Image Slicers - IAG-Usp
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Transcript Image Slicers - IAG-Usp
Instrumentation Concepts
Ground-based Optical
Telescopes
Keith Taylor
(IAG/USP)
Aug-Nov, 2008
Aug-Nov, 2008
Aug-Sep,
2008
IAG/USP (Keith
IAG-USP
(Keith Taylor)
Taylor)
Integral Field Units
Three principal types of IFUs at UV, optical and
near IR wavelengths:
Reflective
Refractive (microlenses)
Optical fibre
Aug-Nov, 2008
Also combinations of microlenses and fibres.
IAG/USP (Keith Taylor)
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.
Aug-Nov, 2008
IAG/USP (Keith Taylor)
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.
Aug-Nov, 2008
IAG/USP (Keith Taylor)
Aug-Nov, 2008
IAG/USP (Keith Taylor)
Lenslet array (example)
LIMO (glass)
Pitch = 1mm
Some manufacturers
use plastic lenses.
Pitches down to
~50m
Used in
SPIRAL (AAT)
VIMOS (VLT)
Eucalyptus (OPD)
Aug-Nov, 2008
IAG/USP (Keith Taylor)
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
Tiger
SIFS
MPI’s 3D
Aug-Nov, 2008
IAG/USP (Keith Taylor)
Mirror Image Slicers
Pioneered by
MPI (3D)
(Gensel)
Compact
Efficient
Slicer of choice
but …
Cannot be
retrofitted to
existing
spectrographs
Aug-Nov, 2008
IAG/USP (Keith Taylor)
Image Slicers
Principle of a simple image
slicer, arranging several
slices of the sky in a line
along the entrance slit of the
spectrograph.
Aug-Nov, 2008
IAG/USP (Keith Taylor)
Reflective Image Slicer
Aug-Nov, 2008
IAG/USP (Keith Taylor)
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.
Aug-Nov, 2008
IAG/USP (Keith Taylor)
3D spectroscopy
• Integral Field Unit:
– How to have a projection of a 3D volume to a
2D plan?
Y
• Spatial reformatting: Slicers
l
X
Aug-Nov, 2008
IAG/USP (Keith Taylor)
How to “slice” the target?
Aug-Nov, 2008
IAG/USP (Keith Taylor)
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
Aug-Nov, 2008
IAG/USP (Keith Taylor)
Optical design (IR Path)
Relay optics
Slicer Unit
Prism
Collimator
Camera
Aug-Nov, 2008
Detector
IAG/USP (Keith Taylor)
Slicer Design (IR)
Slicer Unit
Slicer Unit
Relay optics
Pupil & Slit
mirror
Collimator
Pupil & Slit mirror
Aug-Nov, 2008
IAG/USP (Keith Taylor)
Optical design (IR Path)
Relay optics
Slicer Unit
Prism
Collimator
Camera
Aug-Nov, 2008
Detector
IAG/USP (Keith Taylor)
Hybrids & Exotica
PYTHEAS (Georgelin et al – Marseille)
1.
2.
Based on a cross between
TIGER (lenslet array IFU)
Fabry-Perot
Tunable Echelle Imager (Bland & Baldry)
1.
2.
Aug-Nov, 2008
Based on a cross between
Cross-dispersed Echelle
Fabry-Perot
IAG/USP (Keith Taylor)
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
Aug-Nov, 2008
IAG/USP (Keith Taylor)
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)
The central or
“Jacquinot”
spot
Aug-Nov, 2008
IAG/USP (Keith Taylor)
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 don’t the spectra all overlap?
