these reflectors, and arrange them in a line to
  form a pseudo slit.
• The stacked reflectors need not be plane, often
  they have some power to keep the instrument
  compact.

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3D spectroscopy

• Integral Field Unit
• How to have a projection of a 3D volume to a 2D
  plan?
• Spatial reformatting Slicers

X
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How to slice the target?
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Instrument Status

• New Optical design
• Dichroics earlier possible
• Smallest size (2mm)
• Better instrument optimization (sampling)
• Easier focal plane
• Shorter instrument (300mm)
• Implementation phase in a compact volume
• Shoehorn needed to enter in the shoebox

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Optical design (IR Path)
Relay optics
Slicer Unit
Prism
Collimator
Camera
Detector
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Slicer Design (IR)
Relay optics
Collimator
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Optical design (IR Path)
Relay optics
Slicer Unit
Prism
Collimator
Camera
Detector
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Hybrids Exotica

• PYTHEAS (Georgelin et al Marseille)
• Based on a cross between
• TIGER (lenslet array IFU)
• Fabry-Perot
• Tunable Echelle Imager (Bland Baldry)
• Based on a cross between
• Cross-dispersed Echelle
• Fabry-Perot

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Fabry-Perot (reminder)

• Light enters etalon and is subjected to multiple
  reflections
• Transmission spectrum has numerous narrow peaks
  at wavelengths where path difference results in
  constructive interference
• need blocking filters to use as narrow band
  filter
• Width and depth of peaks depends on reflectivity
  of etalon surfaces finesse

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Fabry Perot (reminder)What you see with your eye
Emission-line lab source (Ne, perhaps) note the
yellow fringes

• Orders
• m
• (m-1)
• (m-2)
• (m-3)

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Tiger (Courtes, Marseille)

• Technique reimages telescope focal plane onto a
  micro-lens array
• Feeds a classical, focal reducer, grism
  spectrograph
• Micro-lens array
• Dissects image into a 2D array of small regions
  in the focal surface
• Forms multiple images of the telescope pupil
  which are imaged through the grism spectrograph.
• This gives a spectrum for each small region of
  the image (or lenslet)
• Without the grism, each telescope pupil image
  would be recorded as a grid of points on the
  detector in the image plane
• The grism acts to disperse the light from each
  section of the image independently

So, why dont the spectra all overlap?
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Tiger (in practice)
Enlarger
Detector
Camera
Lenslet array
Collimator
Grism
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Avoiding overlap
?-direction

• The grism is angled (slightly) so that the
  spectra can be extended in the ?-direction
• Each pupil image is small enough so theres no
  overlap orthogonal to the dispersion direction

Represents a neat/clever optical trick
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Tiger constraints

• The number and length of the Tiger spectra is
  constrained by a combination of
• detector format
• micro-lens format
• spectral resolution
• spectral range
• Nevertheless a very effective and practical
  solution can be obtained

Tiger (on CFHT) SAURON (on WHT) OSIRIS (on
Keck)
True 3D spectroscopy does NOT use time-domain
as the 3rd axis (like FP IFTS) very limited
FoV, as a result
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PYTHEAS

• PYTHEAS (Georgelin et al Marseille)
• Based on a cross between
• TIGER (lenslet array IFU)
• Fabry-Perot
• Goal
• True 3D imaging
• Given by a lenslet array IFU system
• Wide wavelength range
• Given by a classical Grating or Grism
• High Spectral resolution
• Given by a Fabry-Perot

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Scientific Motivation

• Ideal 3D imager should have
• High Spatial Resolution
• Large telescope (with Adaptive Optics)
• Large Field-of-View (comparable with interesting
  sources)
• High Spectral Resolution
• Easily obtained with FPs
• Long wavelength coverage
• Easily obtained classical spectroscopy

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PYTHEAS(Optical Scheme)

• Magnified field imaged onto a mirolens array
• FP dissects spectral information into multiple
  orders
• Grism disperses these orders in same way as TIGER
• FP is scanned over a FSR to give full wavelength
  coverage

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PYTHEAS Combination of

• TIGERs true 3D capability
• Simultaneous 2D Spatial 1D Wavelength
• FPs quasi-3D capability
• through encoding wavelength with time
• In this way one achieves high spectral and
  spatial resolution over a wide wavelength range
• but not simultaneously

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PYTHEAS How it works
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PYTHEAS - Results
Enlargement of Na Doublet range. Local
Interstellar Globular components
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Tunable Echelle Imager(TEI Baldry Bland)
Consider what a spectrograph does to this image
if it is placed at the input aperture of the
spectrograph
Assume galaxy is a continuum, then
becomes
Spectra from each point overlaps total
confusion This is why we use a slit
becomes
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But what if the galaxy ismonochromatic?
Then
becomes
So lets move the slit at the spectrograph input
becomes
and, in fact
becomes
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Crossing gratings with FPs

• So, if we want to do imaging and spectroscopy
  simultaneously
• ie Integral Field Spectroscopy
• We have to make objects appear monochromatic
• Crazy how can we do that?
• So how about making them multi-monochromatic?
• This is exactly what a Fabry-Perot does

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Multi-monochromatic FP images dispersed by
grating spectrograph
becomes
Scan the FP and then
becomes
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Reminder of X-dispersedEchelle

• X-dispersed echelle grating spectrometers
  allow high resolution and lots of spectral
  coverage
• Achieve this by having two orthogonal
  gratings
• One gives the high resolution (in y-axis) the
  other spreads the spectrum across the detector(in
  x-axis)
• Slit is consequently much shorter

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X-dispersion

• Orders are separated by dispersing them at low
  dispersion (often using a prism).
• X-dispersion is orthogonal to the primary
  dispersion axis.
• With a suitable choice of design parameters, one
  order will roughly fill the detector in the
  primary dispersion direction.
• With suitable choices of design parameters it is
  possible to cover a wide wavelength range, say
  from 300-555nm, as shown in the figure, in a
  single exposure without gaps between orders.

• Illustrative cross-dispersed spectrum showing a
  simplified layout on the detector.
• m 10-16
• The vertical axis gives wavelength (nm) at the
  lowest end of each complete order.
• For simplicity the orders are shown evenly
  spaced in cross-dispersion.

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So now replace grating with a cross-dispersed
echelle
Crossed with an FP gives
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A TEI scan
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TEI Option 1
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TEI Option 2
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TEI Option 3
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TEI configurations(from Baldry Bland)
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Highly efficient use of detector
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The neatest trick
OH sky-line suppression imaging
In this example, 90 of OH energy is
suppressed. Huge gain in SNR against sky
continuum