Aug-Nov, 2008 IAG/USP (Keith Taylor) Instrumentation Concepts Ground-based Optical Telescopes Keith Taylor (IAG/USP) Aug-Nov, 2008 Aug-Sep, 2008 IAG-USP

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

Instrumentation Concepts

Ground-based Optical Telescopes

Keith Taylor(IAG/USP)

Aug-Nov, 2008

Aug-Sep, 2008 IAG-USP (Keith Taylor)


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.


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.


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.



Lenslet array (example)

Used inSPIRAL (AAT)VIMOS (VLT)Eucalyptus (OPD)

LIMO (glass)Pitch = 1mm

Some manufacturersuse plastic lenses.

Pitches down to~50m


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

SIFS

Tiger

MPI’s 3D


Mirror Image SlicersPioneered by

MPI (3D)(Gensel)

CompactEfficientSlicer of choice

but …

Cannot be retrofitted to existing spectrographs


Image Slicers

Principle of a simple image slicer, arranging several slices of the sky in a line along the entrance slit of the spectrograph.


Reflective Image Slicer


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.


3D spectroscopy

• Integral Field Unit:– How to have a projection of a 3D volume to a 2D plan?

• Spatial reformatting: Slicers

Y

X


How to “slice” the target?


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


Optical design (IR Path)

Relay optics Slicer Unit

Collimator

Prism

Camera Detector


Slicer Design (IR)

Relay optics

Collimator

Slicer Unit

Pupil & Slit mirror

Slicer Unit

Pupil & Slit mirror


Optical design (IR Path)

Relay optics Slicer Unit

Collimator

Prism

Camera Detector


Hybrids & Exotica

PYTHEAS (Georgelin et al – Marseille)

Based on a cross between 1. TIGER (lenslet array IFU)2. Fabry-Perot

Tunable Echelle Imager (Bland & Baldry)

Based on a cross between1. Cross-dispersed Echelle2. Fabry-Perot


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


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


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?


Tiger (in practice)

Enlarger

Lenslet array Collimator GrismCamera

Detector


Avoiding overlap

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

-direction

Represents a neat/clever optical trick


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


PYTHEAS

PYTHEAS (Georgelin et al – Marseille) Based on a cross between 1.TIGER (lenslet array IFU)2.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


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



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


PYTHEAS = Combination of …

TIGER’s true 3D capability Simultaneous: 2D Spatial + 1D

Wavelength FP’s 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


PYTHEAS – How it works



PYTHEAS - Results

Enlargement of Na Doublet range.Local Interstellar + Globular

components


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


But what if the galaxy ismonochromatic?

Then …

becomes

So lets move the slit at the spectrograph input …

becomes

and, in fact … becomes

x

x

x


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


Multi-monochromatic FP images dispersed by grating spectrograph

becomes

becomes

Scan the FP and then …

x

x+dx


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


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.


So now replace grating with a cross-dispersed

echelleCrossed with an

FP gives


A TEI scan


TEI: Option #1


TEI: Option #2


TEI: Option #3


TEI configurations(from Baldry & Bland)




Highly efficient use of detector


The neatest trick

OH sky-line suppression imaging

In this example, 90% of OH energy is suppressed.

Huge gain in SNR against sky continuum

Documents

Aug-Nov, 2008 IAG/USP (Keith Taylor) Instrumentation Concepts Ground-based Optical Telescopes Keith Taylor (IAG/USP) Aug-Nov, 2008 Aug-Sep, 2008 IAG-USP