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IAG/USP (Keith Taylor)
Tiger (in practice)
Enlarger
Lenslet array
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Detector
Collimator
IAG/USP (Keith Taylor)
Grism
Camera
Avoiding overlap
• The grism is angled (slightly) so that the spectra can be extended
in the l-direction
• Each pupil image is small enough so there’s no overlap orthogonal
to the dispersion direction
Represents a neat/clever optical trick
Aug-Nov, 2008
IAG/USP (Keith Taylor)
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
SAURON
OSIRIS
(on CFHT)
(on WHT)
(on Keck)
True 3D spectroscopy
– does NOT use time-domain as the 3rd axis (like FP & IFTS)
– very limited FoV, as a result
Aug-Nov, 2008
IAG/USP (Keith Taylor)
PYTHEAS
PYTHEAS (Georgelin et al – Marseille)
Based on a cross between
1. TIGER (lenslet array IFU)
2. Fabry-Perot
Goal
True 3D imaging
Wide wavelength range
Given by a lenslet array IFU system
Given by a classical Grating or Grism
High Spectral resolution
Aug-Nov, 2008
Given by a Fabry-Perot
IAG/USP (Keith Taylor)
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
Aug-Nov, 2008
Easily obtained classical spectroscopy
IAG/USP (Keith Taylor)
Aug-Nov, 2008
IAG/USP (Keith Taylor)
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
Aug-Nov, 2008
IAG/USP (Keith Taylor)
PYTHEAS =
Combination of …
TIGER’s true 3D capability
FP’s quasi-3D capability
Simultaneous: 2D Spatial + 1D Wavelength
through encoding wavelength with time
In this way one achieves high spectral and spatial
resolution over a wide wavelength range
but not simultaneously
Aug-Nov, 2008
IAG/USP (Keith Taylor)
PYTHEAS – How it works
Aug-Nov, 2008
IAG/USP (Keith Taylor)
Aug-Nov, 2008
IAG/USP (Keith Taylor)
PYTHEAS - Results
Enlargement of Na Doublet range.
Local Interstellar + Globular
components
Aug-Nov, 2008
IAG/USP (Keith Taylor)
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
Aug-Nov, 2008
IAG/USP (Keith Taylor)
But what if the galaxy is
monochromatic?
Then …
becomes
x
So lets move the slit at the spectrograph input …
becomes
x
and, in fact …
becomes
x
Aug-Nov, 2008
IAG/USP (Keith Taylor)
Crossing gratings with FPs
So, if we want to do imaging and spectroscopy
simultaneously:
We have to make objects appear monochromatic
ie: Integral Field Spectroscopy
Crazy … how can we do that?
So how about making them multi-monochromatic?
This is exactly what a Fabry-Perot does
Aug-Nov, 2008
IAG/USP (Keith Taylor)
Multi-monochromatic FP images
dispersed by grating spectrograph
becomes
x
Scan the FP and then …
becomes
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x+dx
IAG/USP (Keith Taylor)
Reminder of X-dispersed
Echelle
•
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
IAG/USP (Keith Taylor)
Aug-Nov, 2008
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.
Aug-Nov, 2008
IAG/USP (Keith Taylor)
So now replace grating with a
cross-dispersed echelle
Crossed
with an
FP gives
Aug-Nov, 2008
IAG/USP (Keith Taylor)
A TEI scan
Aug-Nov, 2008
IAG/USP (Keith Taylor)
TEI: Option #1
Aug-Nov, 2008
IAG/USP (Keith Taylor)
TEI: Option #2
Aug-Nov, 2008
IAG/USP (Keith Taylor)
TEI: Option #3
Aug-Nov, 2008
IAG/USP (Keith Taylor)
TEI configurations
(from Baldry & Bland)
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IAG/USP (Keith Taylor)
Aug-Nov, 2008
IAG/USP (Keith Taylor)
Aug-Nov, 2008
IAG/USP (Keith Taylor)
Highly efficient use of detector
Aug-Nov, 2008
IAG/USP (Keith Taylor)
The neatest trick
OH sky-line suppression imaging
In this example, 90% of OH
energy is suppressed.
Huge gain in SNR against
sky continuum
Aug-Nov, 2008
IAG/USP (Keith Taylor)